COMPARATIVE STUDY OF FIELD PLATED AlGaN/GaN AND …
Transcript of COMPARATIVE STUDY OF FIELD PLATED AlGaN/GaN AND …
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COMPARATIVE STUDY OF FIELD PLATED
AlGaN/GaN AND InAlN/GaN HEMT FOR
MICROWAVE ELECTRONIC APPLICATIONS
A Project report submitted in partial fulfillment of the requirements for
the award of the degree of
BACHELOR OF ENGINEERING
IN
ELECTRONICS AND COMMUNICATION ENGINEERING
Submitted by
R.Lahari(316126512181) P. Anusha(316126512159)
P.Asha Srikar(316126512158) M.Divya Sri(316126512153)
Under the guidance of
Dr.P.Murugapandiyan,M.E,Ph.D
Associate Professor,ECE
DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING
ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY AND SCIENCES
(Permanently Affiliated to AU, Approved by AICTE and Accredited by NBA & NAAC with ‘A’ Grade)
Sangivalasa, bheemili mandal, visakhapatnam dist.(A.P)
2019-2020
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ACKNOWLEDGEMENT
We would like to express our deep gratitude to our project guide
Dr.P.Murugapandiyan Associate Professor, Department of Electronics and
Communication Engineering, ANITS, for his guidance with unsurpassed knowledge
and immense encouragement. We are grateful to Dr. V. Rajyalakshmi, Head of the
Department, Electronics and Communication Engineering, for providing us with the
required facilities for the completion of the project work.
We are very much thankful to the Principal and Management, ANITS, Sangivalasa,
for their encouragement and cooperation to carry out this work.
We express our thanks to all teaching faculty of Department of ECE, whose
suggestions during reviews helped us in accomplishment of our project. We would like
to thank all non-teaching staff of the Department of ECE, ANITS for providing great
assistance in accomplishment of our project.
We would like to thank our parents, friends, and classmates for their encouragement
throughout our project period. At last but not the least, we thank everyone for
supporting us directly or indirectly in completing this project successfully.
PROJECT STUDENTS
R.Lahari(316126512181),
P.Anusha(316126512159),
P.Asha Srikar(316126512158),
M.Divya Sri(316126512153)
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DEPARTMENT OF ELECTRONICS AND COMMUNICATION
ENGINEERING
ANIL NEERUKONDA INSTITUTE OF TECHNOLOGY AND SCIENCES
(Permanently Affiliated to AU, Approved by AICTE and Accredited by NBA &
NAAC with ‘A’ Grade)
Sangivalasa, bheemili mandal, visakhapatnam dist.(A.P)
CERTIFICATE
This is to certify that the project report entitled “Comparative Study of Field Plated
AlGaN/GaN and InAlN/GaN HEMTs for Microwave Electronic Applications”
submitted by R.Lahari(316126512181), P.Anusha(316126512159),
P.AshaSrikar(316126512158),M.Divya Sri(316126512153) in partial fulfillment of
the requirements for the award of the degree of Bachelor of Engineering in
Electronics & Communication Engineering of Andhra University, Visakhapatnam is
a record of bonafide work carried out under my guidance and supervision.
Project Guide Head of the Department
Dr.P.Murugapandiyan Dr.V.Rajyalakshmi
Associate Professor Department of E.C.E
Department of E.C.E ANITS
ANITS
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CONTENTS
ABSTRACT vi
LIST OF SYMBOLS vii
LIST OF FIGURES viii
LIST OF TABLES ix
LIST OF ABBREVATIONS x
CHAPTER 1 INTRODUCTION 1
1.1 Nitride Based Semiconductors 1
1.1.1 GaN Based Transistor 3
1.1.2 Basic Properties of III-Nitride Semiconductors 3
1.1.3 Band Stricture and Lattice Constant of III-N Semiconductors 4
1.1.4 Spontaneous and Piezoelectric Polarization 5
1.1.5 Advantages of III-N Semiconductors 7
1.2 Project Objective 11
1.2.1 Scope of the project 12
1.3 Silvaco TCAD Tool 13
1.3.1 Atlas Overview 13
1.3.1.1 Atlas inputs and outputs 13
1.3.2 Deckbuild Overview 15
1.3.3 Dev-Edit Overiew 15
1.3.4 Device Basic Construction Layout 15
CHAPTER 2 AlGaN/GaN High Electron Mobility Transistors(HEMTs) 16
2.1 Basic Hemt Structure 17
2.1.1 Field plated HEMT 17
2.1.2 Working Principle 21
2.2 Two Dimensional Electron Gas(2-DEG) in HEMT 23
2.3 Results and Discussions 24
2.3.1 Drain Current Characteristics 24
2.3.2 Transfer Characteristics 26
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2.3.3 Log Id vs Vg Characteristics 27
2.3.4 Gate lekage current Characteristics 29
2.3.5 Breakdown Characteristics 31
2.3.6 Transconductance 33
2.3.7 Cutoff Frequency 34
2.3.8 Maximum Oscillation Frequency 36
CHAPTER 3 InAlN/GaN High Electron Mobility Transistors(HEMTs) 38
3.1 Basic Structure 38
3.2 Results and Discussions 41
3.2.1 Drain Current Characteristics 41
3.2.2 Transfer Characteristics 42
3.2.3 Log Id vs Vg Characteristics 43
3.2.4 Gate lekage current Characteristics 44
3.2.5 Breakdown Characteristics 45
3.2.6 Transconductance 46
3.2.7 Cutoff Frequency 47
3.2.8 Maximum Oscillation Frequency 48
CONCLUSION 49
FUTURE WORK 51
REFERENCES 52
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ABSTRACT
Gallium Nitride (GaN) has recently attracted widespread attention for promising
applications in the RF domain. Due to wider bandgap, GaN based devices show
exceptional performance at higher temperatures. Additionally, they permit device
operation at higher voltages and enable high frequency operation because of higher
bandgap energy (i.e. 3.4 eV). Higher sheet carrier densities, higher thermal
conductivity, higher electron mobility, and higher breakdown field are the major factors
which provide better performance in case of GaN based heterostructures. Additionally,
GaN based heterostructures have unintentionally doped two-dimensional electron gas
(2DEG). Owing to their high sheet carrier density in 2-D Electron Gas (2DEG), GaN
based High Electron Mobility Transistors (HEMTs) have been observed to provide
excellent performance in high–power switching applications and large breakdown field
strength. The device output characteristics and small signal characteristics of the
HEMTs degrades as temperature increases. Moreover, the breakdown characteristics of
the AlGaN/GaN and InAlN/GaN HEMTs show less dependence on temperature
variations.
The power–switching performance is however limited by the conduction losses in the
ON–state resistance RON and the OFF–state breakdown voltage VBR. It is therefore a
necessity to enhance the OFF-state breakdown while keeping RON as low as possible.
The objective of this project is to design Various device configurations in order to
improve the device performance characteristics such as the cut–off frequency ( fT),
power density, power added efficiency, breakdown voltage, etc.
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LIST OF SYMBOLS
E Longitudinal Electric Field
EC Conduction energy band
EV Valence energy band
μ
ɳs
Mobility of electron
Charge Concentration
n(x) Sheet Charge Density
v(x) Channel Potential
Lg Gate Length
Ldg Drain to Gate length
Lsg Source to Gate length
PSP Spontaneous Polarization
PPZ Piezoelectric polarization
Ids Drain saturated current
Ig f Gate Current
Ron On-resistance
Vgs Gate to Source Voltage
Vds Drain to Source Voltage
Vth Threshold voltage
gm Transconductance
fT Cutoff Frequency
fmax Maximum Oscillation Frequency
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LIST OF FIGURES
Fig. 1.1 Potential applications of GaN 2
Fig. 1.2 Wurtzite hexagonal unit cell. 6
Fig. 1.3(a) Total polarization for III-nitride materials grown pseudomorpjically
on Ga-polar c-plane GaN
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Fig. 1.3(b) Corresponding polarization charge density 7
Fig. 1.4(a) Dots: specific ON-resistance vs breakdown voltage for SiC 9
Fig. 1.4(b) Best reported output power density at various microwave frequencies
for AlGaN/GaN HEMTs
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Fig. 1.5 Simulation flow 14
Fig. 1.6 Atlas Command Groups with the primary statements in each group 14
Fig. 2.1(a) Structure of AlGaN/GaN hemt with fieldplate 20
Fig. 2.1(b) Simulated structure of AlGaN/GaN hemt with fieldplate 20
Fig. 2.2(a) Generalized band diagram of HEMT 22
Fig. 2.2(b) Simulated band diagram of AlGaN/GaN HEMT 22
Fig. 2.3 Drain characteristics 24
Fig. 2.4 Transfer characteristics 26
Fig. 2.5 Log Id vs Vg characteristics 28
Fig. 2.6 Log Ig vs Vg characteristics 30
Fig. 2.7 Breakdown field distribution with and without fieldplate. 31
Fig. 2.8 Breakdown curve for different lengths of field-plate 31
Fig. 2.9 𝑔𝑚 vs Vgs characteristics 33
Fig. 2.10(a) HEMT Structure 35
Fig. 2.10(b) Small signal equivalent circuits of HEMT 35
Fig.2.11 Current gain vs cutoff frequency 36
Fig.2.12 Power gain vs cutoff frequency 37
Fig. 3.1 InAlN/GaN HEMT structure with field plated 39
Fig. 3.2 Simulated result of InAlN/GaN HEMT with field plate length 2.25μm 40
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Fig. 3.3 Band diagram of InAlN/GaN HEMT 40
Fig. 3.4 Drain current charcteristics 41
Fig. 3.5 Transfer characteristics 42
Fig. 3.6 Log Id vs Vg characteristics 43
Fig. 3.7 Log Ig vs Vg Characteristics 44
Fig. 3.8 Breakdown characteristics of this device for different field plated
lengths 45
Fig. 3.9 gm vs Vgs characteristics 46
Fig. 3.10 Current gain vs Cutoff frequency 47
Fig. 3.11 Power gain vs frequency characteristics 48
LIST OF TABLES
Table 1.1 Semiconductor materials properties 2
Table 1.2 Band gap Eg and lattice constant a and c of binary nitride and
together with bowing parameter b of ternary nitride alloys 4
Table 3.1 DC and microwave performance of GaN based hemts 50
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LIST OF ABBREVATIONS
FET Field Effect Transistor
HEMT High Electron Mobility Transistor
MESFET Metal Semiconductor Field Effect Transistor
MODFET Modulation Doped Field Effect Transistor
GaN Gallium Nitride
GaAs Gallium Arsenic
AlGaN Aluminium Gallium Nitride
InAlN Indium Aluminium Nitride
2-DEG Two Dimensional Electron Gas
FP Fieldplate
I-V Current-Voltage
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CHAPTER 1
INTRODUCTION
The unique properties of GaN materials in comparison with other
semiconductor materials are fully investigated in this chapter. In this way the questions
should be answered, why the nitride material system has been in the focus of intense
research for more than a decade and still is today.
1.1 NITRIDE BASED SEMICONDUCTORS
In recent 20-30 years, a nitride-based semiconductor technology has been enormously
developed since essential problems preventative its progress, like high quality growth
and p-type doping, are solved through in substantial research work[16,17]. The
excellent material properties of the nitride alloys have encouraged many different
fields, which expands many possible application areas as shown in Figure 1.1.
In optoelectronics, the complete solution for emission and detection from infrared to
ultraviolet are achieved by the nitride based devices thanks to its wide range of potential
energy band gaps from 0.7 eV (InN) to 6.2 eV (AlN).
In 1971, the first GaN electroluminescent diode was demonstrated [18]. In continuation
with that GaN-based light emitting diodes (LED) were developed remarkably [17,19]
and in 1993 the very first commercial LEDs were launched in Nichia Chemical
Industries. Then its application extended to optoelectronics applications[20,21]. In
electronics and telecommunication industry GaN-based devices playing a vital role for
both high speed and high power applications. The key electrical properties of all
semiconductor materials are displayed in Table 1.1. GaN provides a high electric
breakdown field among all other maerials such as Si and GaAs. Moreover, the peak
saturation velocity with higher mobility makes the GaN-based electron devices is the
suitable candidates for high frequency application for handling high power.
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Figure 1.1 Potential applications of GaN
Table 1.1 Semiconductor materials properties.
Even though, SiC electrical properties is similar to GaN, due to critical issues in
heterojunction formation and extreme process condition bounds the applications of SiC.
On the other hand, GaN material can easily form the heterojunctions with other nitride
alloys and this enabling it to design high electron mobility transistors (HEMTs) which
has been used in various fields.
Material Properties Si GaAs SiC GaN Diomand
Band gap 𝐸𝑔 (𝑒𝑉) 1.1 1.4 3.3 3.4 5.5
Permitivity 𝜀𝑟 11.8 13.1 10 9.0 5.5
Electron mobility 𝜇𝑛 (𝑐𝑚 2 𝑉 −1 𝑠−1 ) 1350 8500 700 1200 1900
Saturation velocity 𝑣𝑠𝑎𝑡 (107𝑐𝑚/𝑠) 1.0 1.0 2.0 2.5 2.7
Critical electric field 𝐸𝑐𝑟 (𝑀𝑉/𝑐𝑚) 0.3 0.4 3.0 3.3 5.6
Thermal conductivity 𝜃 (𝑊𝐾 −1 𝑚−1 ) 150 43 330 130 2000
JFoM/JFoMSi 1.0 1.3 20.0 27.5 50.4
BFoM/BFoMSi 1.0 14.4 11.9 20.0 82.0
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Moreover, the availability of polarization induced charge carriers (2DEG) at the
heterojuction interface enables the device fabrication simple, because without the need
of external doping high sheet charge density available in the 2DEG. Due to the
aforementioned key features, GaN material widely used in the electronics in the past
three decades.
1.1.1 GaN based transistor
Since the first GaN-based MESFET demonstrated in 1993[22,23], several substantial
research have been carried out for verifying the existence of 2DEG with higher mobility
by using AlGaN/GaN HEMT structure[24-26]. And hence the HEMT structure thus
becomes the main structure of GaN-based transistors [22,23]. The unique properties of
both high electric breakdown field and high mobility of GaN-based transistor, it has
been captivated for high power switching and high power RF applications.
For high power switching applications, the operating frequency range is varying from
kHZ to MHz, where the breakdown voltage is in the range of 10 to 1000 V. Another
important application of GaN-based transistors is high power RF applications where
the device giving maximum output power while operating at high frequency. The high
power handling capability and high frequency operation of the device is related with
breakdown voltage of the device and saturation velocity of electron in 2DEG. Therefore
GaN-based semiconductor devices are the future forerunner of high power microwave
applications.
1.1.2 Basic properties of III-nitride semiconductors
The III-N compound semiconductors are majorly classified in to 1.Binay (AlN,GaN
and InN) compounds material 2.Ternary compound materials (AlxGa(1-x)N, InxAl(1-x)N
and InxGa(1-x)N) and 3. Quaternary alloys (InxAl(1- x)Ga(1-x-y)N). The GaN is the
interesting material among all in the nitride family as it can be grown with high quality
on sapphire, SiC and Si substrate. This is the major reason why GaN material used as
building blocks of a wide range of nitride-based devices such as, RF power transistor,
high frequency MMICs, laser diode light emitting diodes, MEMS and power
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conversion. However, GaN alone is not creating the devices. It includes the AlN,
AlGaN, InGaN or InAlGaN layers for creating heterostructures for functional
electronics and optoelectronics. In this section we revisit the III-N semiconductors, in
particular more relevant to electronic devices.
1.1.3 Band Structure and Lattice constant of III-N semiconductors
As a consequence of the strength of the metal-nitrogen bonds, the III- nitrides are
characterized, except for InAlN, by larger band gaps and shorter lattice constants
compared to other III-V compound semiconductors families, like arsenides,
phosphides, antimonides and their alloys. The band gap and lattice constant of binary
nitride materials are listed in Table.1.2 band gaps and lattice constant of Nitride
semiconductor alloys are shown in Figure.1.along with other III-V semiconductor
materials. The energy band gaps of AlxGa(1- x)N,InxAl(1-x)N and InxGa(1-x)N are follows
the quadratic equation:
𝐸𝑔,𝐴𝑥𝐵1−𝑥𝑁 = 𝑥𝐸𝑔,𝐴𝑁 +( 1−𝑥) 𝐸𝑔,𝐵𝑁 −𝑏𝐴𝐵𝑁𝑥(1−𝑥) (1.1)
Where A and B stand for Al,Ga or In depending on the compound desired and b is the
bowing parameter. The III-N family bowing parameter is listed in Table 1.2.
Table.1.2. Band gap Eg and lattice constant a and c of binary nitride and together with
bowing parameter b of ternary nitride alloys [27,28]
Material Eg (eV) a (Aº) c(Aº) Alloy b(eV)
AlN 6.14 3.112 4.982 AlGaN 0.7
GaN 3.42 3.189 5.185 InGaN 1.4
InN 0.64 3.545 5.703 InAlN 5.36
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1.1.4 Spontaneous and Piezoelectric Polarization
The basic wurtzite cell structure of III-Nitrides is displayed in Figure1.2. The main
advantage of wurtzite structure, which is not isotropic, is that III-nitrides are strongly
polar materials. The relatively low symmetry of the wurtzite structure give rise to a
spontaneous polarization PSP, directed along c-axis. The magnitude of the spontaneous
polarization of whole nitride family has been obtained from the following expression
[29] given in c/m2:
𝑃𝐴𝑙𝑥𝐺𝑎(1−𝑥)𝑁𝑆𝑃 = [−0.090𝑥 − 0.034(1 − 𝑥) + 0.021𝑥(1 − 𝑥) (1.2)
𝑃𝐼𝑛𝑥𝐺𝑎(1−𝑥)𝑁𝑆𝑃 = [−0.042𝑥 − 0.034(1 − 𝑥)] (1.3)
𝑃𝐼𝑛𝑥𝐴𝑙(1−𝑥)𝑁𝑆𝑃 = [−0.042𝑥 − 0.090(1 − 𝑥) + 0.070𝑥(1 − 𝑥)] (1.4)
The [0001] direction is commonly referred to as the Ga-polar direction, while the [000Ī]
one is normally called the N-polar direction. When considering the pseudomorphic
heterostructures where one layer is under tensile or compressive stress, a piezoelectr ic
component PPZ has to be added to the polarization. This term comes from the fact that
bond lengths are modified by the applied stress, and this in turn affects the polarization.
The piezoelectric polarization term can take complicated forms depending on which
kind of stress is applied to a layer. However, in this research we will consider
exclusively the relatively simple case of heterostructure grown pseudomorphically on
relaxed GaN along the c- axis with GaN-polarity. In this case, GaN has no piezoelectric
polarization, while for the other nitrides PPZ is given by (in C/m2)[29].
𝑃𝐴𝑙𝑥𝐺𝑎(1−𝑥)𝑁𝑃𝑍 = −0.525𝑥 − 0.0282𝑥(1 − 𝑥) + 0.0938(1 − 𝑥) (1.5)
𝑃𝐼𝑛𝑥𝐴𝑙(1−𝑥)𝑁𝑃𝑍 = 0.148𝑥 − 0.0525(1 − 𝑥) + 0.0938(1. 𝑥) (1.6)
𝑃𝐼𝑛𝑥𝐺𝑎(1−𝑥)𝑁𝑃𝑍 = 0.148𝑥 − 0.0424(1 − 𝑥) (1.7)
The total polarization
𝑃 = 𝑃𝑆𝑃 + 𝑃𝑃𝑍 (1.8)
The total polarization 𝑃 = 𝑃𝑆𝑃 + 𝑃𝑃𝑍 for nitrides pseudomorphic to GaN is summerized in
Figure 1.2.
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Figure 1.2 Wurtzite hexagonal unit cell.
The „a‟ and „c‟ lattice constants are indicated, together with the c-axis direction and the
basal c-plane.According to Maxwell‟s laws, a spatial variation of the polarization P is
associated to a fixed charge density ρ, given by the gradient of P:
𝜌 = −𝛻. 𝑃 (1.9)
In heterostructures, the polarization undergoes abrupt variation at interfaces. As a
consequence, interfaces between III-nitride materials are associated with a sheet density
of fixed interface charges σ. For the heterostructures grown along the Ga -polar
direction, the magnitude of σ is simply given by the polarization difference ∆P between
the two materials:
𝒆𝝈 = −∆. 𝑷 (1.10)
The (2DEG) sheet charge density for III-nitride heterojunctions interface is
summarized in Figure 1.3.
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Figure 1.3 (a) Total polarization for III-nitride materials grown pseudomorpjically on
Ga-polar c-plane GaN. (b) Corresponding polarization charge density[29].
1.1.5 Advantages of III-N Semiconductors
The nitrides have several features among the large semiconductor materials, which
makes the interest for various electronics applications where compact devices are
required to efficiently handle high power levels. The wide band gap of GaN and III-
nitride based alloys makes the breakdown voltage of III-nitride based devices
generally higher than analogous devices of the same size fabricated with Si or
classical III-Vs. At the same time, with respect to other wide band gap materials like
SiC, GaN is characterized by a higher saturation velocity, which means that parasitic
resistances can be reduced, making the devices more energetically efficient. A final
strength of III-nitride semiconductors is that they easily allow for heterostructure
based devices. Therefore it is possible to play with band engineering for improving
device performance, as for classical III-Vs, for example through the introduction of
quantum well (QW) or 2DEGs, while this is not possible for SiC or diamond based
devices. As a consequence of their properties, GaN-based electron devices have
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𝑉
found applications mainly in two fields: power electronics and microwave transistors.
Table 1.1 Johnson‟s figure of merit and Baliga‟s figure of merit of semiconductor
materials [29]. The figure of merits normalized to si.
In the field of power electronics, GaN has allowed the realization of devices with
high breakdown voltage (VBR) and very low specific on-resistance (Ron). While a high
breakdown voltage (VBR) is required to the device handling high power densities, a
low Ron is necessary to minimize the power dissipation in the ON-state. The figure
of merit that best measures the potentials of a semiconductor for power electronics
applications is the Baliga‟s figure of merit (BFOM)[30]:
𝐵𝐹𝑂𝑀 = 𝜖𝜇𝐸𝐵𝑅3 (1.11)
Where ε is the dielectric constant of semiconductor, µ is the electron mobility in the
drift region and EBR the electric field for breakdown. As BFOM describes the best
possible trade-off between VBR and Ron, an equivalent and suitable expression, more
suitable for comparing real devices, is the following one:
𝐵𝐹𝑂𝑀 = 𝑉𝐵𝑅
2
𝑅𝑜𝑛 (1.12)
The theoretical 𝐵𝐹𝑂𝑀 for the principal semiconductors are listed in Table 1.1 and the
summery of VBR and Ron data for published devices is presented in Figure 1.5(a) From
this it can be seen that both theoretically and practically, GaN based devices offer
reduced Ron and thus lower dissipation. At present, high performance GaN based P-N
diode have been demonstrated with 3.7 kV breakdown voltages[31], and GaN based
vertical transistors with low specific on-resistance and breakdown voltage as high
1.5 kV have been reported[31]. This indicates that GaN holds great potential for
low/medium voltage power circuits and is being considered seriously for improving the
efficiency of DC-DC converters[32]s, LED[33] and motor drivers[34],etc.
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Figure 1.4 (a) Dots: specific ON-resistance vs breakdown voltage for
SiC[35][36][37][38] and GaN devices[31,33,39], lines: theoretical limits for Si, SiC
and GaN. (b) Best reported output power density at various microwave frequencies for
AlGaN/GaN HEMTs[40-44].
For microwave transistors, which are the objectives of this research work, GaN is an
interesting material as it combines a high breakdown voltage and associated with high
saturation electron velocity. Analogously to power devices, a high breakdown voltage
is fundamental for increasing the power density a transistor can handle. A high
saturation velocity is instead advantageous to reduce the electron transit time under the
gate electrode or across the base region, and therefore the switching time.
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Combining these two features, GaN based microwave transistors and amplifiers hold
the promise of delivering very high output power levels and at the same time highcut-
off frequencies. The suitability of a material for power microwave applications can be
measured by means of Johnson‟s figure of merit (JFOM)[45]:
JFOM= 𝐸𝐵𝑅 𝑣𝑠𝑎𝑡
2𝜋 (1.13)
where 𝑣𝑠𝑎𝑡 is the saturated velocity.
JFOM quantities the trade-off between VBR and device speed, which is quantified by
the cut-off frequency fT. Thus, an equivalent definition suitable for comparing device
is the following one:
JFOM=𝑉𝐵𝑅𝑓𝑇 (1.14)
The 𝐽𝐹𝑂𝑀 for the semiconductors are listed in Table 1.1. It can be seen that GaN JFOM
is ten times that of GaAs and is surpassed only by diomond. However, if we consider
that diamond is not easy to dope and does not easily allow for heterostructures, GaN
emerges as the ultimate semiconductor for RF applications. GaN-based heterostructure
has been used for the realization of 2DEG based devices, in particular HEMTs, which
have demonstrated in the last 15 years breakdown voltages much higher than III-V
based HEMTs or bipolar transistors[46]. As a direct consequence, nitride based HEMTs
have reached record power outputs and efficiencies, and are currently available on the
market for RF systems requiring high power levels, like cellular base stations. Figure
1.4 (b) summarizes the highest power levels reached by AlGaN/GaN HEMTs, which
have been the workhorse for the RF community in the past years.
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1.2 Project objective
To reinforce the next generation high power mixed signal monolithic microwave
integrated circuits (MMIC), the requirement of high power microwave transistors are
more demands. In recent years, GaN-based High Electron Mobility Transistors
(HEMTs) are attracted enormous attention for future deployment of high power RF
applications, which offers high output power density than other materials systems such
as GaAs or InP at high- frequency operation. The unique combination of larger band
gap (3.44eV), breakdown voltage (800V) of GaN-based HEMTs are used in high power
RF applications, such as next-generation wireless communications, development of
satellite telecommunication, high power amplifiers for radar and space research,
microwave image sensing and low noise wide bandwidth amplifiers design.GaN-based
high electron mobility transistors (HEMTs) have successfully proven their potential as
high power microwave amplifiers in wireless communication market.With the upsurge
of the wireless communication, the requirement of high speed and high power
microwave transistors are more demands.
The goal of this project is to optimize the GaN-based HEMTs structure for future high
power and high frequency applications. In order to improve the DC and microwave
characteristics of the HEMT, the following design techniques are used.
Addition of fieldplate to the device structure to attain high breakdown
voltage.
The heavily doped GaN source and region with ohmic contacts effectively
minimize the contact resistances.
Passivation layer for drastic reduction in gate access resistance and
parasitic capacitances to improve the frequency reesponse.
Lattice matched Wide bandgap barrier material to GaN for large sheet
charge density, low gate leakage current, high transconductance and
resistance to short channel effects.
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1.2.1 Scope of the project
This project accords the DC and microwave qualities of the GaN-based HEMTs by
using Silvaco TCAD tool. These transistors to be a optimistic applicant for future high
power millimetre wave applications including wireless communication, satellite
communication and TV broadcasting, high speed data transmission, military
applications, radio astronomy and low noise amplifiers etc.
In this work, the eminent physical properties of InAlN,AlGaN barrier and GaN
channel materials are completely exploited in the sub 400 nm gate length HEMTs with
different field-plate lengths are reported. From the simulation results GaN-based
HEMTs are expected to be the best microwave transistors for future high power
millimetre electronics.
Several design optimization techniques adopted in this work to uplift the
microwave performance of the GaN-based HEMTs. Scaling the gate length in nm
regime enables the low gate capacitance (Cg) while maintaining high transconductance,
ultrathin wide bandgap barrier material (InAlN) provides large sheet charge density and
low gate leakage current, heavily doped GaN soure/drain region minimizes the parasitic
resistance and capacitive effects for enhancing the cut-off frequency, presence of field
plate contributes for high breakdown voltage.
A comparison table of the behavior for different barriers(InAlN,AlGaN) in
properties is prepared and mentioned at the end of the report.
13
1.3 Silvaco TCAD tool
Silvaco TCAD tool is a widely used software package for the simulation of various type
of Si devices.
• There are many sub-packages in Silvaco in order to implement the various
required steps for the Si device simulation:
1. ATHENA (for fabrication process simulation),
2. DevEdit(for defining the structure geometry and mesh editing),
3. DeckBuild (which shows the run time output of the simulations)
4. ATLAS (which is a main device simulator tool in Silvaco)
5. TonyPlot (which can be used to visualize the simulation results)
1.3.1 Atlas overview
Atlas provides general capabilities for physically-based two (2D) and three-
dimensional (3D) simulation of semiconductor devices. It predicts the electrical
behavior of specified semiconductor structures and provides insight into the internal
physical mechanisms associated with device operation.
1.3.1.1 Atlas inputs and outputs
Figure 1.4 shows the types of information that flow in and out of Atlas. Most Atlas
simulations use two input files. The first input file is a text file that contains commands
for Atlas to execute. The second input file is a structure file that defines the structure
that will be simulated. Atlas produces three types of output files. The first type of output
file is the run-time output, which gives you the progress and the error and warning
messages as the simulation proceeds.
The second type of output file is the log file, which stores all terminal voltages and
currents from the device analysis. The third type of output file is the solution file,
which stores 2D and 3D data relating to the values of solution variables within the
device at a given bias point.
14
Figure1.5 Simulation flow
THE ORDER OF ATLAS COMMANDS
Figure1. 6 Atlas Command Groups with the Primary Statements in each Group
15
1.3.2 Deckbuild overview
DECKBUILD is an interactive, graphic runtime environment for developing process
and device simulation input decks. It consists of a window for input deck creation and
editing, a window for simulator output and control, and a set of popups for each
simulator that provide full language and run-time support.
DeckBuild also contains many other convenience features:
• A built-in tool palette allows interactive plotting of the current structure.
• Full interactive control of the simulator, including a history function that
allows you to back up in the deck and try again.
• The ability to define an arbitrary number of stop points where the
simulator is halted automatically.
• An indication in the input deck of the currently executing line.
1.3.3 Dev-edit overview
DEVEDIT is a device structure editor. It can be used to generate a new mesh on an
existing structure and can be used to create or modify a device. These devices can then
be used by 2D 2D and 3D simulators. DEVEDIT can be used as a simulator under
DECKBUILD or through a Graphical User Interface (GUI).
1.3.4 Device basic construction layout
Mesh construction
Regions and electrodes
Doping
Material
Models
Solution method
Solution specification
Data extraction and Plotting
16
CHAPTER 2
AlGaN/GaN HIGH ELECTRON MOBILITY
TRANSISTORS(HEMTs)
The basic concept of the HEMT was introduced by Dingle et al. In their concept Dingle
et al. describe how the electrons transferred from wide band gap semiconductor to
narrow band gap material and forms a heterojunction with a triangular well at its
interface, which confines the electrons in 2DEG. Because the 2DEG is separated from
the ionized donors this technique significantly reduces ionized impurity scattering and
thus results in a high electron mobility and saturation velocity. Because the technique
of Dingle et al. is also referred to as modulation doping the HEMT is also called
Modulation Doped Field Effect Transistor (MODFET).
Focusing on the 2DEG the designation Two-Dimensional-Electron-Gas Field Effect
Transistor (TEGFET) is also convenient. Other labelling are Separately-Doped-Field
Effect Transistor (SEDFET) or Selectively Doped Heterojunction Transistor (SDHT)
or just Heterojunction Field Effect Transistor (HFET). If the barrier layer, e.g. made of
AlGaAs, is undoped the resulting structure is comparable to a MOS structure and the
resulting FET may be referred to as Heterostructure Insulated Gate Field Effect
Transistor (HIGFET)[47].
HEMTs are field effect devices based on 2DEG as current channel. According to the
general field effect transistor construction, they are constituted by two ohmic contacts
to the 2DEG, called the source and drain contacts and a schottky contact, called the gate
contact, placed in drain-source spacing. A vertical cross section of HEMT is shown in
Figure 1.9. The role of the electrode is to control the current flowing between the source
and drain electrodes through the 2DEG.
17
2.1 Basic HEMT structure
A high-electron-mobility transistor (HEMT), also known as heterostructure
FET (HFET) or modulation-doped FET (MODFET), is a field-effect
transistor incorporating a junction between two materials with different band gaps (i.e.
a heterojunction) as the channel instead of a doped region (as is generally the case for
a MOSFET). A commonly used material combination is GaAs with AlGaAs, though
there is wide variation, dependent on the application of the device. Devices
incorporating more indium generally show better high-frequency performance, while
in recent years, gallium nitride HEMTs have attracted attention due to their high-power
performance. Like other FETs, HEMTs are used in integrated circuits as digital on-off
switches. FETs can also be used as amplifiers for large amounts of current using a small
voltage as a control signal. Both of these uses are made possible by the FET’s
unique current–voltage characteristics. HEMT transistors are able to operate at higher
frequencies than ordinary transistors, up to millimeter wave frequencies, and are used
in high-frequency products such as cell phones, satellite television receivers, voltage
converters, and radar equipment. They are widely used in satellite receivers, in low
power amplifiers and in the defense industry.
The dimensions of this structure are:
GaN layer is of length 0.18 micro meter with 1*105/cm3 concentration
Al0.295Ga0.705N layer is of length 0.02 micrometer with 1*1016/cm3
concentration
The 2-DEG charge concentration is 1*1013/cm3
The gate length is 400nm,Ldg=510nm,Lsg=500nm
2.1.1 Feildplated HEMT
In recent years, high electron mobility transistors (HEMTs) based on GaN have
demonstrated a power density up to 9.2 W/mm at 8 GHz and 30 V bias , which is well
beyond the capability of GaAs FETs. Thus, AlGaN/GaN HEMTs have established
themselves as microwave frequency power devices. To explore the full range of
18
applications of the AlGaN/GaN HEMT, it is also worth investigating its potential as a
high- voltage power switch. This is because, GaN has a breakdown field approximately
( ~3MV/cm),which is~3 times of that in Si or GaAs. The Baliga figure of merit gives
the ideal t h e specific on-resistance for a high voltage lateral FET as given
𝑅𝑜𝑛 =𝑉𝑏𝑟
2
𝑞𝜇𝑛𝑠𝐸𝑐2 (2.1)The
mobility AlGaN/GaN HEMTs is an order of magnitude higher than in Si or GaAs FETs,
since polarization effects induce a 2-DEG concentration of 1013/cm at the
heterointerface.
Thus, for a given Vbr, a GaN device has 10–100 times lower Ron than a device made
in other material. Devices with several hundred volts Vbr and low Ron are of paramount
importance in power switching applications such as factory automation,
telecommunications and motor control etc.
Recently,significant improvement in Vbr of an AlGaN/GaN HEMT was experimentally
demonstrated, by employing a field plate (FP) connected to the gate and placed in the
gate to drain separation,Ldg over a uniform insulator. However, a comprehensive
theoretical study of the effects of all the critical geometrical and material variables of the
FP-HEMT on its Vb r and frequency response is lacking. In this paper, all these effects
are identified, their physical basis is explained, and therefrom, a systematic procedure
for device optimization is developed. Also, the parameters of an optimized device are
calculated using a numerical simulation of the internal 2-D po- tential distribution and
the terminal Id-Vd (drain current–drain voltage) characteristics.
The application of the FP technique of Vbr improvement to HEMTs follows its
application to other lateral FETs such as LD-MOSFETs and MESFETs, after its original
proposal in the context of high voltage planar p-n junctions. The structure of the
AlGaN/GaN hemt with field plate is shown in figure 2.1(a) and simulated result of structure
field-plate lengths 2.25μm in figure 2.1(b).Thus, extensive literature on this technique is
available. Still, the present work is necessary for several reasons. First, the FP work on
MESFETs and LD-MOSFETs was restricted to specific effects. A systematic and
quantitative account of all the various FP effects on Vbr is available for planar junc-
19
tions only, and this account based on numerical simulations did not discuss the physical
basis of all the effects . Second, this account of “vertical” planar junctions cannot be
transferred to lateral FETs, since the structural and potential pictures of the crucial
semiconductor region below the FP in the two types of devices are different. Third,
although these pictures of the various lateral FETs are broadly similar, the charge
concentration and the material (i.e., Si, GaAs, or GaN, etc.) of the crucial region can
differ significantly. This calls for a separate study for each of these types of devices.
For example, unlike in MOSFETs and MESFETs, in AlGaN/GaN HEMTs, the
polarization and interface charges rather than doping determine the channel charge and
need to be modeled properly. Further, the Vbr values of a few tens of volts in GaAs
MESFETs are much lower than the several hundred volts target of the GaN HEMTs in
the present study. Finally, in contrast to the practice in earlier FP studies, of inferring
the breakdown condition indirectly from the simulated peak electric field value, the
present work extracts the Vbr directly from the simulated Id-Vd characteristics.
The main purpose of the field plate is to reshape the electric field distribution in the
channel and to reduce its peak value on the drain side of the gate edge. Introducing a
field plate in AlGaN/GaN HEMTs reduces the current collapse which in turn increases
the power performance. The benefit is an increase of the breakdown voltage and a
reduced high-field trapping effect. Overall the power density is increased. Various field
plate methods like gate field plate, a floating field plate, multiple field plate, etc have
been reported so far. It has been found that by increasing the gate to drain distance and
by reducing the field plate length, breakdown voltage increases for a particular
structure.
In The field plate technique, an additional lithography is done in order to place a metal
plate covering the gate and extending to the access region on the drain side. This metal
plate is electrically connected to the gate. It tracks the potential of the gate electrode,
acting as a quasi-electric field plate.The field distribution at the drain of the gate region
splits and reshape the distribution of the electric field, hence the peak electric field
reduces and increases the device breakdown. The field-plate length dictates the size of
the field-reshaping region. This not only increases device breakdown voltage but also
reduces the high-field trapping effect, hence enhancing current-voltage swings at high
20
frequencies. The trade-off of the field plate structure includes addition of the gate-drain
capacitance at low voltages and extension of the gate-drain depletion length at high
voltages, which reduces gain as well as frequency of the device.
Figure2.1(a):Structure of AlGaN/GaN hemt with fieldplate
Figure2.1(b):Simulated structure of AlGaN/GaN hemt with fieldplate
21
2.1.2 WORKING PRINCIPLE
HEMTs are essentially heterojunctions formed by semiconductors having dissimilar
band‐ gaps. When a heterojunction is formed, the conduction band and valence band
throughout the material must bend to form a continuous level. The wide band element
has excess elec‐ trons in the conduction band as it is doped with donor atoms (or due to
polarization charge in GaN‐based HEMTs). The narrow band material has conduction
band states with lower energy. Therefore, electrons will diffuse from wide bandgap
material to the adjacent lower bandgap material as it has states with lower energy. Thus,
a change in potential will occur due to movement of electrons and an electric field will
be induced between the materials. The induced electric field will drift electrons back to
the conduction band of the wide bandgap element. The drift and diffusion processes
continue until they balance each other, creating a junction at equilibrium like a p‐n
junction. Note that the undoped narrow bandgap material now has excess majority
charge carriers, which yield high switching speed. An interesting fact is that the low
bandgap undoped semiconductor has no donor atoms to cause scattering and thus
ensures high mobility.
Another interesting aspect of HEMTs is that the band discontinuities across the
conduction and valence bands can be engineered to control the type of carriers in and
out of the device. This diffusion of carriers leads to the accumulation of electrons along
the boundary of the two regions inside the narrow bandgap material. The accumulation
of electrons can lead to a very high current in these devices. The accumulated electrons
are also known as 2DEG. Figure 2.2(a) shows the generalized band diagram formed at
the heterojunction for typical HEMTs and 2.2(b) shows the simulated bandgiagram for
400nm gate length AlGaN/GaN HEMT with 2.25micro meter length field-plate .Both
the conduction band (Ec ) and valence band (Ev ) bend with respect to the Fermi level
(EF ) resulting in a quantum well filled with 2DEG and eventually, a conducting channel
is formed.
22
Figure:2.2(a)Generalized band diagram of HEMT
Figure 2.2(b):Simulated band diagram of AlGaN/GaN HEMT
23
2.2 Two Dimensional Electron Gas (2DEG) in HEMT
A schematic band diagram of a modulation doped heterostructure is shown in Figure 2.2(a).
It consists of a wide gap semiconductor (AlGaN) and a semiconductor with narrower gap
(GaN). At the interface a triangular quantum well is formed in the undoped narrow gap
material. Electrons from the wider band gap material fall into this potential well and are
confined within the well. Because of such quantum mechanical confinement in a very
narrow dimension, they form a high density of electron gas in two dimensions. Electrons
can move freely within the plane of the heterointerface, while the motion in the direction
perpendicular to the heterointerface is restricted to a well-defined space region by energy,
momentum, and wave function quantization, thus forms the so-called 2DEG. As the narrow
gap material is undoped and these electrons are away from the interface, the electron
mobility can be simultaneously increased with high concentration of carriers. Within the
AlGaN layer there are two polarization vectors Ppz, AlGaN and Psp, AlGaN for the
piezoelectric and spontaneous polarizations respectively. The polarization in the AlGaN
causes dipole charges to form at the borders of the material with a negative sheet charge at
the surface and an equal positive sheet charge at the AlGaN/GaN junction. The polarization
in the GaN layer causes a negative sheet charge at the AlGaN/GaN junction and an equal
positive sheet charge on the bottom surface. Since the total polarization in the AlGaN is
larger, the overall result is a net positive sheet charge at the AlGaN/GaN interface. Notice
that the bottom GaN and AlGaN/GaN interfaces are both positive while the charge density
at the top AlGaN interface is negative. The critical point to remember is that the interface
sheet charges here are not free carriers. They are induced charges as in a typically polarised
dielectric. However, it is the presence of these charges and the polarization induced electric
field in the AlGaN which allows for 2DEG formation without barrier doping.
24
2.3 Results and Discussions
2.3.1 Drain current characteristics
When a dc voltage applied to the drain electrode with the source electrode grounded, a
current Id, whose magnitude depends on the total resistivity of the 2DEG channel, flows
between two electrodes. The gate electrode, by acting on the 2DEG density below it,
modulates the channel resistivity and thus Ids. The V-I characteristics, also known as the
output characteristics, can be obtained by imposing the constant current condition along
the channel at a given gate voltage Vgs.
The Id vs Vds characteristics of this device is simulated and shown in figure 2.3.At constant
Vgs ,Vds is swept from 0V to 20V.The device remained in off-state(i.e Ids=0 A)
when Vgs=-3V.The drain current when Vgs=0V is 1.8A/mm.
Figure 2.3:Drain characteristics
Vgs=-3V
Vgs=-2V
Vgs=-1V
Vgs=0V
Vgs=1V
V
Vgs=2V
25
The drain current Ids depends on the mobility of electron (µ), longitudinal electric field (E)
and sheet charge density ɳs. Therefore the expression for Ids as follows:
𝐼𝑑𝑠 = 𝑤𝑒𝑛(𝑥)𝐸(𝑥)𝜇 (2.2)
W is the gate length of the device and the sheet charge density n(x) in the gate region as a
function of channel potential V(x):
𝑛(𝑥) = 1
𝑒𝐶𝑏(𝑉𝑔𝑠−𝑉(𝑥)−𝑉𝑜𝑓𝑓) (2.3)
Considering 𝐹(𝑥) =𝑑𝑉(𝑥)
𝑑𝑥, Equation (2.2) can be written as:
𝐼𝑑𝑠 = 𝑊𝜇𝐶𝑏(𝑉g𝑠 −𝑉( 𝑥) −𝑉𝑜𝑓𝑓)𝑑𝑉(𝑥)
𝑑𝑥 (2.4)
Integrating this equation over the gate length, we finally obtain a relation between 𝐼𝑑𝑠 and
𝑉𝑑𝑠:
𝐼𝑑𝑠 = 𝑤𝜇𝐶𝑏2𝐿𝑔
[(𝑉𝑔𝑠−𝑉𝑜𝑓𝑓)2−(𝑉𝑔𝑠−𝑉𝑜𝑓𝑓−𝑉𝑑𝑠)2] (2.5)
This expression is valid only for 𝑉𝑑𝑠<𝑉𝑔𝑠−𝑉𝑜𝑓𝑓 and for 𝑉𝑔𝑠> 𝑉𝑜𝑓𝑓, i.e in the so called linear
regime. At 𝑉𝑑𝑠=𝑉𝑔𝑠−𝑉𝑜𝑓𝑓 the channel at the drain edge is fully depleted. In this case, the
current does not increase further with 𝑉𝑑𝑠 and remains approximately constant. This is the
saturation regime. Finally, for 𝑉g𝑠< 𝑉𝑜𝑓𝑓the channel is completely depleted whatever 𝑉𝑑𝑠
and the device is in the pinch-off state.
26
2.3.2 Transfer characteristics
The Ids vs Vgs characteristics of this device is simulated and shown in figure 2.4 which
describes the Ids-Vgs dependence at a fixed 𝑉𝑑𝑠, usually in saturation regime. At constant
Vds(the value where current is saturated),Vgs is swept from -10V to 4V.The minimum gate-
to-source voltage VGS (th) that is needed to create a conducting path between the source and
drain terminals is called threshold voltage(Vth).
The Vth for this device is determined as -2.989V.
Figure 2.4:Transfer characteristics
Vds=15V
Vds=17V
27
2.3.3 Log Id vs Vg Characteristics
The log Id vs Vg characteristics of this device is simulated and shown in figure 2.5.
The slope of this curve is called subthreshold slopeand detrmined as 26.7594mV/dec.
Short channel effects can be also important causes of non-ideal behaviour. If the gate
does not have a very strong electrostatic control over the channel, the drain voltage can
still strongly influence the drain-source current even at the saturation regime. This
happens particularly when the gate to channel distance,i.e. the barrier thickness, is not
negligible with respect to Lg. short channel effects have a strong effect in the pinch-off
region. At high Vds, the gate can no longer block the current flow and a non negligible
Ids can appear. Short channel effects can be therefore assimilated to a Vds dependent Voff
and are measured through the drain induced barrier lowering (DIBL) parameters.
Drain induced barrier lowering(DIBL):
Drain induced barrier lowering is a short channel effect in HEMTs which refers to the
reduction of the threshold voltage of the transistor at higher drain voltages. The origin of
the threshold decrease can be understood as a consequence of charge neutrality: the Yau
charge-sharing model. The combined charge in the depletion region of the device and that
in the channel of the device is balanced by three electrode charges: the gate, the source and
the drain. As the drain voltage is increased,the depletion region of the p-n junction between
the drain and body increases in size and extends under the gate, so the drain assumes a
greater portion of the burden of balancing depletion region charge, leaving a smaller burden
for the gate. As a result, the charge present on the gate retains charge balance by attracting
more carriers into the channel, an effect equivalent to lowering the threshold voltage of the
device. In effect, the channel becomes more attractive for electrons. In other words, the
potential energy barrier for electrons in the channel is lowered. Hence the term "barrier
lowering" is used to describe these phenomena.
28
Figure 2.5 Log Id vs Vg characteristics
Unfortunately, it is not easy to come up with accurate analytical results using the barrier
lowering concept. Barrier lowering increases as channel length is reduced, even at zero
applied drain bias, because the source and drain form pn junctions with the body, and so
have associated built-in depletion layers associated with them that become significant
partners in charge balance at short channel lengths, even with no reverse bias applied to
increase depletion widths. As channel length is reduced, the effects of DIBL in
the subthreshold region (weak inversion) show up initially as a simple translation of the
subthreshold current vs. gate bias curve with change in drain-voltage, which can be
modeled as a simple change in threshold voltage with drain bias. However, at shorter
lengths the slope of the current vs. gate bias curve is reduced, that is, it requires a larger
change in gate bias to effect the same change in drain current. At extremely short lengths,
the gate entirely fails to turn the device off.
𝐷𝐼𝐵𝐿 =𝜕𝑉𝑜𝑓𝑓
𝜕𝑉𝑑𝑠 (2.6)
DIBL of this device is determined as 0.0985
In order to keep the SCEs to acceptable level, Lg should be atleast 5 times of the barrier
thickness or even more [48]. This imposes a limit of scaling of a transistor dimensions.
29
Ion/Ioff
For a device working as a switch it conducts an on current Ion when it is on and off current
Ioff when it is switched off. For an ideal switch the off current is ideally zero and the on
current is limited by the external circuit.
In case of solid sate switches as the HEMTs. the off current is not zero but it is a very small
current. It is the current passing in the transistor when its gate to source voltage equals zero
and Vds= Vdd. It is observes as the transistor is scaled down, the off current increases
appreciably because of the short channel effects with the most effective one is the induced
barrier lowering.
The on current is limited by the W/L ratio, and the excess voltage of the transistor Vgs-Vth
in addition to the mobility. Therefore, because of this trend one has to maximize the Ion/Ioff
ratio for static transistor losses.
Ion/Ioff of this device is 1014
2.3.Gate lekage current characteristics
The gate lekage current charcteristics of this device is simulated and shown in figure 2.6.
Gate lekage current
As stated previously, AlGaN/GaN HEMT devices are well suited to high-power high-
frequency applications such as high power amplifiers and applications for wireless base
stations. For such cases there is a general requirement for a low input gate current and a
high reliability figure for the device.The role played by the degradation with time in the
gate leakage current is important in the understanding of the reliability issue for the
device.From a physical point of view the degradation, and hence changes observed with
the device, arise from defects under the gate region. These become more evident at a critical
point in the value of the electric field. Trap formation in the device at either the
semiconductor surface or within the bulk is also a performance-limiting issue. The gate
leakage current surges as a consequence of surface processing and passivation issues.
30
Figure 2.6 Log Ig vs Vg characteristics
In Field Effect devices quantum mechanical tunnelling is an important effect to be
accounted . For example, electrons tunnelling from the gate can create a gate-to-drain
leakage current by hopping from trap to trap. Alternatively, the electrons can accumulate
on the surface next to the gate or move through the AlGaN layer to the conducting
channel.As the number of electrons increases at the gate edge as a function of time the gate
leakage current reduces due to the feedback. In addition, the increased electron density on
the AlGaN surface decreases the number of 2DEG electrons and this causes the gate current
to decrease.
We found a strong correlation between the gate leakage current and the transistor
subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and
the steeper the subthreshold slope. In order to reduce off-state power consumption, it is
necessary to reduce leakage current.
The gate leakage current of this device is determined as 10-17 A/mm.
31
2.3.5 Breakdown characteristics
The breakdown voltage is the drain-source voltage, which causes the transistor to enter the
breakdown region.It is the region where the device receives too much voltage across its
drain-source terminal, which causes the drain-source terminal to break down, which makes
the grain current drastically increase.Figure 2.7 shows a HEMT without a FP, the breakdown
field distribution along the 2-DEG over , is confined over a small distance from the gate
edge. So, a high field is reached even for small values of and is low.
Figure 2.7:Breakdown field distribution with and without fieldplate.
Inclusion of a FP reduces and spreads the field along the 2-DEG.The breakdown
characteristics of this device for different field-plate lengths 1,1.25,1,5,1.75,2,1.25(μm) is
simulated at 300ok and shown in figure2.8.
Figure 2.8 Breakdown curve for different lengths of field-plate
32
Breakdown is an important problem for high power HEMT devices. In order to tackle
breakdown issue of HEMT many investigations are carried out to understand its physical
origin. Some of the relevant breakdown mechanism in HEMT devices are,
Source-drain breakdown due to short channel effect
Hetero structure fabrication defects and trap related problem.
Impact ionisation mechanism, sudden increase in drain current due to carrier
collapse.
High gate electric field, due to the schottky contact leakage current or through
surface related conduction.
There are different techniques available to improve the Breakdown voltage of a transistor,
some of the common techniques and its review is given.
Schottky source/drain contact HEMT
Field plated HEMT
Passivation layer in HEMT
The breakdown down charcteristics is also simulated for 400o k and 5000k for 2.5 micro
meter fieldplte length and the breakdown volatage values are determined as:
Vbr=819.443V (300ok)
Vbr=813.961V (400ok)
Vbr=810.282V (500ok)
The breakdown voltage decreases with increase in temperature and increase in field-plate
length beyond 2.25μm length.
33
2.3.6 Transconductance(gm)
Transconductance is the ratio of the change in drain current to the change in gate voltage
over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
𝑔𝑚 =𝜕𝐼𝑑𝑠
𝜕𝑉𝑔𝑠 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉𝑑𝑠 (2.6)
As for the output characteristics, what is usually represented is the Vgs dependence of gm
in the saturation regime. The transconductunce vs Vgs characteeristics for this device is
simulated and shown in figure 2.9.
In an ideal HEMT, both Ids and gm increase indefinitely with Vgs. In real devices,
however, several non linearities contribute to modify the output and transfer
characteristics.
Figure 2.9:𝑔𝑚 vs Vgs characteristics
A first effect comes from the fact that µ depends on the longitudinal electric field, in
particular it decreases as the field increases. This reduces the current the transistor can
handle in the saturation regime. A second important effect comes from the presence of an
access and output region on the source and drain side of the gate, respectively. The
resistance of the two regions, which is composed of the contact resistance and the
34
resistance of the 2DEG portion connecting the source or drain electrode to the gate region,
increases the on resistance of the device. As a consequence, the saturation regime is
reached for Vds values higher than Vgs-Voff. Source and drain resistances induce
furthermore a significant reduction of the saturation current, the effect becoming stronger
with higher resistances.
A second important phenomenon in GaN based HEMTs is an access and output resistances
with increasing current levels[49][50]. This is due to nonlinearity velocity field
characteristics of GaN (1.7), and its major consequence is a sublinear at high Vgs (Figure
1.10(b)). Vgs is finally limited by the onset of forward conduction in the schottky contact,
which usually take place for Vgs>1V.
The transconductance of this device is maximum at Vgs=0V with and determined
as 0.0525 in value.
2.3.7 Cutoff frequency(fT)
The simplest way for understanding the evolution of the transistor properties with
frequency is to start with its behaviour under small signal conditions. The small signal
equivalent circuit of HEMT is shown in Figure 2.10. The Rg and Lg are the resistance and
inductance of source resistance and access region. Rd, Rg, Ld and Lg are the resistances and
inductances of the drain and gate electrodes. Cgs, Cgd and Cds are, respectively, the gate-
source, gate-drain and drain source capacitances. Rds is the drain-source resistance and is
related to short channel effects and buffer conductivity.Rgs has a less evident physical
origin. Among all these components, the most important ones are by far 𝑔𝑚 and Cgs.The
other components are less important and can be considered as parasitic components. If
these parasitic components are negligible, the current gain G I can be easily worked out:
𝐺𝐼 =𝑔𝑚
2𝜋𝑓𝐶𝑔𝑠 (2.7)
Where f is the frequency. From this equation, we can see that the current gain decreases
with frequency.
35
Figure 2.10 (a) HEMT structure, (b) Small signal equivalent circuits of HEMT
If expressed in dB, the gain is found to decrease at a rate of 20 dB/decade and becomes
unity at the transition frequency fT:
𝑓𝑇 =𝑔𝑚
2𝜋𝐶𝑔𝑠 (2.8)
Therefore, for 𝑓>𝑓𝑇 the transistor no more amplifies the current. In the ideal case, all the
gate-source capacitance is used to modulate the 2DEG electrons. If<𝑣𝑒> is the mean
electron velocity under the gate electrode, we have 𝑔𝑚 = 𝐶𝑔𝑠 < 𝑣𝑒 > 𝐿𝑔 .
Writing now < 𝑣𝑒 >= 𝐿𝑔/ 𝜏 , where 𝜏 is the transit time for electrons under the gate, we
obtain:
𝑓𝑇 =1
2𝜋𝜏 (2.9)
Therefore, the faster the electrons transit under the gate, the higher 𝑓𝑇 will be. High 𝑓𝑇 can
be thus obtained by reducing 𝐿𝑔 and by increasing the electric field under the gate so that
electrons reach their saturation velocity. The highest possible value for 𝑓𝑇 is therefore
𝑓𝑇,𝑚𝑎𝑥 =𝑣𝑠𝑎𝑡
2𝜋𝐿𝑔 (2.10)
Considering that for GaN 𝑣𝑠𝑎𝑡 = 2.5𝑋107 cm/s, and taking 10 nm as the lower bound for
Lg, transition frequencies in excess of 1 THz can be theoretically reached. In real devices,
however, 𝐶𝑔𝑠 contains also fringing parasitic components that lower the 𝑔𝑚/ 𝐶𝑔𝑠 ratio . A
second point is that the electron velocity cannot be 𝑣𝑠𝑎𝑡 under the whole gate. Electrons
enter the gate at relatively low velocities and are accelerated to 𝑣𝑠𝑎𝑡 only at the right side
36
of the gate. This further lowers the maximum value of 𝑓𝑇. Current gain vs frequency
characteristics is simulated for the device to determine the cutoff frequency as shown in
figure 2.11.Furthermore, the effect of the other parasitic elements is not completely
negligible. An expression for 𝑓𝑇 taking into account the parasitic components is the
following one:
𝑓𝑇 = 𝑔𝑚2𝜋
1
(𝐶𝑔𝑠+𝐶𝑔𝑑)+(𝑅𝑠+𝑅𝑑
𝑅𝑑𝑠)(𝐶𝑔𝑠+𝐶𝑔𝑑)+𝑔𝑚𝐶𝑔𝑑(𝑅𝑠+𝑅𝑑)
(2.11)
Figure 2.11:Current gain vs cutoff frequency
The cutoff frequency of this device is determined as 23GHz.
2.3.8 Maximum Oscillation Frequency(fmax)
Parasitic components can have a significant impact on 𝑓𝑇 , especially in deeply scaled
transistors where the low 𝐿𝑔 makes 𝐶𝑔𝑠 small, enhancing the effect of the parasitic source
and drain resistances. Regarding the power gain, the situation is more complicated as many
definitions have been given for this parameter. The Mason‟s unilateral power gain U is the
most commonly used and is defined as the power gain when the drain to gate feedback is
suppressed. Its frequency behavior is not different from the current gain. It also decreases
37
with frequency at a rate of 20 dB/decade and becomes equal to one at a frequency indicated
as 𝑓𝑚𝑎𝑥 . Therefore, as for the current gain, the higher 𝑓𝑚𝑎𝑥 is, the higher the gain at a given
frequency will be. However, 𝑓𝑚𝑎𝑥 can be quite different from 𝑓𝑇 (Schwierz et al. 2002):
𝑓𝑚𝑎𝑥 =𝑓𝑇
2√𝑅𝑠+𝑅𝑔+𝑅𝑔𝑠
𝑅𝑑𝑠+2𝜋𝑓𝑇𝐶𝑔𝑑𝑅𝑔
(2.12)
For power amplifier applications, 𝑓𝑚𝑎𝑥 is more critical than 𝑓𝑇 as amplifiers have to amplify
power rather than current. In order to obtain a high 𝑓𝑚𝑎𝑥 , it is required to have a high 𝑓𝑇
and low parasitic resistances.The power gain vs frequency of this device is simulated and
shown in figure2.12.
The frequency of this device is improved by:
Si3N4 passivation,which decrease the effect of parasitic capacitances
GaN doping,which reduces contact resistances
Figure 2.12:Power gain vs frequency characteristics
The maximum oscillation frequency of this device is extracted as 100GHz.
38
CHAPTER 3
InAlN/GaN HIGH ELECTRON MOBILITY
TRANSISTORS(HEMTs)
3.1 Basic Structure
The structure is similar to the structure mentioned in previous chapter(AlGaN/GaN) except
the barrier layer is replaced with In0.17Al0.83 N as shown in figure 3.1 and the device
characteristics are simulated.The simulated structure and band diagram are simulated and
shown in figure 3.2 and 3.3 respectively.
Kuzmik proposed an alternative approach of replacing AlGaN by InAlN as the barrier layer
material in the GaN high-electron-mobility transistor (HEMT) structure to improve the
device performance. The improvement is mainly due to the superior material properties of
In0.18Al0.82N, which is lattice-matched to GaN and has a stronger polarization effect.
Without lattice strain, In0.18Al0.82N grown on GaN has a lower possibility of defect
formation. Additionally, the larger spontaneous polarization field in the InAlN barrier layer
also induces higher sheet carrier density at the InAlN/GaN interface than that in the
AlGaN/GaN structure. Nevertheless, for nitride-based HEMTs, the issue of high leakage
current resulting from the low-resistivity GaN layer has restricted the device performance.
In particular, GaN grown by metal organic vapor deposition (MOCVD) exhibits n-type
conductivity owing to the incorporation of oxygen (O), silicon (Si), and other impurities.
On the other hand, high-resistivity GaN is required to ensure complete channel pinch-off,
high-voltage operation, and proper drain–source current saturation on a GaN HEMT
device.
InAlN/GaN-based high-electron mobility transistors (HEMTs) are excellent candidates for
high power and frequency applications, primarily because of the very high polarization-
induced two-dimensional gas (2DEG) in the channel (n2DEG). On the other hand
AlGaN/GaN-based HEMTs are known to suffer from the trapping effects related to the
surface states.It was suggested that the existence of surface states is coupled with
polarization effects because the surface states provide the charge neutrality on the device
39
surface and eliminate strong electric field in AlGaN. Filling of surface states in the gate–
drain area by electrons emitted from the reverse-biased gate may deplete n2DEG in the
AlGaN/GaN quantum well (QW) channel. Subsequently a gate-lag (GL) i.e. collapse in
drain current is observed if positive pulse on the gate is applied. Alternatively it was
suggested that interface states behave as hole traps during the GL transients but that
happens also in the source-gate area. Strong modulation of the AlGaN surface potential
due electrons emitted from the ohmic contacts was reported elsewhere and this resembles
situation during the pulse applied on the drain called a drain lag (DL) effect.
Figure3.1:InAlN/GaN HEMT structure with field plated
40
Figure3.2:Simulated result of InAlN/GaN HEMT with field plate length 2.25μm
Figure 3.3:Band diagram of InAlN/GaN HEMT
41
3.2 Results and Discussions
3.2.1 Drain Current Characterisctics
When a dc voltage applied to the drain electrode with the source electrode grounded, a
current Id, whose magnitude depends on the total resistivity of the 2DEG channel, flows
between two electrodes. The gate electrode, by acting on the 2DEG density below it,
modulates the channel resistivity and thus Ids. The V-I characteristics, also known as the
output characteristics, can be obtained by imposing the constant current condition along
the channel at a given gate voltage Vgs.
The Id vs Vds characteristics of this device is simulated and shown in figure 3.4.At constant
Vgs ,Vds is swept from 0V to 20V.The device remained in off-state(i.e Ids=0 A)
when Vgs=-7V.The drain current when Vgs=0V is 3.8A/mm.
Figure 3.4:Drain current charcteristics
Vgs=-7V
Vgs=2V
42
3.2.2 Transfer characteristics
The Ids vs Vgs characteristics of this device is simulated and shown in figure 3.5 which
describes the Ids-Vgs dependence at a fixed 𝑉𝑑𝑠, usually in saturation regime. At constant
Vds(the value where current is saturated),Vgs is swept from -10V to 4V.The minimum gate-
to-source voltage VGS (th) that is needed to create a conducting path between the source and
drain terminals is called threshold voltage(Vth).
The Vth for this device is determined as -8.5V.
Figure 3.5:Transfer characteristics
Vds=15V
Vds=17V
43
3.2.3 Log Id vs Vg Characteristics
The log Id vs Vgs characteististics of this device is simulated and shown in figure 3.6.
The slope of this curve is called subthreshold slopeand detrmined as 13.91mV/dec.
Short channel effects can be also important causes of non-ideal behaviour. If the gate
does not have a very strong electrostatic control over the channel, the drain voltage can
still strongly influence the drain-source current even at the saturation regime. This
happens particularly when the gate to channel distance,i.e. the barrier thickness, is not
negligible with respect to Lg. Short channel effects have a strong effect in the pinch-off
region. At high Vds, the gate can no longer block the current flow and a non negligible
Ids can appear. Short channel effects can be therefore assimilated to a Vds dependent Voff
and are measured through the drain induced barrier lowering (DIBL) parameters.
𝐷𝐼𝐵𝐿 =𝜕𝑉𝑜𝑓𝑓
𝜕𝑉𝑑𝑠 (3.1)
DIBL of this device is determined as 0.116297
Ion/Ioff of theis device is 1014
Figure 3.6:Log Id vs Vg characteristics
44
3.2.4 Gate lekage current Characteristics
The gate lekage current charcteristics of this device is simulated and shown in figure 3.7.
The gate leakage current surges as a consequence of surface processing and passivation
issues.
Figure:3.7 Log Ig vs Vg Characteristics
We found a strong correlation between the gate leakage current and the transistor
subthreshold characteristics: the lower the gate leakage, the higher the ON/OFF ratio and
the steeper the subthreshold slope. In order to reduce off-state power consumption, it is
necessary to reduce leakage current.
The gate leakage current of this device is determined as 10-17 A/mm.
45
3.2.5 Breakdown Characteristics
The breakdown voltage is the drain-source voltage, which causes the transistor to enter the
breakdown region.It is the region where the device receives too much voltage across its
drain-source terminal, which causes the drain-source terminal to break down, which makes
the grain current drastically increase.The breakdown characteristics of this device is
simulated and shown in figure3.8.
The breakdown down charcteristics is also simulated for 400o k and 5000k for 1.75 micro
meter fieldplte length and the breakdown volatage values are determined as:
Vbr=85.8771V (300ok)
Vbr=89.4763V (400ok)
Vbr=95.428V(500ok)
Figure 3.8:Breakdown characteristics of this device for different field plated lengths
46
3.2.6 Transconductance(gm)
Transconductance is the ratio of the change in drain current to the change in gate voltage
over a defined, arbitrarily small interval on the drain-current-versus-gate-voltage curve.
𝑔𝑚 =𝜕𝐼𝑑𝑠
𝜕𝑉𝑔𝑠 𝑎𝑡 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 𝑉𝑑𝑠 (3.2)
As for the output characteristics, what is usually represented is the Vgs dependence of gm
in the saturation regime. The transconductunce vs Vgs characteeristics for this device is
simulated and shown in figure 3.9.
In an ideal HEMT, both Ids and gm increase indefinitely with Vgs. In real devices,
however, several non linearities contribute to modify the output and transfer
characteristics.
Figure 3.9:gm vs Vgs characteristics
The gm is maximum when Vgs=-2V with a value of 0.0625S/mm.
47
3.2.7 Cutoff frequency(fT)
The fT is based on the short-circuit current gain (h21) which is the ratio of the small-signal
output current (drain current) to input current (gate current) with the output short-circuited.
Generally, the magnitude of the current gain rolls off with a slope of -10 dB/decade at high
frequency range.The current gain vs frequency characteristics of this device is simulated
and shown in figure 3.10.
The fT is the frequency that the magnitude of the current gain becomes unity.
𝑓𝑇 = 𝑔𝑚2𝜋
1
(𝐶𝑔𝑠+𝐶𝑔𝑑)+(𝑅𝑠+𝑅𝑑
𝑅𝑑𝑠)(𝐶𝑔𝑠+𝐶𝑔𝑑)+𝑔𝑚𝐶𝑔𝑑(𝑅𝑠+𝑅𝑑)
(3.3)
Neglcting the parasitic capacitances and resistances by GaN doping and Si3N4 passivation
the cutoff frequency is improved.
𝑓𝑇 =𝑔𝑚
2𝜋𝐶𝑔𝑠 (3.4)
Figure 3.10:Current gain vs Cutoff frequency
The cutoff frequency of this device is 90GHz.
48
3.2.8 Maximum Oscillation Frequency(fmax)
Fmax is defined as the frequency that the unilateral power gain (U) equals unity.The power
gain vs frequency characteristic of this device is simulated and shown in figure 3.11.The
unilateral power gain (U) is the power gain under the assumption of no feedback from the
output to the input and it rolls off with the -20 dB/decade slope at high frequency range.
𝑓𝑚𝑎𝑥 =𝑓𝑇
2√𝑅𝑠+𝑅𝑔+𝑅𝑔𝑠
𝑅𝑑𝑠+2𝜋𝑓𝑇𝐶𝑔𝑑𝑅𝑔
(3.5)
Figure 3.11:Power gain vs frequency characteristics
The maximum oscillation frequency(fmax) is determined as 160GHz.
49
Table 3.1:DC and microwave performance of GaN based HEMTs
The comparison with various experimental results of GaN-HEMTs along with
our work are presented in table 3.1.The high Vbr along with high fmax make the
asymmetric recess technology particulary well-suited for the design of high-
speed and efficient power amplifiers.This result exhibits that these devices are
excellent candidates for ultra-high-frequency power applications.
Parameters AlGaN/
GaN
[this
work]
AlGaN/
GaN
[15]
AlGaN/
GaN
[13]
AlGaN/
GaN
[14]
AlGaN/
GaN
[13]
InAlN/
GaN
[this
work]
Gatelength
(Lg)(nm)
400 600 250 170 150 400
Drain current
(Ids) (A/mm)
1.8 1.59 1.40 - 2.7 3.8
Threshold
volage(Vth)(V)
-2.989 - -5.6 -2.6 -6.0 -8.5
Cutoff
frequency(fT)(GHz)
60 19 42 46 50 70
Maximum Oscillation
frequency(fmax)(GHz)
180 50 100 150 121 210
Transconductance(gm)
(mS/mm)
525 374 255 245 280 625
Breakdown voltage
(V)
819.443 - - - - 85.8771
50
CONCLUSION
In our project, the DC and microwave characteristics of GaN-based high
electron mobility transistors are analysed by using Silvaco TCAD tool. The primary
focus of our project is to enhance the drain and microwave characteristics of GaN-
based HEMTs with high breakdown voltage for future high power electronic
applications.
The essential key parameters for high power microwave applications is
maximum frequency oscillation (fmax). In this project, several optimization
techniques are adopted for improving the maximum frequency oscillation, such as
heavily doped n++ GaN source/drain region for drastic reduction in the source/drain
contact resistances,addition of fieldplate of length 2.25μm to the structure for
improving the breakdown voltage,Si3N4 passivation techniques for reducing the
parasitic capacitances for enhancing the high frequency performance of the HEMTs
for microwave electronics. The InAlN/GaN is of particular interest of this project
work due to their superior performance in terms of high frequency, high power. The
summary of this work is presented in Table 3.1.
The proposed novel GaN-based high electron mobility transistors shows
preeminent drain current and microwave characteristics and the objectives of this
project work is attained. These devices are expected to be the most optimistic future
forerunners of millimeter wave electronics such as, satellite telecommunications,
high power amplifiers for radar, microwave image sensing, military electronic
systems and low noise wide bandwidth amplifiers design for future high power
microwave communications.
51
FUTURE SCOPE
The rapid development of high power millimeter wave electronics such as next
generation cellular communication, satellite communication, high speed data
transmission and radar applications requires both high breakdown voltage and high
electron velocity transistors. GaN-based high electron mobility transistors offering
high electron mobility and high carrier concentration results in high current density
and high frequency operation.
Although the microwave performance of the GaN-based HEMTs is pushed towards
the sub-millimetre wave region, a novel device structures such as metal oxide
semiconductor high electron mobility transistors (MOSHEMT) are needed for
future ultra-high power, high frequency applications.
52
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