96603724 5th Chapter Thesis 2nd

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HEMT Simulations

1Department of ECE, NITC

CHAPTER 1

INTRODUCTION

1.1 Introduction:

High Electron Mobility Transistor (HEMT) is also known as Heterostructure

Field Effect Transistor (HFET) or Modulation-doped Field Effect Transistor

(MODFET), is a Field Effect Transistor (FET) incorporating a junction between two

different materials with different band gaps (i.e. a heterojunction) as the channel

instead of a doped region, as is generally the case for Metal Oxide Field Effect

Transistor (MOSFET). It is a device that exploits the high electron mobility in an

undoped region to achieve high speed operation. Film deposition technique like

epitaxy is used to create undoped region, then narrow undoped region, when applied

a MBE electron well which forms the channel for current flow. Electrons from

suitable bias, work as a quantum mechanical surrounding doped regions of the device

are trapped in the quantum well resulting in a high concentration of electrons in the

channel. This channel is below the surface of the device and separated from surface

which reduces surface scattering. As the doping in the channel is uninitiated, lack of

scattering sites in the channel results in high electron mobility. In addition, the

channel itself is normally constructed from a material which possesses high mobilitysuch as InGaAs. A commonly used material combination is GaAs with AlGaAs, in

this work combination of GaAs (substrate), InGaAs (channel) with AlGaAs used.

This combination is known as pseudomorphic HEMT (PHEMT) [1]. InGaAs channel

is epitaxially grown on a GaAs substrate. Second variant is AlInAs/GaInAs HEMTS

grown lattice-matched on InP substrates. These two combinations used in

pseudomorphic HEMT.



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CHAPTER 2

COMPOUND SEMICONDUCTORS

This chapter discusses the classification of Semiconductors and then

compound semiconductor properties are discussed. Hetero junction function isdiscussed along with HEMT operation. Semiconductors can be broadly classified intotwo categories.

1. Elemental semiconductors2. Compound semiconductors.

Elemental semiconductors C, Si, Ge, Sn are widely used in VLSI application.However, Due to their inferior properties of compound semiconductors are when bothelectronic and mechanical properties are better when compared to elementalsemiconductors.

2.1 Types of Compound Semiconductors:

Compound semi conductors are semiconductor materials obtained by mixing

two or more elements. To get a compound semiconductor from different elements the

following criterions must be satisfied.

1. The average valence should be four.

2. Mole fraction contribution group should be same that of its complimentary

group.

3. There shouldn’t be any chemical any chemical compound formation or

chemical reaction between different elements which are used in compound

semiconductor.

4. Band gap of resultant material should be in the range of semiconductor

band gap.

5. Conductivity modulation by doping.

6. The bonding should be of more covalent than ionic.

Depending up on the number of elements, compound semiconductors are classified

into three categories. They are

1. Binary

2. Ternary



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3. Quaternary.

2.1.1 Binary Compound Semiconductors:

These are the alloys formed between two compatible elements of periodic

table. Binary compound result from II – VI, III – V and IV – IV group conversions.

Group Element Compound semiconductor

II Zn, Cd ZnS, ZnSe, ZnTe, CdS, CdSe

CdTeVI S,Se,Te

III Al, Ga, In AlN, AlP, AlAs, AlSb, GaN, GaP,

GaAs, GaSb, InN, InP, InAs, InSbV N, P, As, Sb

IV C, Si, Ge Si, SiGe

Table.2.1. Provides a comprehensive list of Binary compound

semiconductors

Band gap and lattice constant of a binary compound semiconductors is constant.

2.1.2 Ternary Compound Semiconductors:

Ternary compounds are generally an extension of binary compound

semiconductors, one of the elements replaced with two elements from the same group

and the atomic composition or mole fraction under consideration between those

elements is varied from 0 to 100%. Few examples of ternary compound

semiconductors are InxGa1-xAs and AlxGa1-xAs compounds. x is known as mole

fraction.

As the atomic composition in varied, the chemical composition varies as

electronic composition varies. This result is different electrical and mechanical

properties of the semiconductor. As a result the lattice constant, band gap, dielectric

constant and intrinsic carrier concentration variation. This unique property can be

used in designing and controlling field in a compound semiconductor.



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2.1.3 Quaternary Compound Semiconductors:

Quaternary compound semiconductors are made by combining from elements

generally from columns III and V are mixed to obtain compound semiconductor.

Widely used compound semiconductor is GaxIn1-xAs1-yPy. As the mole fraction isvaries different properties can be varies. The In/Ga and As/P ratios may be varied

independently. GaInAsP is used for near-infrared laser diodes employed in optical

communication [2].

2.2 General Properties:

GaAs, InxGa1-xAs and AlxGa1-xAs are the semiconductors considered in this

study. So the properties of their semiconductors are studied in detail. Fig. 2.1 shows

energy band gap versus lattice constant for various semiconductors. It can be seen

that lattice constant of Si and GaAs are very close. In HEMT devices, generally GaAs

is used as substrate and InGaAs or GaAs is used as the channel region. In this work

mole fraction discusses In 0.15 and mole fraction of Al is 0.2.



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Fig.2.1. Energy band gap Vs lattice constant [3]

2.2.1. Energy Band Gap:

The energy band gap as a function of In content ‘x’ in In xGa1-xAs at room

temperature given by eqn. (2.1) [4]

E( x) = 1 .4 2 5 − 1 .5 0 1x + 0 .4 3 6x (2.1)

Eqn. (2.1) is plotted for different mole fraction x and is shown in Fig.2.2.

Energy Band Gap of In0.15Ga0.85As is 1.209 eV.



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Fig.2.2. variation of Eg in InxGa1-xAs alloys

The energy band gap as a function of Al content ‘x’ in AlxGa1-xAs at roomtemperature is given by eqn. (2.2) and eqn. (2.3). [5]

E,( x) = 1 .4 2 2 eV + x 1 .2 4 75 eV (x<0.45) (2.2)

E,( x ) = 1 .9 eV + 0 .1 25 x eV + 0 .1 43 x eV (x>0.45) (2.3)

In Fig.2.3 where eqn. (2.2) shows direct band gap and eqn. (2.3) shows indirect it is

worth to note that the semiconductor changes from direct to indirect band gap on the

Al mole fraction in curve beyond 0.45.

Energy Band Gap of Al0.2Ga0.8As is 1.675 eV.



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Fig.2.3. variation of Eg in AlxGa1-xAs alloys

2.2.2 Lattice Constants:

Lattice constant of the binary compound semiconductors are fixed and ternary

compound semiconductors changes with a change in mole fraction. Lattice constant

(a) as a function of In content ‘x’ in InxGa1-xAs given by eqn. (2.4)

a = 6 .058 3 − 0.405x (2.4)

Lattice Constant of In0.15Ga0.85As is 5.714Å.

Lattice Constant as a function of In content ‘x’ in AlxGa1-xAs given by eqn. (2.5)

a = 5 .653 3 + 0 .0078x (2.5)

Lattice Constant of Al0.2Ga0.8As is 5.65486 Å.

2.2.3 Dielectric Constant:

Dielectric constant of the binary compound semiconductors are fixed and

ternary compound semiconductors are varies with mole fraction. Dielectric constant

() as a function of In content ‘x’ in InxGa1-xAs given by eqn. (2.6)

= 1 5 .1 − 2 .8 7 x + 0 .6 7 x2 (2.6)



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Dielectric Constant of In0.15Ga0.85As is 13.144.

Dielectric Constant as a function of In content ‘x’ in AlxGa1-xAs given by eqn. (2.7)

r = 1 2 .9 0

−2.84x (2.7)

Dielectric Constant of Al0.2Ga0.8As is 12.322.

2.2.4 Intrinsic Carrier Concentration:

Intrinsic carrier concentration (ni) of the binary compound semiconductors are

fixed and ternary compound semiconductors are varies with mole fraction. ni as a

function of In content ‘x’ in InxGa1-xAs given by eqn. (2.8)

n = 4 .8 2 x 1 0 ( 0.41 − 0.09x) + ( 0 .0 2 7 + 0 .0 4 7x) ( 0 .0 2 5 +

0.043x) / Texp

( ) 1 +

. +. − . (2.8)

Where v =

Where it should be noted that Eg varies as x.

Intrinsic carrier concentration of In0.15Ga0.85As is 6.99x10

7

/cm

3

Intrinsic carrier concentration as a function of AlxGa1-xAs given by eqn. (2.9) andeqn. (2.10)

n = 4 .8 2 x 1 0 ( 0 .3 2 13 + 0 .0 5 80 8 + 0 .0 2 07 5x ) T

exp( Eg

2k T) x<0.41 (2.9)

Where it should be noted that Eg varies as x.

n = 4 .8 2 x 1 0 (

0.85 − o . 1 4 x) T

exp

(

Eg

2k T)

x>0.41 (2.10)

Where eqn. (2.9) is a direct band gap and eqn. (2.10) is an indirect.

Intrinsic carrier concentration of Al0.2Ga0.8As is 4.8806 x 104 /cm3



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Different semiconductors are formed between different materials, there

junctions will be discussed.

2.3 Hetero junctions:

Hetero junction is a junction formed by two dissimilar semiconductormaterials, across the junction, one semiconductor material will have larger band gap

(denoted by capital letter) and other material will have narrow band gap (denoted by

small letter). Depending upon the conductivity type across the junction, hetero

junction can be classified into iso type and aniso-type. Depending upon the band

alignment, Hetero junctions can be classified into three categories. They are

1. Type I

2. Type II3. Type III

2.3.1 Type I:

Type I heterostructure is the most common. In type I hetero junction, both

conduction band and valence band of narrow band gap material lie completely

between conduction band and valence band of wide band gap material. Schematic

band gap diagram of type I hetero junction is shown in Fig.2.4. An important example

of a type I heterostructure is the GaAs–AlGaAs materials system. In a type I

Heterostructure, the sum of the conduction band and valence band edge

discontinuities is equal to the energy gap difference,∆E = ∆E + ∆E (2.11)

Fig.2.4. Type I hetero junction [6]



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2.3.2 Type II:

In Type II hetero junction, only one of conduction band or valence band of

narrow band material is in between the conduction band and valence band of the wide

band gap material. Schematic band representation of type II material is shown in

Fig.2.5. The band gap discontinuity in this case is given as the difference between the

conduction band and valence band edge discontinuities. A type II Heterostructure is

formed by Al0:48In0:52As and InP.

Fig.2.5. Type II hetero junction

2.3.3 Type III:

Type III heterostructure, both conduction band and valence band of narrowband material never lie between conduction band and valence band of wide band gap

material. Schematic band representation of type III is shown in Fig.2.7. Example of

type III hetero structure is GaSb and InAs.

As for the type II case, the band gap discontinuity is equal to the difference

between the conduction band and valence band edge discontinuities. Certainly, one of

the most important Heterostructure is that formed between GaAs and AlAs or its

related ternary compounds, AlGaAs. The GaAs–AlGaAs heterostructure has the

additional feature of close lattice matching. Two materials that have nearly identical

lattice constants are when used to form a Heterostructure. If the materials are not

lattice matched, the lattice mismatch can be accommodated through strain or by the

formation of misfit dislocations.



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Fig.2.6. Type III hetero junction

2.4 Band diagram and conduction band discontinuity:

Fig.2.7. n-p junction energy band diagram (equilibrium condition).

When the doping applied n+

on n side, lightly doped on p type then apply agate voltage, Fermi level reach above the conduction band edge (∆Ec). The

discontinuity and band bending takes place such that the charge at the n side interface

is a minimum. This forces the neutral levels E0 on either side of the junction to align

and lineup and in-turn determines band offset. E0 that position may not exact but

close to each other. With respect to Donor level occupied is neutral and unoccupied is

positive.

Fig2.8. band diagram with neutral level



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With respect to acceptor level, unoccupied is neutral and occupied is negative.

The neutral level lineup does not occur, charge transfer takes place between the states

on either side of the interface and this would create a dipole field. From the energy

considerations the system favors dipole minimization and therefore equalization of E0 levels. In the hetero junction a band bending take place such that the conduction band

edge in the neutral region of InGaAs is raised by ∆Ec so that net electron flow in

thermal equilibrium is zero. Therefore the potential drop S in InGaAs in thermal

equilibrium is higher than in the homogeneous junction by ∆Ec /q.

cEHetero Homo

S S q

(2.12)

Due to the discontinuity ∆Ec in the conduction band of InGaAs and GaAs, theband bending in the undoped InGaAs is more in the GaAs homojunctions of similar

doping levels. Due to this effect, large concentrations of electrons are present at the

InGaAs surface adjacent to GaAs and they remain there due to notch in the

conduction band. The electrons have actually been supplied from GaAs layer which

has been doped. These electrons are in the In0.2Ga0.8As region where doping

concentration is low.

2.5 Strain at Heterointerfaces:

Bulk crystalline semiconductor is that it exhibits perfect or nearly perfect

translational symmetry. In other words, suitable translations of the basic unit cell of a

crystal restore the crystal back into itself. Atoms within the crystal are regularly

spaced throughout the entire bulk sample. This assumption is true for bulk materials.

However, two important exceptions can arise. The first is that a bulk crystal can

include impurities and dislocations such that the perfect periodicity of the material is

disrupted locally. The crystal can still retain its overall highly ordered structure, yet

contain local regions in which perfect periodicity is disrupted by impurities ordislocations. These impurities and dislocations can significantly affect the properties

of the material. The second situation arises in multilayered structures. Crystal growth

technology has enabled the growth of thin layers of heterogeneous semiconductor



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material called Heterostructure. Using exacting crystal growth procedures,

Heterostructure can be grown with atomic layer precision. A very thin layer of

material can be grown on top of or sandwiched between layers grown with a different

type of semiconductor material, even materials in which the lattice constant is

different. When a thin layer of material is grown either on or between layers of a

different semiconductor that has a significantly different lattice constant, the thin,

epitaxial layer will adopt the lattice constant of the neighboring layers provided that

the lattice mismatch is less than about 10% as can be seen from Figure 2.9, When the

thin, epitaxial layer adopts the lattice constant of the surrounding layers, it becomes

strained, i.e., it is either compressed or expanded from its usual bulk crystal shape.

There exists a maximum thickness of the thin layer below which the lattice mismatch

can be accommodated through strain. For layer thickness above the critical thickness,the lattice mismatch cannot be accommodated through strain, dislocations are

produced and the strain relaxes as is seen in Fig.2.10.

The strain within the layer is homogeneous. The strained layer can be in either

compressive or tensile strain. If the lattice constant of the strained layer is less than

that of the surrounding layers the system is in tension. Conversely, if the lattice

constant of the strained layer is greater than that of the surrounding layers, the

strained layer is in compression. Heterojunctions may be comprised of

Pseudomorphic layers in which one of the layers is lattice mismatched and coherent,

or elastically strained. For a given strain, determined by the lattice mismatch, a

maximum thickness exists, called the critical thickness hc, for the strained layer to be

completely coherent with the substrate. The critical thickness has a minimum under

conditions of thermodynamic equilibrium, but can be enhanced by the epitaxial

process. For thicknesses greater than the critical thickness, dislocations are created to

reduce the energy of the system. These dislocations can significantly degrade device

performance and reliability.



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Fig.2.9. Thin epitaxial layer strained to accommodate the various lattice constants of

the underlying semiconductor layer

Fig.2.10. The epitaxial layer is thicker than the critical thickness and dislocations

appear at the interface



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2.6 Pseudomorphic Growth on GaAs Substrate:

Pseudomorphic growth was first investegated by methews and

Blakeslee.he is experiments with alternating layers of GaAs and GaAsP to derive a

theoritical expression for critical layer thickness (hc) which canot be exceeded if misfit dislocations must be avoided.the thickness hc upto which the mismatch of the

InxGa1-xAs layer can be accomodated by elastic strain is given by eqn. (2.13)

0 .0 7 x =[ √ ]

√ ( )(2.13)

Where x is the indium content, a is GaAs lattice constant (a=0.565nm), and is

poisson’s ratio (0.23 for GaAs). For x=0, the growth is lattice matched, magnitude of hC is infinite (same as AlGaAs/GaAs HEMT). For large x critical thickness is only a

few nanometers. Quantum wells that are too narrow contain only a small number of

charge carriers, and the transport properties of these carriers are degraded due to

excessive interface scatering. Transistor performnance is found x=0.2 and a thickness

12nm. Transsistor performnance is number of transistors in the channel and carrier

mobility.

2.7 Compound Semiconductor Properties:

Properties of compound semiconductors will be discussed below. In this work

I will discuss GaAs, AlxGa1-xAs, InxGa1-xAs material properties. Mole fraction of

AlxGa1-xAs is 0.2 and InxGa1-xAs is 0.15.



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2.7.1 Basic parameters:

Property GaAs Al0.2Ga0.8As In0.15Ga0.85As

Crystal structure Zinc Blende Zinc Blende Zinc Blende,

CubicGroup of symmetry Td -F43m Td -F43m Td -F43m

Number of atoms in (/cm ) 4.42x10 4.386x10 2.8845x10

Debye temperature(K) 360 381.68 373.5

Density(g/cm ) 5.32 5.008 5.3655

Dielectric constant (static ) 12.9 12.332 13.144

Dielectric constant (high frequency) 10.89 10.344 11.11

Effective electron mass me 0.063mo 0.0796 mo 0.0566 mo Effective hole masses mh 0.51mo 0.56mo 0.495 mo

Effective hole masses mlp 0.082mo 0.0956 mo 0.0736 mo

Electron affinity ( eV) 4.07 3.85 4.1945

Lattice constant(Å) 5.65325 5.65486 5.714

Optical phonon energy (eV) 0.035 0.03725 -

Table.2.2. Compound semiconductor basic parameters [4,7, 8]

2.7.2 Electrical properties:

Property GaAs Al0.2Ga0.8As In0.15Ga0.85As

Breakdown field (V/cm) ≈4x10 (4-6) x10 (2-4)x10

Mobility electrons (cm /V.sec) ≤8500 4000 33,000

Mobility holes (cm /V.sec) ≤400 205.6 ~300-400

Diffusion coefficient electrons (cm /s) ≤200 100 171.6

Diffusion coefficient holes (cm /s) ≤10 5.14 7-12

Electron thermal velocity (m/s) 4.4x10 3.98x10 4.56x10Table.2.3. Compound semiconductor electrical parameters



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2.8 Potential Well and Two Dimensional Electron Gas:

Different compound semiconductors are epitaxially grown on device their

Band gaps and electron affinity are different so that the interface forms a

discontinuity and potential well is created. AlGaAs and InGaAs forms Triangularpotential well. Electrons in the interface of two regions are confined in this well it

forms Two-Dimensional Electron Gas (2DEG). 2DEG is a gas of electrons free to

move in two dimensions, but tightly confined in the third. This tight confinement

leads to quantized energy levels for motion in that direction, which can then be

ignored for most problems. Thus the electrons appear to be a 2D sheet embedded in a

three dimensional (3D) world. Dopants placed in AlGaAs layer. Due to the energy

band structure, electrons from these dopants are confined to the channel layer where

they form 2DEG. The sheet carrier density of 2DEG and the electron mobility

measured at room temperature are 2.1x1012 cm2 and 8600 cm2 /V s, respectively.

Electrons are confined in the channel device operated in inversion region.

2.9 HEMT Structure:

HEMT channel is undoped and physically removed from the ionized donors,

and because the electrons travel in the quantum well parallel to the heterointerface,

the HEMT electron mobilities are more typical of ultra-pure bulk semiconductors. Itshould be noted that p-type modulation doping is also feasible though there is wide

variation, dependent on the application of the device. Devices incorporating more

indium generally show better high-frequency performance [9], while in recent years;

GaN HEMTs have attracted attention due to their high-power performance [10]. To

allow conduction, semiconductors are doped with impurities which donate mobile

electrons (or holes). However, these electrons are slowed down through collisions

with the impurities (dopants) used to generate them in the first place. HEMTs avoidthis through the use of high mobility electrons generated using the heterojunction of a

highly-doped wide-band gap n-type donor-supply layer (AlGaAs in our example) and

a non-doped narrow-band gap channel layer with no dopant impurities (InGaAs in

this case).



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The electrons generated in the thin n-type AlGaAs layer drop completely into

the InGaAs layer to form a depleted AlGaAs layer, because the heterojunction created

by different band-gap materials forms a quantum well (a steep canyon) in the

conduction band on the InGaAs side where the electrons can move quickly without

colliding with any impurities because the InGaAs layer is undoped, and from which

they cannot escape. The effect of this is to create a very thin layer of highly mobile

conducting electrons with very high concentration, giving the channel very low

resistivity (or to put it another way, "high electron mobility"). This layer is called a

Two-Dimensional Electron Gas (2DEG). As with all the other types of FETs, a

voltage applied to the gate alters the conductivity of this layer. Ordinarily, the two

different materials used for a heterojunction must have the same lattice constant

(spacing between the atoms). In semiconductors, these discontinuities form deep-level traps, and greatly reduce device performance. HEMT formed from different

components of elements are called compound semiconductors. Basic HEMT diagram

is shown in below fig.2.11.

Fig.2.11. HEMT basic block diagram

Different combinations of HEMT are as shown in Table.2.4.



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Cap layer GaAs GaAsInyGa1-yAs(y=0.53)

InyGa1-yAs(y=0.53)

BarrierAlxGa1-xAs

(x<0.23)AlxGa1-xAs

(x<0.23)InxAl1-xAs(x=0.52)

InxAl1-xAs(x=0.52)

spacer AlxGa1-xAs AlxGa1-xAs InxAl1-xAs InxAl1-xAs

Channel GaAs(latticematched) InyGa1-yAs(y<0.2) InyGa1-yAs(y=0.53) InyGa1-yAs(y>0.53)Substrate GaAs GaAs InP InP

Conduction banddiscontinuity (eV)

0.2 0.3 0.52 >0.52

Valence banddiscontinuity (eV)

0 0.12 0.52 >0.52

RemarksHistorically first

HEMT

SingleHeterojunction

PHEMT onGaAs

Standard HEMTon InP

AdvancedHEMT on InP

Table.2.4. HEMT different combinations [1]

In this work single hetero junction pseudomorphic HEMT technique is used.

2.10 Working of HEMT:

In this topic Band diagram, Gate functionality, operation of HEMT discussed.

2.10.1 Band Diagram:

Two semiconductors with different energy band gaps are grown on each other,

discontinuities of the conduction and valence band edges arise at the heterointerface.

Two semiconductors with different band gap energies are joined together the

difference is divided up into a band gap offset in the valence band ∆EV and a band

gap offset in the conduction band ∆EC. In most cases conduction band offset between

adjacent layers ∆EC is close to 2/3 of the total band gap difference ∆Eg. One of the

most common made assumptions for the AlGaAs/InGaAs material system is 40 %

valence band offset and 60 % conduction band offset. This is only valid for Alcontents below about 45 %. For higher Al contents the band gap of AlGaAs changes

from direct to indirect. Energy band diagram with gate voltage is -0.6 V as shown in

Fig.2.12. AlGaAs layer electron concentration increased from top to bottom. Sudden



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discontinuity is due to change of compound semiconductors. The conduction band of

the channel relative to the Fermi level EF is determined by ∆EC, the doping level ND,

the barrier height of the Schottky contact (qFB), the gate to channel separation, and

the applied voltage on the gate VGS. Gate voltage is 0 compare to -0.6 volts

conduction band discontinuity is more and more number of electrons are confined in

the channel. Up to 0.6 um AlGaAs layer there electron concentration increases and

later 0.6 to 0.7 um InGaAs channel where 2DEG formed and after GaAs substrate

region electron concentration decreases. InGaAs layer is to provide better electron

confinement and superior electron transport characteristics. In these devices, the sheet

concentration of 2DEG is increased over comparable AlGaAs /GaAs structures due to

the increase of the conduction band-edge discontinuities at the heterojunctions.

Electron mobility and steady state saturation velocity in InGaAs are intrinsicallyhigher than those in GaAs. There are only two controllable parameters that exert a

strong influence on the electron density: the donor density and the conduction-band

discontinuity. Band diagram of HEMT applied voltages -0.6 and 0 volts as shown in

Fig.2.12 and Fig.2.13.

Fig.2.12 Band Diagram applied gate voltage is -0.5 V



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Fig.2.13. Band Diagram applied gate voltage is 0 volts

2.10.2 Gate Functionality:

A Schottky barrier is a rectifying metal semiconductor contact while ohmic

contact has linear relationship between the voltage and current, Schottky contact in

equilibrium. There is no current flow in equilibrium and that gradient of Fermi levelis zero. So Fermi level is flat everywhere in equilibrium. At the metal-semiconductor

in equilibrium the Fermi levels must align. Consider the behavior of metal first Fermi

level lies above the conduction band edge. The energy needed that an electron be

ionized, escape from the metal and enter the vacuum level, is called the work function

energy or qm. Most of the conduction electrons within the metal are at energy

reasonably close to the Fermi level. Therefore Work function is the energy difference

between the vacuum level and Fermi level. In a nondegenerate semiconductor theirFermi level doesn’t lie above the conduction band edge but some is somewhere in the

forbidden gap. The semiconductor work function is also difference between the Fermi

level and vacuum level. Since most of the electrons in semiconductors are not at the

Fermi level, it is known as electron affinity.



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2.10.3 HEMT Operation:

Current flowing through the device is depends on electron concentration in the

channel. Its controlled gate Schottky barrier diode formed on AlGaAs. Drain current

flow due to electron transport through the n-type AlGaAs layer should be avoided byensuring that this layer is fully depleted wider in gate region. This voltage is known

OFF-voltage. It is same as like threshold voltage in MOSFET. Voltage required to

turnoff the depletion. HEMT AlGaAs doped layer should not be use for conducting

otherwise current flows through it. Only channel layer is used for current transport.

Current flow in the device should take place only through electron in the notch. They

are high mobility electron. Electron concentration can be reduced by decreasing the

potential drop on the P-side. It is possible by forward bias the AlGaAs and InGaAs.That voltage is EC /q. OFF-voltage is the difference between voltage required to

depleted to entire region is Vbi-Vpo where Vpo is pinch of voltage, Vbi is the build

in potential and Remove charges in InGaAs(no conduction of electrons). voltage

required to depleted to entire region is Vbi-Vpo where Vpo is pinch of voltage, Vbi is

the build in potential. and Remove charges in InGaAs(no conduction of electrons) is

EC /q. Vpo is pinch of voltage depletes the channel completely.

V( OFF) = V

−V

−(

∆) (2.11)

V = l n

(2.12)

V = ∈∈ (2.13)

where a is GaAs layer thickness and ND is dopant density of AlGaAs. Whenever

reverse gate voltage is applied, the conducting channel between the source and drain

can be choked off completely, under these conditions, the device is said to be in

pinch-off.

Depends on OFF-voltage weather the device is depletion or enhancement decided.

Gate voltage is less than OFF-voltage device is depletion mode and gate voltage is

greater than OFF-voltage device in enhancement mode. –ve voltage on gate will



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widen the depletion layer belongs to schottky barrier and narrow down the depletion

layer width of heterojunction. So band bending in In0.2Ga0.8As layer reduces causing a

reduction in the electron concentration in the 2DEG channel. charge at the channel is

Cs(VGS-V(OFF))

Fig.2.14 Charge distribution and band diagram of fully depleted HEMT [11]

Drain current I = ( V − V) (2.14)

Transconductance g =µ ( V − V) (2.15)

=Є Є

(2.16)

Drain current and Trans conductance are depends on mole fraction and

AlGaAs layer thickness and doping concentration of barrier. To Fix the OFF-voltage

it is depends on pinch-off voltage. Pinch off voltage depend on AlGaAs layer doping

concentration. If increase the thickness AlGaAs layer doping is also increase then

only fix OFF-voltage. Doping is up to 1018 /cm3 maximum possible. Spacer layer

thickness increases their CS falls and gm also falls, so spacer layer thickness is not

more increases 20 to 40Å. Conduction band discontinuity higher 2DEG

concentration increases so improves the mobility. It depends on Al content.

∆E = 0 .6 ( E − E) (2.17)



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Eg1 varies with Al content. Band gap increases with Al content. Al mole fraction

greater than 0.3 the layer defects increases. Dx centers are introduced, which are

electron traps. And also very difficult to form a ohmic contact to AlGaAs layer. Metal

work function is equal 0.67(Eg1-Eg2).

Finally spacer layer thickness is 20 to 40Å.

AlGaAs doping layer upper limit is 1018 /Cm3, Al mole fraction is 0.2.

2.11 Comparisons of Compound Semiconductors:

Compound semiconductors Gallium Arsenide, Indium Phosphate and Gallium

Nitride compare with Silicon and Germanium. Mobility of Compound

semiconductors is high except silicon carbide. Mobility is high GaAs is

8500cm2 /V.sec. Hole mobility is almost same for all materials except germanium

1900 cm2 /V.sec. Saturation velocities and breakdown fields are more for compound

semiconductors. Energy gap of SiC is 3.26 electron volts is more and Ge is 0.66 is

least and also compound semi conductors band gaps are more. Structures of Si and

Ge are diamond lattice, InP and GaAs are zincblende, SiC and GaN are wurtzite

lattice structure. HEMT, MESFET, HBT are prepared from Compound

semiconductors while MOSFET and BJT prepare from Silicon and Germanium. Cost

prototype is high for GaN and SiC, low for silicon



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Properties Si Sic Inp GaAs GaN Ge

Electron Mobility

(cm2 /V-s)1500 700 5400 8500

1000-

20003900

Hole Mobility

(cm2 /V-s)475 - 150 400 - 1900

Peak saturated

electron velocity

(107 cm/sec)

1 2 2 2.1 2.1 1

Energy Gap at

300K (eV)

1.12 3.26 1.35 1.424 3.49 0.66

Breakdown Field

(V/cm)3 x 105 3 x 106 .5 105 4 x 105 3 x 106 1 x 105

Thermal

Conductivity at

300 K (W/cm.K)

1.5 4.5 0.7 0.5 >1.5 0.46

Dielectric

Constant

11.9 10 12.5 12.8 9 16.0

Substrate resi-

stance (ohm.cm)1-20 1-20 >1000 >1000 >1000 1-20

Crystal Structure Diamond Wur t z i t e Zincblende Zincblende Wur t z i t e Diamond

Number of

transistors>1 bilion <200 <500 <1000 <50 -

Transistors

MOSFET,

BJT,

HBT

MESFET

HEMT

MESFET

HEMT

HBT

MESFET

HEMT

HBT

MESFET

HEMT

MOSFET,

BJT,HBT

Cost prototype / high/low Very High/very Low/high Very high/low



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mass prototype high/- high high/-

Ecological

compatibilitygood good Bad bad bad bad

Lattice Constant(Å)

5.43095 3.086 5.8686 5.6533 3.1 5.64613

Table.2.5. Comparisons of compound semiconductors [12]

2.12 Comparisons of MOSFET and HEMT:

MOSFET(NMOS) HEMT

Substrate is p-type silicon Substrate is a semi insulated GaAsGate is a polysilicon Gate is a schoktty barrier

SiO2 is insulating layer No insulating

it is a metal oxide semiconductor device It is a metal semiconductor device

Capacitance is less than HEMT for same

thickness because dielectric constant is

less

Capacitance is more than MOSFET for

same thickness because dielectric

constant is more

Drain current

I = ( V − V)

Drain current

I = ( V − V)

Transconductance

g =µ ( V − )

Transconductance

g =µ ( V − V)

Mobility is 5 times less than HEMT Mobility is 5times more than MOSFET

Drain current and gm are low Drain current and gm are high

More parasitic Less parasitic

Table.2.6. MOSFET and HEMT differences



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2.13 Applications:

The HEMT was originally developed for high speed applications. It was only

when the first devices were fabricated that it was discovered they exhibited a very

low noise figure. This is related to the nature of the 2DEG and the fact that there are

less electron collisions. As a result of their noise performance they are widely used in

low noise small signal amplifiers [13], power amplifiers, oscillators [14] and mixers

operating at frequencies up to 60 GHz and more and it are anticipated that ultimately

devices will be widely available for frequencies up to about 100 GHz. In fact HEMT

devices are used in a wide range of RF design applications including cellular

telecommunications, direct broadcast receivers - DBS, radar, radio astronomy, and

any RF design application that requires a combination of low noise and very high

frequency performance. HEMTs are manufactured by many semiconductor devicemanufacturers around the globe. They may be in the form of discrete transistors, but

now a days they are more usually incorporated into integrated circuits. These

Monolithic Microwave Integrated Circuit chips (MMIC) are widely used for RF

design applications, and HEMT based MMICs are widely used to provide the

required level of performance in many areas.

The GaAs-based PHEMTs are used extensively for solid state power amplifier

[15] applications at microwave and millimeter-wave frequencies.



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CHAPTER 3

HEMT PROCESS AND DEVICE SIMULATIONS

3.1 SYNOPSYS TCAD Simulations:

In this section, brief overviews on the process and device simulations using

TCAD software from SYNOPSYS® are presented. TPROCESS or TSUPREM-4 used

for process simulation and MEDICI is used for device simulations are widely used to

develop and optimize the semiconductor processes. Both MEDICI and TPROCESS

are physical based simulators offering predictive capability and insight into the

viability of a device/process. Physical simulations are advantageous, because they are

faster and cheaper. They are

3.2 Process Simulation: [16]

Process simulation is a sequence of commands which describes the necessary

process steps to be performed in a given sequence to generate a given structure. For

example, in a HEMT, these are different semiconductor layers which are deposited

epitaxilly. The process sequence starts with initialization of a substrate of required

thickness, orientation and resistivity/conductivity along with the doping type. As

GaAs is the substrate which is commonly used, 50nm thick GaAs substrate doped

with selenium is initiated. Selenium is an n-type dopant in GaAs [17] and the doping

concentration is 102 /cm3. On top of the substrate, InGaAs layer of 10nm in thickness

is epitaxially deposited. This layer is p-type doped with Zinc [17] at a doping

concentration of 102 /cm3 which works as channel. Next an n-type AlGaAs layer

which works as spacer is epitaxially deposited. The thickness of the layer is 10nm and

doping concentration is 102 /cm3. Above the spacer 25nm thick highly doped AlGaAs

layer is deposited. The doping concentration is 1018 /cm3. This layer source electron in

to the potential well which is formed in the channel region. p-type InGaAs layer is

used to stop electron reaching the ground terminal, then stopping the leakage of

carriers from the potential well. Finally Two GaAs layers are used as source/drain

formed by implantation on the right and left side top of the AlGaAs layer. Si3N4 is a



HEMT Simulations


cap layer for implantation. Source and drain implants have a dose of 2×1020 /cm2 and

energy 15keV. After source or drain implantation, metallization and contact

formation finalizes the process. The ohmic contacts source and drain can be identified

by a rough metal/semiconductor interface or Schottky contact to the AlGaAs barrier.

Materials, type of doping, type of dopants, thickness and concentrations of various

layers are tabulated in table.3.1. The process simulated structure is shown in Fig.3.1.

Fig.3.1 Structure of HEMT



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Region Material typeDopant

name

Thickness

(nm)

Concentration

(/cm3)

Substrate GaAs n Selenium 50 10

Channel InGaAs p Zinc 10 10

Spacer AlGaAs n Selenium 10 10

Source of

electronsAlGaAs n Selenium 25 1018

Source/drain

regionsGaAs n Selenium 25

2x10 /cm ,

15keV

Table.3.1 Specifications of HEMT process simulations

3.3 Device Simulations:

The process simulated structure is then exported in to device simulator

MEDICI. The exported file consists region information, dopant information, contact

information and structure information of the process simulated structure. To

successfully simulate the electrical characteristics, one has to consider and choose

appropriate model. The following subsection discusses various models.

3.3.1 Mobility Models [18]

Carrier mobility in estimating the currents play an important role. There arevarious mobility models which can be used in simulating the currents. The following

mobility models are considered in this study.

1. Constant mobility

2. Concentration dependent mobility

3. Analytical mobility

4. Arora mobility

5. Carrier-carrier mobility

3.3.1.1 Constant Mobility:

In the constant mobility model, mobility values are constant with respect to

doping concentration, temperature etc. Electron mobility for InGaAs layer (µ on) is

2.73x104 cm2 /V.sec and hole mobility for InGaAs layer (µ op) is 480 cm2 /V.sec.



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We know that the mobility values changes when doping comes and

temperature are increased, the constant mobility model is not considered in the device

simulation.

3.3.1.2 Concentration Dependent Mobility:

In concentration dependent mobility model, the mobilities depend upon the

local total impurity concentrationN ( x , y ) . The mobility values at a given

temperature (generally room temperature) are modeled byμ = μ( N ( x , y ) ) [18] (3.1)

μ = μ( N ( x , y ) ) [18] (3.2)

3.3.3 Analytical Mobility:

Analytical mobility of an electron and hole are modeled by eqn. (3.3) and eqn.

(3.4)

μ = M UN. M IN +. .

(3.3)

μ = M UP. M IN + .

.

(3.4)

Where the empherical value of Ntotal is 2x1020 /cm3 in eqn. (3.3) and eqn. (3.4) for

Different parameters for GaAs, AlGaAs and InGaAs are listed in Table.3.2. [18]



HEMT Simulations


Parameter GaAs AlGaAs InGaAs

MUN.MIN (cm /V.sec) 0 2.37 x10 4x10

MUN.MAX (cm /V.sec) 8.5x10 9.89 x10 2.73x104

MUP.MIN(cm /V.sec) 0 0 0

MUP.MAX(cm /V.sec) 400 400 480

NUN -1 0 0

NUP -2.1 0 0

XIN 0 0 0

XIP 0 0 0

NREFN(/cm ) 1.69x10 3.63x10 3.63x10

NREFN2(/cm ) 10 1.75x10 1.75x10

NREFP(/cm ) 2.75x101 2.75x101 10

NREFP2(/cm ) 10 10 10

ALPHAN 0.436 1 1

ALPHAP 0.395 0.395 1

Table.3.2 Analytical mobility parameters

By substitution various values, eqn. (3.3) and eqn. (3.4) can be re written as

μ = M UN. M IN + .. (3.5)

μ = M UP. M IN +.

(3.6)

3.3.4 Arora Mobility Model:

Using Arora mobility model, an electron and hole mobilities of an electron

and hole are given by

μ = M UN 1. AR0 . + . .

(,)

. . [18] (3.7)



HEMT Simulations


μ = M UP1 . AR0 .+

. .

(,)

. . [18] (3.8)

With α = A N. A RORA T

300. (3.9)

α = A P. A RORA T

300.

(3.10)

This model considers both temperature and doping concentration dependencies on

mobility. Different parameters are used in eqn. (3.7) to eqn. (3.10) for various

materials are listed in Table.3.3. [18]

Parameter GaAs AlGaAs InGaAsMUN1.ARO(cm /V.sec) 8.5x10 9.69x10 2.73x104

MUP1.ARO(cm /V.sec) 400 400 480

MUN2.ARO(cm /V.sec) 0 0 0

MUP2.ARO(cm /V.sec) 0 0 0

EXN1.ARO -0.57 0 0

EXP1.ARO 0 0 0

EXN2.ARO 0 0 0

EXP2.ARO 0 0 0

AN.ARORA 1 1 1

AP.ARORA 1 1 1

EXN3.ARO 0 0 0

EXP3.ARO 0 0 0

CN.ARORA(/cm ) 1.26x101 10 10

CP.ARORA(/cm ) 2.35x101 10 10

EXN4.ARO 0 0 0

EXP4.ARO 0 0 0

Table.3.3 Arora mobility parameters



HEMT Simulations


Values in to eqn. (3.7) to eqn. (3.10) the mobility model result isμ = M UN 1. AR0 (3.11)

μ = M UP1 . AR0 (3.12)

Then from eq. (3.11) and eqn. (3.12) it can be observed that arora mobility

model transform in to constant mobility model for all temperature and doping

concentration. Then arora mobility model is not considered for device simulations.

3.3.5 Carrier-carrier mobility

The carrier-carrier consideration mobility model also takes carrier-carrier

scattering effects, in to this is based on work by Dorkel and Leturcq [19]. While

estimating mobility values the carrier-carrier scattering effects are important whenhigh concentrations of electrons and holes are present in a device. The model also

takes into account the effects of lattice scattering and ionized impurity scattering. The

electron hole mobilities are modeled by eqn. (3.13) and eqn. (3.14) respectively.

μ = μ (.

. μμ. − C.LIC) (3.13)

μ = μ ( .. μμ. − C.LIC) (3.14)

Where the superscripts L, I and C stand for lattice scattering, ionized impurity

scattering and carrier-carrier scattering, respectively; n and p stand for electron and

hole respectively.

μ = μ +μ (3.15)

μ = μ + μ (3.16)

Eqn. (3.14) and eqn. (3.15) is Effective mobility calculated using matthiessen’s rule.

The carrier-carrier scattering mobility given by



HEMT Simulations


μ =. T

30 01. 5

(. T

3 002

() / )(3.17)

The ionized impurity scattering mobility term for electron and hole are given by eqn.

(3.17) and eqn. (3.18) respectively.

μ =. T

3 001. 5

. g[. T

30 02

] (3.18)

μ =. T

3 001. 5

. g[. T

3 002

] (3.19)

Where g( x ) = [ ln ( 1 + x) − ] (3.20)

And lattice scattering terms for electron and holes are given by eqn.

μ = M UN 0. LAT T

300−EXN.LAT

(3.21)

μ = M UP0 . LAT T

300−EXP.LAT

(3.22)

Different parameters are listed in Table.3.4



HEMT Simulations


Parameter GaAs AlGaAs InGaAs

A.LIC 1 1 1

B.LIC 0 0 0

C.LIC 0 0 0

EX.LIC 0 0 0

A.CCS (/cm ) 1.04x10 10 10

B.CCS(/cm ) 7.45x101 10 10

AN.IIS(/cm ) 2.4x10 1 10 10

BN.IIS(/cm ) 1.37x10 10 10

AP.IIS(/cm ) 5.2x10 10 10

BP.IIS(/cm ) 5.63x10 10 10

MUN0.LAT(cm /V.sec) 8.5x10 9.89x10 2.73x104

MUP0.LAT(cm /V.sec) 400 400 480

EXN.LAT 0 0 0

EXP.LAT 0 0 0

Table.3.4 Carrier-carrier mobility parameters

After simplifying eqn. (3.13), various equations from eqn. (3.14) to eqn. (3.22), the

electron and hole mobilities can be simplified in to eqn. (3.23) and eqn. (3.24) and

further simplified in to eqn. (3.27) and eqn. (3.28) respectively.μ = μ (3.23)μ = μ (3.24)

μ = M UN 0. LAT (3.25)μ = M UP0 . LAT (3.26)

μ = μ = M UN 0. LAT (3.27)

μ=

μ = M UP0 . LAT (3.28)

Then carrier-carrier mobility model is simplified in to constant mobility

model. Hence carrier-carrier mobility model is not considered for device simulations.



HEMT Simulations


In the above four mobility models, expect analytical model equation other

model gets simplified in to constant mobility model, which is constant with respect to

temperature and doping concentration. The mobility of carriers in the channel with

respect to doping can be treated as constant, as the doping levels are very low. But

when high currents are flowing and at elevated temperature, other scattering

mechanism cannot be neglected. This as analytical models depends on maximum,

minimum mobility, temperature and total concentrations, analytical mobility model

considered in this simulation work.

3.4 Device simulation results:

3.4.1 Channel electron concentration:

When a bias is applied on the gate, the metal semiconductor contact between the

spacer layer and gate metal contact is reverse biased. Then the electrons from the

electron source layer will be confined only in the channel region, as the depletion

region layer is extended fully in to the electron source layer. Thus a triangular

potential well formed. The electron concentration in the channel perpendicular to the

direction of potential well formation (i.e. electrons in various energy levels is shown

in Fig. 3.2.). Now applying suitable voltage between source and drain results in a

channel current. The contours of current flow in the device are shown in fig. 3.3.



HEMT Simulations


Fig.3.2 Channel doping and electrons and device on

Fig.3.3 Current flow when the device is on



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3.4.2 I (drain) vs. V (gate) characteristics:

In gate voltage, drain current characteristics simulation, voltage between drain

and source are fixed and the voltage between gate and source varied between -1V to

1V. When the gate voltage approach -1V, the transistor will be turned off. When gate

voltage exceeds 1V, metal semiconductor contact between the channel and gate will

be forward biased and the device looses its functionality. As this device conducts

when there is no bias applied on the gate and requires a gate bias to switch off the

device, this is a depletion mode HEMT. The gate voltage and drain current

characteristics of a HEMT with different channel length are shown in Fig.3.4. And

corresponding VT values are tabulated in Table.3.5.

Fig.3.4 Gate vs Drain characteristics

Gate length(um) OFF voltage (V) Sub VT (mV/decay)0.2 -0.2179V 107.6

0.15 -0.2582 113.810.13 -0.2790 118.2

Table.3.5 OFF voltage and Sub threshold for different Gate length



HEMT Simulations


Gate length scaling is to decrease the threshold voltage in short channel

devices due to two-dimensional electrostatic charge sharing between the gate and the

source, drain region. Short channel effect plays important role in deciding the lowest

acceptable threshold value. To scale down the effective gate length corresponding

gate controlled depletion width must also be scaled down by same factor. To reduce

depletion width one should increase the doping concentration. But increasing doping

concentration leads to higher depletion region charge in the AlGaAs layer. Due to this

potential across AlGaAs layer and hence threshold voltage rises. HEMT

psedomorphic devices have a doping concentration upper limit of 1018 cm-3 for 0.2

mole fraction. So reduction of depletion width is not possible. Halo doping in the

lateral direction gives the freedom of reduction of VT, but reduces the mobility

considerably.3.4.3 Transconductance Characteristics:

The transconductance (Gm) of a HEMT in linear and saturation region can be

using eqn. (3.29) and eqn. (3.30)

G =()()()()

(3.29)

G

=

()

(

)

(

)

(3 .30 )

Transconductance depends on gate voltage and off voltage and drain bias. Fig.3.5.

shows the Transconductance characteristics of a HEMT. Trans conductance is

defined as

G = | (3.31)



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Fig.3.5. Transconductance vs. gate voltage

3.4.4 Drain current and drain voltage characteristics:

Drain current through HEMT in linear region can be modeled as modeled

using eqn. (3.31). Here V (OFF) is the threshold voltage.

I ( D) = μC ( V( D) V( G) − V( OFF) − () ) (3.31)

And V( D) < () − () (3.32)

In Saturation region drain current is can be modeled as

I ( D) = μC V( G) − V( OFF) (3 .33 )

And V( D) > V( G) − V(OFF) (3.34)

The drain current characteristics of a HEMT with various channel lengths are

shown in Fig.3.6.



HEMT Simulations


Fig.3.6. Drain current vs. drain voltage characteristics

3.5 Comparison between HEMT and MOSFET:

A comparison study between HEMT and MOSFET for following

characteristics is made.

1. Drain current

2. Mobility

3. Electron velocity

3.5.1 Drain Current:

It can be noted HEMT when compared to MOSFET, drain current for HEMT

is more. A comparison of drain current characteristics of a HEMT and MOSFET is

plotted in Fig.3.8.



HEMT Simulations


Fig.3.8. current voltage characteristics for HEMT and MOSFET

3.5.2 Mobility:

Mobility of carriers in HEMT and MOSFET is shown in Fig.3.9. In HEMT

the mobility of carriers is 6 times more than MOSFET.

Fig.3.9. MOSFET and HEMT electron mobility Comparisons



HEMT Simulations


3.5.3 Electron velocity:

Peak saturation velocity is more for HEMT devices compare MOSFET.

Electron velocity is as shown in Fig.3.10. HEMT peak saturation velocity is 15 to 20

times higher than MOSFET. Then resulting is high speed operation of device.

Fig.3.10. MOSFET and HEMT electron velocity Compressions



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CHAPTER 4

CONCLUSION

In this work, I studied and compare the materials of compound

semiconductors. These components are useful for my device. Study the different layer

structures, process of HEMT and device simulations are generated. This work has

been demonstrated that using an easily available two-dimensional device simulation

tool (MEDICI) can be easily carried out in order to realize the target device

performance. Current, voltage characteristics, threshold voltage for different channel

lengths, transconductance and band diagrams are investigated. Finally the

comparisons of HEMT and MOSFET are discussed.



HEMT Simulations


REFERENCES

[1] R. Lee Ross, Stefan P. Svensson, Paolo Lugli, “Pseudomorphic HEMTtechnology and applications,” kluver the language of science, erice, 1994.

[2] Hans Zappe, “Fundamentals of Micro-Optics,” Cambridge university press, New

York, 2010.

[3] J. C. Bean, “Materials and Technologies in High-Speed Semiconductor Devices”,

S. M. Sze,Editor, John Wiley & sons, New York, 1990.

[4] http://www.ioffe.ru/SVA/NSM/Semicond/GaInAs/basic.html

[5] Sadao Adachi, “GaAs and Related Materials”, World Scientific Publishing Co., 1994.

[6] Kevin F. Brennan, April S. Brown, “Theory of modern electronic semiconductor

devices,” John Wiley & sons, New York, 2002.

[7] http://www.ioffe.ru/SVA/NSM/Semicond/GaAs/basic.html

[8] http://www.ioffe.ru/SVA/NSM/Semicond/AlGaAs/bandstr.html

[9] Norman Fadhil Idham M., Ahmad Ismat A.I., Rasidah S., Asban D., “Effect of

Indium Content in the Channel on the Electrical Performance of Metamorphic

High Electron Mobility Transistors”, Semiconductor Electronics, Kuala Lumpur,

Malaysia,2006.[10] Vetury, R., Wei, Y., Green, D.S., Gibb, S.R., Mercier, T.W., Leverich, K.,

Garber, P.M., “High power, high efficiency, AlGaN/GaN HEMT technology for

wireless base station applications”, Microwave Symposium Digest, USA, 2005.

[11] http://www.nptel.iitm.ac.in/video.php?subjectId=117106089.

[12] Frank Ellinger, “Radio Frequency Integrated Circuits and Technologies”, 2nd

edition, springer , 2008.

[13] Malmkvist M., Lefebvre E. , Borg M. , Desplanque L. , Wallart x., DambrineG., Bollaert S., Grahn, J., “Electrical Characterization and Small-Signal Modelingof InAs/AlSb HEMTs for Low-Noise and High-Frequency Applications,” Microwave Theory and Techniques,56, pp.2685-2691, 2008.



HEMT Simulations

[14] Kudszus, S. Haydl, W.H. Tessmann, A. Bronner, W. Schlechtweg, M.Fraunhofer Inst. for Appl. Solid State Phys., Freiburg, “Push-push oscillators for94 and 140 GHz applications using standard pseudomorphic GaAs HEMTs,” Microwave Symposium Digest , 3, 2001.

[15] “Taurus TSUPREM-Taurus Process Reference Manual” synopsis, Version X-

2005.10, October 2005.

[16] Sorab K Ghandhi,” Vlsi Fabrication Principles: Gallium Arsenide” 2nd edition John Wiley & sons, New York, 1994.

[17] “Taurus Medici Medici User Guide” synopsis, Version D-2010.03, March2010.

[18] M. Dorkel and Ph. Leturcq, “Carrier Mobilities in Silicon Semi-EmpiricallyRelated to Temperature, Doping, and Injection Level,” Solid-State Electronics,24,pp. 821-825, 1981.

[19] Nhan, E. , Sheng Cheng, Jose M.J. , Fortney S.O. , Penn J.E. , “ Recent test

results of a flight X-band solid-state power amplifier utilizing GaAs MESFET,HFET, and PHEMT technologies,” GaAs Reliability Workshop,2002.

96603724 5th Chapter Thesis 2nd

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