ELCT 871: Advanced

Semiconductor Devices

Course Outline

lemental vs. compound semiconductors

• GaN general properties

• GaN growth techniques

• GaN doping and materials characterization

• Metal contacts to GaN

• Nitride based heterostructures and HFETs

• GaN optoelectronic devices: LEDs and Lasers

• SiC material properties

• SiC device applications

Semiconductor electronic devices tree

Semiconductor devices

Elemental semiconductors Compound semiconductors*

Low power

High speed

High power

Low speed

Low power

High speed

High power

High speed

SiC MESFETs

Wide bandgap

Nitride HEMTs

Si and strained

Si MOSFETs,

SiGe HBTs

Power BJT,

IGBTs,

Thyristors etc.

LDMOS

DMOS

High power

High speed

pHEMTs,

HBTs

*Optoelectronic devices : Lasers and LEDs

Lattice constant vs. bandgap for

common semiconductors

Common semiconductors comparison

Properties Si

(----)

GaAs

(AlGaAs/

InGaAs)

InP

(InAlAs/

InGaAs)

4H- SiC

(----)

GaN

(AlGaN/

GaN)

Bandgap (eV) 1.11 1.42 1.35 3.26 3.42

µe (cm2/Vs) 1500 8500

(10000)

5400

(10000)700 900

(2000)

Vsat

(× 107 cm/s)

1 1

(2.1)

1

(2.3)

2 1.5

(2.7)

2DEG density

(cm-2)

NA < 4×1012 < 4×1012 NA 1-2 ×1013

EB (106 V/cm) 0.3 0.4 0.5 2 3.3

Dielectric

constant

11.8 12.8 12.5 10 9

Wide bandgap applications

GaN is projected to be a $3 billion industry by 2007 !

Figures of merit for high frequency/high power devicesFigures of merit for high frequency/high power devicesFigures of merit for high frequency/high power devicesFigures of merit for high frequency/high power devices

7008024.63.49.51500GaN

600603.12.99.7260SiC

3003.59.61.413.15000GaAs

3001.01.01.111.41300Si

Tmax (°C)JFM

Ratio

BFOM

Ratio

BandgapEg (eV)

Relative

permittivity

ε

Electron mobility (cm2/Vsec)

Semiconductor

BFOM: Baliga’s figure-of-merit3

n CREκµ 2CR satE v π

CFOM =χε oµvsECR

2

χεoµvs ECR2( )

silicon

χ : thermal conductivityE

CR: breakdown field

µ : low field mobilityvs : saturation velocity

εo: dielectric constant

0

200

400

Si GaAs 6H-

SiC

4H-SiC

GaN

Fig

ure

of

Merit

JFM: Johnson’s figure-of-merit

Advantages for Nitride Electronic DevicesAdvantages for Nitride Electronic DevicesAdvantages for Nitride Electronic DevicesAdvantages for Nitride Electronic Devices

• High mobility

• High saturation velocity

• High sheet carrier concentration

• High breakdown field

• Wide bandgap ( )

• Growth on SiC substrate

• Chemical inertness

• Good ohmic contacts

• No micropipes

High microwave power,

Power electronic devices

High temperature operation

Holds promise for reliable

device fabrication

Properties Advantages

• SiO2/AlGaN and SiO2/GaN good

quality interfaces

Insulated Gate

transistors possible

kTE

VCiGNNn

2/exp−=

Power densities for AlGaN/GaN HEMTs vs. Time

Highest reported value of power density of 32 W/mm, EDL, April, 2004

0

5

10

15

20

25

30

35

96 98 00 02 04 06

Ou

tpu

t P

ow

er d

ensi

ty (

W/m

m)

Years

Best GaAs device

~1.2 W/mm

Best SiC device ~5.5

W/mm

Field-plated gate used to

enhance breakdown voltage

Size reduction with same output power

Applications in power electronics

Schottky metal Low doped GaN n+ substrate Ohmic metal

VB = Reverse breakdown voltage

EM = Maximum electric field strength at breakdown

ρs = substrate resistivity

Ws = thickness of substrate

A = cross-sectional area

CSMBON RWEVR +⋅+⋅⋅= µε )/4(32 (per unit area)ρs

Advantage: Higher VB for same Ron, and lower Ron for the same VB

( ) [ ]( )FONBFF JARnTAJenkTV ⋅⋅++= ∗∗ φ2/ln/

Optoelectronic applications: LEDs

Pd/Ag/Au

Al2O3

n+-GaN

n-GaN

n-InGaN MQW

p-GaN

p+-GaN

Ni/Au

Ti/Au

Ti/Al/Ti/Au

100 µm

t= 1-5 µm

100 µm

• Recent applications include high brightness white LEDs

Nitride based lasers

• Purple - Blue CW Lasers (> 104 hour lifetime easily achieved)

• Blue lasers are used for increasing storage capacities of DVDs

Bio-agent and missile plume detection

Ozone

layer

290 nm

Bio organismDetectorDeep UV Light Spectrometer

Solar Spectrum

A. Bio-agent detection due to UV induced fluorescence

B. Solar blind photo-detectors for missile plume detection

Non line-of-sight communication

• Uses UV LEDs with ~280 nm wavelength

• Due to lower wavelength the UV light is scattered very strongly by the atmosphere and particulates

• Useful in areas where the parties concerned cannot see each other, but very fast and highly reliable communication is essential

Advantages of nitride based light source

• Compact, light, inexpensive, efficient, and robust light sources can be made of III-nitrides

Gallium nitride structure

• 2 interpenetrating HCP structures of Ga and N atoms each displaced from the other by 3/8 c

• Structure is 2H type

• Atoms of only Ga or N lie on any single plane normal to the c-axis (<0001> or <000-1>, called c-planes), but not for a-planes

• Very strongly polar bond as N is the most electronegative of the Group V materials

• The crystal structure is non-centrosymmetric, i.e. lacks inversion symmetry along the c-axis

SiC polytypes

Band structures of GaN

Zinc BlendeZinc Blende

WurtziteWurtzite