1. Review of Semiconductor Devices

2. Si vs. GaAs: crystal lattice
Both GaAs and Si have FCC lattice with two basis at (0,0,0) and (¼, ¼, ¼)
First brillouin zone for fcc lattice

3. Band Structure
For Si, there are six equivalent minima at 0.8X, along the (100) direction (in the reciprocal lattice).
For GaAs, secondary minimum at the L point about 0.3 eV above the  minima

4. Ingot Growth and Epitaxy
Substrate growth
Si : Czochralski
GaAs, GaP : Liquid encapsulated Czochralski with arsenic or phosphorus over-pressure
Semi-insulating properties:
Si : difficult to get semi-insulating wafers ( ~ 104 -cm)
GaAs: deep acceptors compensated wafers have  ~ 108 -cm. Recently semi-insulating wafers realized without compensation
Epitaxial growth (mainly for compound semiconductors)
MBE
CVD
Czochralski technique

5. Processing Doping Diffusion, ion implantation, and oxidation Etching
Si : As (54 meV), P (45 meV), B (45 meV)
GaAs: Si (5.8 meV), Be (28 meV), Mg (28 meV)
Diffusion, ion implantation, and oxidation
Standard for Si
Difficult for compound semiconductors
Etching
Si: very anisotropic etch
GaAs: fairly isotropic

6. Mobility and velocity-field plots
For Si:
2107 cm2V-1s-1
107 cm2V-1s-1
107
Carrier drift velocity (cm/s)
~ 2, for electrons
~ 1, for holes
106
For GaAs:
105
103
104
105
Electric field (V/cm)
vs = 1x107 cm/s for GaAs
Room temperature low field mobility of GaAs is 8500 and of Si is 1500 cm2V-1s-1 (for undoped samples).

7. Transferred Electron Effects
Oscillations due to TE effect I
GaAs sample
Vdc
Pulse width
Time
Electrons get transferred from the (0,0,0) minima at  point to a higher minima at L point along the 111 direction having mobility of 100 cm2V-1s-1

8. MOSFET G S D n+ n+ p 0,Si > 0,SiO2 Ec EF Ev qVg qVg
Vg < 0 Accumulation
Vg > 0 Depletion/Inversion
Vg = 0
Flat band

9. Gate Contacts and Current
Thermionic emission
(Reverse bias sat. current)
Direct (MOS)
Fowler-Nordheim (MOS)
qB
qB
qVg
qVg
qVg
A* = effective Richardson’s constant
Vg > 0, d > 4.5 nm
Vg > 0, d < 4.5 nm

10. MOSFETs vs. MESFETs
Both MOSFETs and MESFETs are homo-structure devices
MOSFET is enhancement mode, while MESFET is depletion mode device
MOSFET uses MOS junction to control channel current while MESFET uses Schottky junction
MOSFET gate leakage can be very low while MESFET gate leakages can be considerable
MESFETs are usually much faster than MOSFETs
Difficult to realize complimentary devices for GaAs because of very low mobility of p-type devices and lack of MOS type junctions
Inversion difficult to achieve in GaAs base devices due to absence of good insulator and high surface trap density
GaAs based devices are radiation hard

11. HEMTs vs. HBTs AlGaAs/GaAs HEMT n+ AlGaAs/p-GaAs/n- GaAs HBT Emitter
Vn
Vp
Emitter
(n+ AlGaAs)
Base
(p-GaAs)
Gate
metal
AlGaAs
donor
layer
GaAs buffer
Collector
(n-GaAs)
AlGaAs
spacer
Due to separation of the electrons from the parent donors the mobility is higher
Due to wider emitter bandgap, the base current is smaller and gain is higher

12. Integration Issues
Difficult to make a Si device based MMIC due to parasitic capacitances due to lack of semi-insulating substrate (lower bandgap)
MMICs are usually made of depletion mode compound semiconductor devices (higher bandgap, semi-insulating substrate)
Difficult to have very high integration with GaAs due to unavailibility of complementary devices and higher leakages
Non-planar devices such as HBTs are quite difficult to integrate at a very large scale

13. Detectors, Emitters, Memory Devices
Photo-detectors such as avalanche and p-i-n photodiodes can be made of both Si and GaAs depending on the applications
Solar cells are made primarily of Si due to lower cost and smaller bandgap
LEDs and Lasers are made primarily from III-arsenide and III-phosphide compounds (also recently III-nitrides)
Floating gate devices are usually made of Si and Si based insulators. Typical examples are floating gate device (metal-insulator-oxide-semiconductor) are flash memories

14. Common Semiconductors
 Properties
 Si
(----)
 GaAs
(AlGaAs/
InGaAs)
 InP
(InAlAs/
 4H- SiC
 GaN
(AlGaN/
GaN)
 Bandgap (eV)
1.11
1.42
1.35
3.26
3.42 
e (cm2/Vs)
1500
8500
(10000)
5400
700
900
(2000)
Vsat
( 107 cm/s) 1
(2.1)
(2.3) 2
1.5
(2.7)
2DEG density
(cm-2) NA
< 41012
1-2 1013
EB (106 V/cm)
0.3
0.4
0.5
3.3
Dielectric constant
11.8
12.8
12.5 10 9

15. Wide Bandgap Applications
GaN is projected to be a $3 billion industry by 2007

16. Figures of merit for high frequency/high power devices
Semiconductor
Electron mobility (cm2/Vsec)
Relative permittivity e
Bandgap Eg (eV)
BFOM Ratio
JFM Ratio
Tmax (°C) Si
1300
11.4
1.1
1.0
300
GaAs
5000
13.1
1.4
9.6
3.5
SiC
260
9.7
2.9
3.1 60
600
GaN
1500
9.5
3.4
24.6 80
700
BFOM: Baliga’s figure-of-merit
JFM: Johnson’s figure-of-merit 2 ce m v E
Figure of Merit o s B
CFOM = ( ) 2 ce m v E o s B
silicon
c is thermal conductivity
EB is breakdown field
m is low field mobility
vs is saturation velocity
eo is dielectric constant

17. Nitride Advantages for Electronic Devices
Properties
Advantages
High mobility
High saturation velocity
High sheet carrier concentration
High breakdown field
High microwave power,
Power electronic devices
Wide bandgap ( )
Growth on SiC substrate
High temperature operation
Chemical inertness
Good ohmic contacts
No micropipes
Holds promise for reliable
device fabrication
Insulated Gate transistors possible
SiO2/AlGaN and SiO2/GaN good quality interfaces

18. Power Densities for GaN and SiC Microwave Devices
Output power density (W/mm)
Highest reported value of power density of 12 W/mm at DRC, 2003

19. Size Reduction with Same Output Power