ELCT 871: Advanced Semiconductor Devices
Course Outline lemental vs. compound semiconductors • GaN general properties • GaN growth techniques • GaN ...
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*
High power Low power High power Low power High speed High speed Low speed High speed
High power High speed
LDMOS Si and strained Power BJT, pHEMTs, HBTs DMOS Si MOSFETs, IGBTs, Wide bandgap Thyristors etc. SiGe HBTs *Optoelectronic devices : Lasers and LEDs SiC MESFETs Nitride HEMTs
Lattice constant vs. bandgap for common semiconductors
Common semiconductors comparison Properties
Si (----)
GaAs (AlGaAs/ InGaAs)
Bandgap (eV)
1.11
µe (cm2/Vs) Vsat (× 107 cm/s)
InP (InAlAs/ InGaAs)
4H- SiC (----)
GaN (AlGaN/ GaN)
1.42
1.35
3.26
3.42
1500
8500 (10000)
5400 (10000)
700
900 (2000)
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 devices Semiconductor
Electron mobility (cm2/Vsec)
Relative permittivity ε
Bandgap Eg (eV)
BFOM Ratio
JFM Ratio
Tmax (°C)
Si
1300
11.4
1.1
1.0
1.0
300
GaAs
5000
13.1
1.4
9.6
3.5
300
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
κµ n ECR 3
JFM: Johnson’s figure-of-merit ECR vsat 2π
Figure of Merit
CFOM = 400 200 0 Si GaAs 6H- 4H- GaN SiC SiC
2 χε o µv s E CR 2 (χε o µvs ECR )silicon
χ : thermal conductivity ECR : breakdown field µ : low field mobility vs : saturation velocity εo: dielectric constant
Advantages for Nitride Electronic Devices Properties • • • •
Advantages
High mobility High saturation velocity High sheet carrier concentration High breakdown field
• Wide bandgap ( ni = N C NV exp − E • Growth on SiC substrate
G
High microwave power, Power electronic devices / 2 kT
)
High temperature operation
• Chemical inertness • Good ohmic contacts • No micropipes
Holds promise for reliable device fabrication
• SiO2/AlGaN and SiO2/GaN good quality interfaces
Insulated Gate transistors possible
Output Power density (W/mm)
Power densities for AlGaN/GaN HEMTs vs. Time 35 30
Field-plated gate used to enhance breakdown voltage
25 20 15 10 5 0 96
98
00
02
04
06
Best SiC device ~5.5 W/mm Best GaAs device ~1.2 W/mm
Years
Highest reported value of power density of 32 W/mm, EDL, April, 2004
Size reduction with same output power
Applications in power electronics Schottky metal
RON
VF
+ Low doped GaN n substrate
Ohmic metal
= (4VB 2 / ε ⋅ µ ⋅ E M 3 ) + ρs ⋅ WS + RC (per unit area)
= (nkT / e )(ln[J
∗∗
F
2
]
/ A T + nφB + RON ⋅ A ⋅ J F
)
VB = Reverse breakdown voltage EM = Maximum electric field strength at breakdown ρs = substrate resistivity Ws = thickness of substrate A = cross-sectional area Advantage: Higher VB for same Ron, and lower Ron for the same VB
Optoelectronic applications: LEDs Ti/Au
t= 1-5 µm
Ni/Au
Pd/Ag/Au p+-GaN p-GaN n-InGaN MQW Ti/Al/Ti/Au
n-GaN n+-GaN
Al2O3
100 µm
• Recent applications include high brightness white LEDs
0 10
µm
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 A. Bio-agent detection due to UV induced fluorescence
Deep UV Light
Bio organism
Spectrometer
Detector
B. Solar blind photo-detectors for missile plume detection
Solar Spectrum
Ozone layer
290 nm
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 ( or , 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