1. MOSFET Device Simulation
MOSFET Device Structure
Semiconductor Equations
Poisson Equation:
Electron current continuity equation:
Hole current continuity equation:
Electron current equation:
Hole current equation:

2. Simulation Methodology
Set up the device dimensions, material properties, temperature, bias voltages, doping profile, etc.
Newton’s Method for better accuracy Y
Converged? N
Current Continuity?
Discretization of the semiconductor equations
Iterative Gummel Block Method. Solve for f, n, p N Y
Extract f, electron and hole concentrations, mobility, current density, IV characteristics, etc.
Initial Guess for f, fn and fp

3. Mobility Models Low field mobility: High field mobility: Oxide
Bulk
Electron Flow
Electron
Surface Phonon
Surface Roughness
Trap
Fixed Charge
Matthiessen's rule
mLF = Low Field Mobility mB = Bulk Mobility
mSP = Surface Phonon Mobility
mSR = Surface Roughness mobility
mC = Trapped interface charge mobility
Low field mobility:
High field mobility:
High Field Mobility:

4. Caughey – Thomas Model for bulk mobility:
Temperature dependence:
Doping dependence:
Surface Phonon Mobility:
tac = Surface acoustic phonon relaxation time
E┴ = Perpendicular E. Field
an, bn = calculated from phonon scattering equation

5. Surface Roughness Mobility:
rSR = Surface roughness parameter.
Higher the value of rSR, smoother is the surface and lesser is the degradation in total mobility
Interface Trap Charge Mobility:
Corresponds to effect of coulomb scattering of mobile charged carriers by fixed charge and interface trap charge. The term also accounts for the screening of these charges by electrons at strong inversion.
nf = Fixed oxide charge
ne = Inversion layer electron concentration
screen_fit, screen_factor = fitting parameters for the screening effect
Nit = Occupied interface trap density
atemp = Temperature dependence
Git = from Coulomb Scattering model

6. 4H SiC 200mm x 200mm MOSFET: Id-Vgs Simulation Fit at T=27oC

7. 4H SiC 200mm x 200mm MOSFET: Id-Vds Simulation Fit at T=27oC

8. Bulk Mobility …. Bulk mobility at Room Temperature and D ~ 1015 is
Parameter
6H SiC
4H SiC
mn0 in cm2/Vs
500.0
1071.0
mnmin in cm2/Vs
0.0
5.0 an
2.4
2.5
Nref
1.1e18
1.9e17 gn
0.45
0.40
Bulk mobility at Room Temperature and D ~ 1015 is
4H SiC: ~ 800 cm2/Vs
6H SiC: ~ 400 cm2/Vs

9. Surface Phonon Mobility ….
Units 6H 4H
m1, m2, m3 -
0.22, 0.90, 1.43
0.29, 0.58, 0.33 m┴
0.44
0.41 m║
1.43
0.33 mc
0.35
0.39 m* ZA eV
17.5
15.0
rbulk
gm/cm3
3.2 an
(cm/s)-1
2.99e-9
2.29e-9 bn
(V/cm)-2/3K
0.1217
0.1246

10. Surface Roughness Mobility ….
Parameter 6H 4H
rSR (V/s)
1e13
5.82e14
4H rSR Value is taken from Linewih (2002) paper
Effect of surface roughness is negligible as compared to the effect of interface traps on the total mobility.

11. Interface Trap Charge Mobility ….nf
5.4 x 1011
2.2 x 1012
Nit at RmT
~ 2 x 1012
~3 x 1012
Git
1.5 x 1011
screen_fit
1.5 x 1018
1 x 1018
screen_factor
0.8
0.7

12. Occupied interface trap density (Nit)
Dit = Density of traps per unit energy
f(E) is the probability density function. It is directly proportional to the mobile charge concentration (ne). Hence as MOSFET goes towards stronger inversion, the occupied interface trap density increases.
4H SiC has a higher bandgap than 6H SiC (by 0.2eV). Ditedge value for 4H SiC is obtained by extrapolating the Dit-E curve for 6H SiC by 0.1eV. This gives a very high Ditedge value for 4H SiC because of the exponential relation between Dit and E near the band edge. Hence 4H SiC has much higher interface traps than 6H SiC.

13. Extrapolation of Dit-E curve for 6H SiC to get Dit-E characteristics for 4H SiC
Ditmid (cm-2eV-1)
1 x 1013
2.19 x 1013
Ditedge (cm-2eV-1)
8 x 1011
Dit_edge = 2.15 x 1013 cm-2eV-1
Dit_mid = 6.5 x 1011 cm-2eV-1
Final Dit-E curve for 4H that is used:

14. Nit vs. position for different Vgs. T=27oC
Occupied interface trap density increases with increase in Vgs. This is because the inversion layer electron concentration increases with increase in Vgs causing more traps to get filled
Device: 4H SiC MOSFET W/L: 200 mm / 200 mm Bias: Vgs = 2 to 4V Vds = 4V

15. Nit vs. position for different Temperatures
Occupied interface trap density decreases with increase in temperature because trapped electrons can escape by gaining sufficient energy at higher temperatures.
So as the temperature increases, effect of interface trap charge decreases, increasing overall mobility
Device: 4H SiC MOSFET W/L: 200 mm / 200 mm Bias: Vgs = 6V Vds = 1V

16. Comparing effects of Surface Roughness and Interface traps at different Temperatures
The change in Id values for a tenfold improvement of the surface roughness factor, is very small at all three temperatures. Thus surface roughness does not change the current with change in temperature.
The increase in current with temperature is caused by the reduction of filled interface trap density as temperature increases.
Device: 4H SiC MOSFET W/L: 200 mm / 200 mm Bias: Vgs = 6V Vds = 0-8V

17. Future Work…
Better screening model based on Brooks-Herring ionized impurity scattering model
Surface roughness calculation to get proper value for rSR
Fitting data at higher temperatures
High power MOSFET simulation
Investigating gate leakage in SiC MOSFETs
Building a Graphical User Interface for the simulator