The analysis of 4H-SiC as compared to Si as a semiconductor material for power MOSFET has been shown in favor of 4H-SiC due to its superior material properties. An improvement of two-order magnitude in the specific on resistance of ideal 4H-SiC MOSFET over ideal Si MOSFET is projected. However, a review of the state of the art of SiC power MOSFETs indicates that the performance progress of SiC power devices have been hampered by MOS interface related issues which resulted in high channel resistance and oxide breakdown. Numerical device simulation-based optimization efforts for this novel device have been performed which resulted in optimum device with blocking voltage more than 1.2kV.
Parameter extraction for numerical device simulation of 4H-SiC unipolar devices is the first major topic developed in this thesis. Using 2D numerical device simulation MEDICI , in the models describing electronic devices, the material parameters of Si are replaced by respective parameters of 4H-SiC reported in literature. Using the MEDICI two-dimension simulator, with already existing models to design and optimization of 4H-SiC MOSFETs.
As the main objective of the MOS device research is to bring the channel mobilities in SiC MOS devices as high as the bulk mobilities in SiC and improve reliability of SiO2 layer.
The future generation of SiC MOS based devices should the necessary quality dielectric layers and low defect between dielectric and semiconductor interface.
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Fig.1.1 Applications for power devices in relation to their voltage and current ratings
Fig.1.2 Lateral n-channel MOSFETs cross section.
Fig. 1.3 The cross section of various power MOSFET structure
Fig.1.4 The operation of power DMOS
Fig. 2.1 Low-field electron mobility as a function of doping concentration in 4H-SiC (perpendicular to the c-axis, T = 300 K).
Fig. 2.2 Drift velocity of electron in 4H-SiC as functions of the applied field.
Fig. 2.3 Mobility between the experimental data and simulation with the parameter values listed in Table 2-2
Fig. 2.4 Temperature dependence of the coefficients ap and bp for 4H-SiC
R
N+Rc R
AR
JR
DR
SGate
N+
P-base
N-Drift Region
N+ Substrate Source
DRAIN
Fig. 3.1 A cross section of a power DMOS MOSFETs structure
n+
Gate
P-well
n-n+
Drain Source
Fig. 4.1 The cross section of trench MOSFET
Fig. 4.2 The cross section of accumulation DMOS
0 1 2 3 4 5 6 7 8 6
8 10 12 14 16 18 20
Ron( ohm ic )
P-base spacing(10
-6m)
Fig. 4.3 The relationship between P-base spacing (LP) and Ron
Fig. 4.4 The cross section of Innovative SiC ACCUFET
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1000
1100 1200 1300 1400 1500 1600 1700 1800 1900 2000
M a x imum Bloc k ing Volt age(V)
Peak Trench Concentration( 10
17cm
-3) NA=1.1x10 17 cm -3
Fig. 4.5(a) Effect of peak trench-region concentration on maximum operating voltage.
1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 2
3 4 5 6 7 8 9 10 11 12
Ron(ohm ic )
Peak Trench Concentration (10
17cm
-3) N
A=1.1x10
17cm
-3Fig. 4.5(b) Effect of peak trench-region concentration on Ron resistance
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 3.0
3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
BF OM ( 10
5V
2/ohm ic )
Peak Trench Concentration( 10
17cm
-3) N
A=1.1x10
17cm
-3Fig. 4.5(c) Effect of peak trench-region concentration on BFOM
1.5 1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 800
900 1000 1100 1200 1300 1400 1500
M ax im un Bl oc k ing Volt age (V)
Peak trench concentration ( 10
17cm
-3) NA=1.6x10 17
cm -3
Fig. 4.6(a) Effect of peak trench-region concentration on maximum operating voltage.
1.6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4 2.0
2.5 3.0 3.5 4.0 4.5 5.0
Ron( ohm ic )
Peak Trench Concentration( 10
17cm
-3) N
A=1.6x10
17cm
-3Fig. 4.6(b) Effect of peak trench-region concentration on Ron resistance.
1.6 1.7 1.8 1.9 2.0 2.1 4.0
4.4 4.8 5.2 5.6 6.0 6.4 6.8
BFOM( 105 V2 /ohmic )
Peak trench concentration( 10
17cm
-3)
NA=1.6x10
17cm
-3Fig. 4.6(c) Effect of peak trench-region concentration on BFOM.
1.2 1.4 1.6 1.8 2.0 2.2 2.4 800
1000 1200 1400 1600 1800
M ax im um Blo c k ing Vol tag e(V)
P-base Thickness (10
-6m)
Fig. 4.7(a) Effects of P-base thickness on blocking voltage.
1.2 1.4 1.6 1.8 2.0 2.2 2.4 1.5
2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0
Ron( ohm ic )
P-base Thickness (10
-6m)
Fig. 4.7(b) Effects of P-base thickness on Ron resistance.
1.2 1.5 1.8 2.1 2.4 3.0
3.5 4.0 4.5 5.0 5.5 6.0
BF OM ( 10
5V
2/ohm ic )
P-base Thickness(10
-6m)
Fig. 4.7(c) Effects of P-base thickness on Ron resistance.
Table 1.1 Comparison of Si and SiC material properties
Table 2.1 Parameter of low field model for 4H-SiC at 300K
Parameter 4H-SiC
MUN.MIN 40cm
2/Vs
MUN.MAX 950 cm
2/Vs
NRFEN 2x10
17cm
-3ALPHAN 0.76
Table 2.2 Parameters for 4H-SiC MOSFET mobility models.
parameter Unit 4H-SiC
μmincm
2/Vs 40
μmax
cm
2/Vs 950 N
refcm
-32x10
17α
0.76
B
1.0x10
6C
1.74x10
5α
l 0.0516
D
V/s 5.82x10
14Table 3.1 Values of doping concentration, electron mobility, drift layer thickness, and specific on-resistance as a function of breakdown voltage for ideal 4H-SiC and Si power MOSFET at room temperature condition.
Breakdown
Table 4.1 Oxide electric field in the trench corner as a functions of trench width, for rectangular and round corner.
Trench width(μm) 4 8 12
Oxide electric field (
10 6
V/cm)Rectangular corner
7.8 7.1 6.4
Oxide electric field (
10 6
V/cm) Rounded corner6.9 6.3 5.8
Table 4.2 The dependence of the oxide electric field at the trench corner of the oxide
thickness.
Oxide thickness(μm) 0.1 0.2 0.3
E oxide
(10 6
V/cm)Trench width
(8
μm)
6.3 5.7 4.9
E oxide
(10 6
V/cm)Trench width (12
μm)
5.8 5.1 4.3
Table 4.3 Comparison of ACCUFET structure