This document summarizes a study of a 4H-SiC superjunction power diode through simulation. It discusses the methodology, which involves a literature survey and simulations using semiconductor simulation software. It provides an introduction to superjunction concepts and comparisons between different materials like SiC and Si. The results section shows electric field simulations of Si diodes with and without superjunction structures. The work plan is to extend the simulations to 4H-SiC diodes and later to SiC MOSFETs.

1. MICROELECTRONICS & VLSI DESIGNMONSOON 2013

2. OBJECTIVE
 Study of 4H-SiC Superjunction power diode by
simulation
2

3. METHODOLOGY
 Literature survey
 Simulations using semiconductor simulation software
Sentaurus
3

4. Overview
 Introduction
 Breakdown voltage(BV) & Specific on-resistance(Ronsp)
 Superjunction concept
 Different material comparison
 Benefits of Silicon Carbide(SiC)
 Results
 Work plan
4

5. Introduction
 In conventional power devices, there is a well known trade-
off between specific on resistance and breakdown voltage [1]
 The idea of a superjunction has been used to improve this
relationship from power law to linear [2]
[1] C. Hu, “Optimum doping profile for minimum ohmic resistance and high breakdown
voltage,” IEEE Trans Electron Devices, Vol.ED-26, pp.243-245, Mar. 1979.
[2] Jian Chen, Weifeng Sun et al, “A Review of Superjunction Vertical Diffused MOSFET”,
IETE Technical review, Vol29, Issue1, Jan-Feb 2012.
5.2
BVRonsp
5

6. How breakdown occurs?
 BV of a power device is an important parameter
governing reverse blocking capability
 How breakdown occurs?
 Impact ionization, a multiplicative phenomenon leads
to avalanche of carriers when breakdown voltage is
reached
 BV and ND (donor concentration in the uniformly doped
n region) relation in a P+N diode is given by [3]
4/315
100.3)4( DNSiCHBV
6
[3] B.J. Baliga, “Breakdown Voltage,” in Silicon Carbide Power Devices, World Scientific Publishing,
Singapore 2005, pp. 42-43

7. Specific on resistance
 Inverse relation between Ronsp and ND in a P+N diode is
given by[3]
 A higher Ronsp adversely affects the performance of the
device by increasing conducting loss and lowering
switching speed
 In conventional power devices the ideal trade-off
between Ronsp and BV
Dn
D
onsp
Nq
W
R
5.2
BVRonsp Si limit
7
[3] B.J. Baliga, “Breakdown Voltage,” in Silicon Carbide Power Devices, World Scientific Publishing,
Singapore 2005, pp. 42-43

8. Superjunction concept
PiN diode
n p
n drift region
p-pillar
h
WW
p+
n+
n drift region
h
2W
p+
n+
8
PiN superjunction diode

9. Superjunction concept
 The drift region of superjunction device is formed of
alternate n and p semiconductor stripes
 Poisson’s equation for 1D electric field
 In a superjunction device, electric field is 2D
 For a same applied voltage, peak electric field is
reduced for a superjunction diode
E
y
Ey
y
E
x
E yx
x
E
y
E xy
9

10. Superjunction concept
 p pillar does not contribute to on-state conduction in the
on-state
 For a given breakdown voltage, a higher doped drift
region can be used and specific on resistance can be
reduced
 The relation between Ronsp and BV now becomes
 Width(W) of the p and n pillar are should be small as
compared with the height(h), so that horizontal depletion
takes place at a relatively low voltage
BVRonsp
10

11. MATERIAL PARAMETERS
11
MATERIAL 6H-
SiC
4H-
SiC
3C-
SiC
Si GaAs
Dielectric constant 9.66 9.7 9.72 11.8 13.1
Band gap(eV) at 300K 3.0 3.2 2.3 1.1 1.42
Intrinsic carrier concentration(cm-3) 10-5 10-7 10 1010 1.8*106
Mobility(μn)(cm2/Vs)
ND=1016 cm-3
par:60
per:400
par:800
per:800
750 1200 6500
Mobility(μp)(cm2/Vs)
ND=1016 cm-3
90 115 40 420 320
Breakdown field (MVcm-1)
at ND=1017 cm-3
par:3.2
per: >1
par:3.0 >1.5 0.6 0.6
Thermal conductivity(Wcm-1K-1) 3-5 3-5 3-5 1.5 0.5
[4] http://www.tf.uni-

12. Why SiC?
 Electronics benefits of SiC
 Maintain semiconductor behavior at much higher
temperature than silicon
 Intrinsic carrier concentrations are negligible, so
conductivity is controlled by intentionally introduced
dopant impurities
 Low junction reverse bias leakage currents
 Permits device operation at junction temperatures
exceeding 800°C, whereas for Si it is 300°C
12

13. Why SiC?
 Allows device to be thinner and doped heavily, which
implies decrease in blocking region resistance
 More efficient removal of heat from active device
 More efficient cooling, so cooling hardware requirement for
the device is less
 Advantages 4H-SiC
 Carrier mobility substantially higher compared with 6H SiC
 More isotropic nature compared to other polytypes
13

14. RESULTS
Pravin N. Kondekar and Hawn-Sool Oh, “Analysis of the Breakdown Voltage, the On-
Resistance, and the Charge Imbalance of a Super-Junction Power MOSFET”, Journal of
the Korean Physical Society, Vol. 44, No. 6, June 2004, pp. 1565-1570
n
7*1014
/cm3
p
7*1014
/cm3
30 μm
5μm5 μm
p+ 3*1019 /cm3
n+ 3*1019 /cm3
1 μm
1 μm
14
n
7*1014 /cm3
30 μm
10 μm
p+ 3*1019 /cm3
n+ 3*1019 /cm3
1 μm
1 μm

15. RESULTS
15

16. RESULTS
16

17. RESULTS
17
Fig 1: Electric field along Y direction at breakdown voltage(326.48 V) of Si diode

18. RESULTS
18

19. WORK PLAN
 Works completed
 Literature survey
 Started simulations in Si diodes with and without
Superjunction
 Works to be done
 3rd Semester
 4H-SiC diode simulations with and without Superjunction
 4th Semester
 Analysis will be extended to SiC VDMOSFET
19

20. 20

21. SiC polytypes
 SiC occurs in different crystal structures, called
polytypes
 Polytypes – different stacking sequence of Si-C bilayers
 All SiC polytypes chemically consists of 50% carbon
atoms covalently bonded with 50% silicon atoms
 Common polytypes 3C-SiC, 4H-SiC, 6H-SiC
 Electrical device properties are non isotropic with
respect to crystal orientation, lattice site, and surface
polarity
21

22. APPENDIX
22
 Baliga’s power law approximation for the impact
ionization coefficients for 4H-SiC for analytical
derivations
 Avalanche breakdown condition is defined by the impact
ionization rate becoming infinite
 The avalanche breakdown defined to occur when the
total number of electron-hole pairs generated within the
depletion region approaches infinity, corresponds to M
becomes infinity
742
109.3)4( ESiCHB
W x
pnp
x
pn
dxdx
dx
xM
0 0
0
)(exp1
)(exp
)(

23. APPENDIX
23
C
D
E
qWN hEV CBR
WzNq
h
R
Dn
ON
zWA )2(

24. APPENDIX
24
D
i
P
P
A
i
N
N
N
n
L
D
N
n
L
D
qAJ
22
0 ..

25. APPENDIX
25

26. Superjunction concept
 Width(W) of the p and n pillar are should be small as
compared with the height(h), so that horizontal depletion
takes place at a relatively low voltage
 n and the p pillars will be completely depleted well
before the breakdown voltage is reached
 Doping and widths of p and n pillar are chosen such a
way that breakdown happens at the p+ -n drift layer
junction
26

27. 27
 High breakdown field + High thermal conductivity + High
operational junction temperatures = High power density
and efficiency