A concept that breaks the silicon limit in semiconductor power devices is implimented in SiC

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