Study of 4H-SiC Superjunction Diodes

MICROELECTRONICS & VLSI DESIGNMONSOON 2013

RICHU JOSE CYRIAC M120128EC

2

OBJECTIVE

Study of 4H-SiC Superjunction power diode by simulation

3

METHODOLOGY

Literature survey

Simulations using semiconductor simulation software Sentaurus

4

Overview

Introduction Breakdown voltage(BV) & Specific on-resistance(Ronsp)

Superjunction concept Different material comparison Benefits of Silicon Carbide(SiC) Results Work plan

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.

5.2BVRonsp

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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]

4/315100.3)4( DNSiCHBV

[3] B.J. Baliga, “Breakdown Voltage,” in Silicon Carbide Power Devices, World Scientific Publishing, Singapore 2005, pp. 42-43

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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

Donsp

Nq

WR

5.2BVRonsp Si limit

[3] B.J. Baliga, “Breakdown Voltage,” in Silicon Carbide Power Devices, World Scientific Publishing, Singapore 2005, pp. 42-43

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Superjunction concept

PiN diode

n

pn drift region

p-pillar

h

W

W

p+

n+

n drift region h

2W

p+

n+

PiN superjunction diode

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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

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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

MATERIAL PARAMETERS11

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:60per:400

par:800per: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-kiel.de/matwis/amat/semi_en/kap_a/illustr/ia_1_2.html .

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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

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

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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

5μm 5 μm

p+ 3*1019 /cm3

n+ 3*1019 /cm3

1 μm

1 μm

n

7*1014 /cm3

30 μm

10 μm

p+ 3*1019 /cm3

n+ 3*1019 /cm3

1 μm

1 μm

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RESULTS

16

RESULTS

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RESULTS

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

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RESULTS

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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

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THANK YOU

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

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APPENDIX

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

742109.3)4( ESiCHB

W x

pnp

x

pn

dxdx

dx

xM

0 0

0

)(exp1

)(exp

)(

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APPENDIX

CD

EqWN

hEV CBR

WzNq

hR

DnON

zWA )2(

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APPENDIX

D

i

P

P

A

i

N

N

N

n

L

D

N

n

L

DqAJ

22

0 ..

25

APPENDIX

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

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