CONTENTS

• INTRODUCTION

• CRYSTAL STRUCTURE AND POLYTYPISM OF SiC

• PROPERTIES OF WBG SEMICONDUCTORS

• HIGH ELECTRIC BREAKDOWN FIELD

• HIGH SATURATED DRIFT VELOCITY

• HIGH THERMAL STABILITY

• COMPARISON OF COMMERCIAL SiC SCHOTTKY DIODES WITH Si PN

DIODES

• SYSTEM LEVEL BENEFITS

• APPLICATIONS OF SiC

• COMMERCIAL AVAILABILITY

• FORECASTING THE FUTURE

• REFFERENCES

INTRODUCTION

The present Si technology is reaching the material’s theoretical limits and can not

meet all the requirements of the transportation industries. New semiconductor

materials called wide band gap(WBG) semiconductors, such as Silicon

Carbide(SiC),Gallium Nitride(GaN) and Diamond are the possible materials for

replacing Silicon in transportation application.

SiC is a perfect material between silicon and diamond.

The crystal lattice of SiC is exactly similar to silicon and diamond, but exactly half

the lattice sites are filled by silicon atoms and remaining lattice sites by Carbon

atoms. Like diamond SiC has electronic properties better properties to silicon.

WHY NOT SILICON?

• Thermal stability of Si is lower than WBG semiconductors. The maximum

junction temperature limit for most Si electronics is 150ºC.

• Conduction and switching loss is more than WBG semiconductors.

• Lower breakdown voltage than WBG semiconductors.

• Lower saturation drift velocity than WBG semiconductors.

WHY WBG SEMICONDUCTORS ?

Increasing the effectiveness of Si to meet the needs of the transportation industry is notviable because it has reached its theoretical limits. Some of the advantages comparedwith Si based power devices are as follows:

• WBG semiconductor-based unipolar devices are thinner and have lower on-resistance.Lower Ron also means lower conduction losses; higher overall converter efficiency isattainable.

• WBG semiconductor-based power devices have higher breakdown voltages because oftheir higher electric breakdown field; thus, while silicon schottky diodes arecommercially available typically at voltages lower than 300V, the first commercial SiCschottky diodes are already rated at 600V.

• WBG semiconductor-based power devices can operate at high temperatures. Theliterature notes operation of SiC devices up to 600°C.On the other hand, Si devices canoperate at a maximum junction temperature of only 150°C.

• Forward and Reverse characteristics of WBG semiconductor-based power devices varyonly slightly with temperature and time; therefore, they are more reliable.

CRYSTAL STRUCTURE AND

POLYTYPISM OF SiC

The SiC’s crystalline structure and its polytypic nature influence of polytypism onthe physical properties of SiC. SiC is a binary compound containing equal amountof ‘Si’ and ‘C’, where Si-C bonds are nearly covalent with an ionic contribution of12% (Si positively, ‘C’ negatively charged). The smallest building element of anySiC lattice is a tetrahedron of a Si (C) atom surrounded by four C(Si) atoms instrong SP3-bonds. Therefore, the first neighbour shell configuration is identical forall atoms in any crystalline structure of SiC. The basic elements of SiC crystals areshown in figure:

Basic elements of SiC crystals: Tetrahedrons containing a) one C and four Si b) one Siand four C atoms

PROPERTIES OF WBG

SEMICONDUCTORS

WBG materials have superior electrical characteristics compared with Si.

• High electric breakdown field

• High saturation drift velocity

• High thermal stability

• Superior physical and chemical stability

Comparison of semiconductor characteristics are shown in table:

Property Si GaAs 6H-SiC 4H-SiC GaN Diamond

Bandgap, Eg (eV) 1.12 1.43 3.03 3.26 3.45 5.45

aDielectric constant, εr 11.9 13.1 9.66 10.1 9 5.5

Electric breakdown field,Ec(kV/cm)

300 400 2,500 2,200 2,000 10,000

Thermal conductivity, λ(W/cm⋅K)

Saturated electron drift velocity, vsat

(×107 cm/s)

1.5 0.46 4.9 4.9 1.3 22

1 1 2 2 2.2 2.7

Ec

Eg

Forbidden Band

Ev

.

Simplified energy band diagram of a semiconductor

7

Valence Band

Conduction Band

HIGH ELECTRIC BREAKDOWN FIELD

Vb≈ ᵋ2

Ec

2qNd

VbSi

≈ 2.96 X 1017

Nd

Vb

4H-SiC≈

135 X 1017

Nd

Vb

6H-SiC≈

166.7 X 1017

Nd

Vb

Vb

GaN

diamond

≈

≈

99.4 X 10

1519.2 X 10

Nd

Nd

17

17

a)Width of the drift region for each material

at different breakdown voltages

W(Vb) ≈2 Vb

Ec

Wd =Si

Wd =

Wd =

Wd =

Wd =

4H-SiC

6H-SiC

GaN

diamond

6.67 X10^-6 Vb

0.91 X10^-6 Vb

0.81 X10^-6 Vb

1 X10^-6 Vb

0.2 X10^-6 Vb

b) Resistance of the drift region for each material at

different break- down voltages.

R on,sp =4 Vb

2

ᵋs Ec3 µn

HIGH SATURATED DRIFT VELOCITY

• The high-frequency switching capability of a semiconductor material is directly

proportional to its drift velocity.

• The drift velocities of WBG materials are more than twice the drift velocity of

silicon; therefore, it is expected that WBG semiconductor-based power devices

could be switched at higher frequencies than their Si counterparts.

• Moreover, higher drift velocity allows charge in the depletion region of a diode to

be removed faster; therefore, the reverse recovery current of WBG semiconductor-

based diodes is smaller, and the reverse recovery time is short.

HIGH THERMAL STABILITY

Junction-to-case thermal resistance, Rth-jc, is inversely proportional to the thermal

conductivity,

Where, λ is the thermal conductivity,

d is the length

A is the cross-sectional area.

R th-jc = d

λA

COMPARISON OF COMMERCIAL SiC

SCHOTTKY DIODES WITH Si PN DIODES

1)Conduction losses

2)Switching losses

R IIDUT

+Vdc V F

-

Fig. 3.1. I-V characterization circuit.

7

6

5

4

3

2

1

01.70.6 0.8 1 1.2 1.4 1.60.5

Diode Forward Voltage, V

Experimental I-V characteristics of the Si and SiC diodes in an operating temperature range of27°C to 250°C.

Dio

de

Fo

rward

Cu

rre

nt,

A

Arrows point at the increasing temperature 27-250C

Si SiC

F

CurrentProbe

DUT

oven

Conduction Loss

0.2 1

0.18 0.9

0.16 0.8

0.14 0.7

0.12 0.6

0.1 0.5

0.08 0.4

0.06 0.3

Si0.04 0.2

0.02 0.1

0 00 50 100 150 200 250 0 50 100

150T oven, ° C

(b)

200 250

Toven, °C

(a)Variation of (a) RD and (b) VD with temperature in Si and SiC diodes.

RD,

Ω

VD

,V

SiC

Si

SiC

V SiC= 0.2785 e−0.0046 T + 0.7042 ,D

R SiC

= −0.1108 e−0.0072T + 0.2023 ,D

V Si = 0.3306 e−0.0103T+ 0.5724 ,D

R Si = 0.2136 e−0.0293T + 0.0529

.

D

where M is the modulation index and φ is the power factor angle.

Pcond,D = I .R D ( 1/8 – (1/3π) M cos φ) + I V D (1/ 2π - 1/8 M cos φ)2

iLProbe

L1Isolator

-Vdc R1

Reverse recovery loss measurement circuit.

SiC Schottky d iode

Si pn d iode

Typical reverse recovery waveforms of the Si pn and SiC Schottky diode (2 A/div.).

17

Q

idoven

Current

+vd

Voltage

D=DUTDUT

i

Switching Losses

6

5

Si4

3

2

1

01 1.5 2 2.5 3 3.5 4 4.5

Peak Forward Current, A

Peak reverse recovery values with respect to the forward current at

different operating temperatures.

Prr = f s ⋅VR ⋅ ∫ id dt .a

2.5

2.25

2

1.75

1.5

1.25

1

0.75

0.5

27, 61, 107, 151, 200, 250°C0.25

01 1.5 2 2.5 3 3.5 4 4.5

Peak Forward Current, A

Diode switching loss of Si and SiC diodes at different operating temperatures.

Pe

ak

Re

ve

rse

Reco

very

Cu

rre

nt,

A

Dio

de

Sw

itc

hin

gL

oss

,W

151°C

Si 107°C 61°C

27°C

SiC

151°C

107°C

61°C

27°C SiC

27, 61, 107, 151, 200, 250°C

b

SYSTEM LEVEL BENEFITS

The use of SiC power electronics instead of Si devices will result in system level

benefits like reduced losses, increased efficiency, and reduced size and volume.

When SiC power devices replace Si power devices, the traction drive efficiency in a

Hybrid Electric Vehicle (HEV) increases by 10 percentage points, and the heat sink

required for the drive can be reduced to one-third of the original size.

APPLICATIONS OF SiC

1)Microelectronic Applications

2)High voltage devices

3)RF power devices

4)Optoelectronics

5)Sensors

COMMERCIAL AVAILABILITY

• As of October 2003, only GaAs and SiC Schottky diodes are available for

low-power applications.

• SiC Schottky diodes are available from four manufactures at ratings up to

20A at 600V or 10A at 1200V.

• Silicon Schottky diodes are typically found at voltages less than 300V.

GaAs Schottky diodes, on the other hand, are available at rating up to 7.5 A

at 500V.

• Some companies have advertised controlled SiC switches, but none of

these are commercially available yet.

FORECASTING THE FUTURE

• WBG semiconductors have the opportunity to meet demanding power converter

requirements. While diamond has the best electrical properties, research on

applying it for high power applications is only in the preliminary stages. Its

processing problems are more difficult to solve than for any of the other materials.

• GaN and SiC power devices show similar advantages over Si power devices. GaN’s

intrinsic properties are slightly better than those of SiC; however, no pure GaN

wafers are available, and thus GaN needs to be grown on SiC wafers.

• SiC power devices technology is much more advanced than GaN technology and is

leading in research and commercialization efforts. The slight improvement GaN

provides over SiC might not be sufficient reason to use GaN instead of SiC. SiC is

the best suitable transition material for future power devices.

REFFERENCES

1) https://en.wikipedia.org/wiki/Wide-bandgap_semiconductor

2) http://web.eecs.utk.edu/~tolbert/publications/iasted_2003_wide_bandgap.pdf

3) http://web.ornl.gov/sci/ees/transportation/pdfs/WBGBroch.pdf

4) http://web.ornl.gov/~webworks/cppr/y2001/rpt/118817.pdf

5) https://www.fairchildsemi.co.kr/Assets/zSystem/documents-

archive/collateral/technicalArticle/Overview-of-Silicon-Carbide-Power-Devices.pdf

6) http://www.sciencedirect.com

10/24/2016