For high power requirements Wide Band Gap (WBG) semiconductors are used like SiC instead of Si.

2. 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
• COMMERCIALAVAILABILITY
• FORECASTING THE FUTURE
• REFFERENCES

3. 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.

4. 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.

5. WHY WBG SEMICONDUCTORS ?
Increasing the effectiveness of Si to meet the needs of the transportation industry is not
viable because it has reached its theoretical limits. Some of the advantages compared
with 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 is
attainable.
• WBG semiconductor-based power devices have higher breakdown voltages because of
their higher electric breakdown field; thus, while silicon schottky diodes are
commercially available typically at voltages lower than 300V, the first commercial SiC
schottky diodes are already rated at 600V.
• WBG semiconductor-based power devices can operate at high temperatures. The
literature notes operation of SiC devices up to 600°C.On the other hand, Si devices can
operate at a maximum junction temperature of only 150°C.
• Forward and Reverse characteristics of WBG semiconductor-based power devices vary
only slightly with temperature and time; therefore, they are more reliable.

6. CRYSTAL STRUCTURE AND
POLYTYPISM OF SiC
The SiC’s crystalline structure and its polytypic nature influence of polytypism on
the physical properties of SiC. SiC is a binary compound containing equal amount
of ‘Si’ and ‘C’, where Si-C bonds are nearly covalent with an ionic contribution of
12% (Si positively, ‘C’ negatively charged). The smallest building element of any
SiC lattice is a tetrahedron of a Si (C) atom surrounded by four C(Si) atoms in
strong SP3-bonds. Therefore, the first neighbour shell configuration is identical for
all atoms in any crystalline structure of SiC. The basic elements of SiC crystals are
shown in figure:
Basic elements of SiC crystals: Tetrahedrons containing a) one C and four Si b) one Si
and four C atoms

7. 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
a
Dielectric 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

8. Ec
Eg
Forbidden Band
Ev
.
Simplified energy band diagram of a semiconductor
7
Valence Band
Conduction Band

9. HIGH ELECTRIC BREAKDOWN FIELD
Vb≈ ᵋ
2
Ec
2qNd
Vb
Si
≈ 2.96 X 10
17
Nd
Vb
4H-SiC
≈
135 X 10
17
Nd
Vb
6H-SiC
≈
166.7 X 10
17
Nd
Vb
Vb
GaN
diamond
≈
≈
99.4 X 10
1519.2 X 10
Nd
Nd
17
17

10. 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

11. b) Resistance of the drift region for each material at
different break- down voltages.
R on,sp =
4 Vb
2
ᵋs Ec
3 µn

12. 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.

13. 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

14. COMPARISON OF COMMERCIAL SiC
SCHOTTKY DIODES WITH Si PN DIODES
1)Conduction losses
2)Switching losses

15. R IIDUT
+
Vdc V F
-
Fig. 3.1. I-V characterization circuit.
7
6
5
4
3
2
1
0
1.70.6 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 of
27°C to 250°C.
DiodeForward
Current,A
Arrows point at the increasing
temperature 27-250C
Si SiC
F
Current
Probe
DUT
oven
Conduction Loss

16. 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 0
0 50 100 150 200 250 0 50 100
150
T 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
RSiC
= −0.1108 e−0.0072T + 0.2023 ,D
V Si = 0.3306 e−0.0103T+ 0.5724 ,D
RSi = 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

17. iL
Probe
L1
Isolator
-
Vdc R1
Reverse recovery loss measurement circuit.
SiC Schottky
diode
Si pn
diode
Typical reverse recovery waveforms of the Si pn and SiC Schottky diode (2 A/div.).
17
Q
id
oven
Current
+
vd
Voltage
D=DUT
DUT
i
Switching Losses

18. 6
5
Si4
3
2
1
0
1 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 = fs ⋅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
0
1 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.
Peak
Reverse
Recovery
Current,A
DiodeSwitching
Loss,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

19. 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.

20. APPLICATIONS OF SiC
1)Microelectronic Applications
2)High voltage devices
3)RF power devices
4)Optoelectronics
5)Sensors

21. COMMERCIALAVAILABILITY
• 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.

22. 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.

23. 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

24. 10/24/2016