SiC FOR HIGH TEMPERATURE APPLICATIONS

SiC as a wide-band gap material for
high temperature applications
11/8/2016 1

Contents
• Introduction
• Crystal structure and polytypism of SiC
• Properties of WBG semiconductors
• Why SiC operate at higher temperature?
• High thermal stability
• Intrinsic carriers
• p-n junction leakage and thermionic leakage
• Conduction and switching loss
• System level benefits
• Applications of SiC
• Commercial availability
• Forecasting the future
• References
11/8/2016 2

Introduction
• The present Si technology is reaching the material’s theoretical limits
and can not meet all the requirements of the transportation and energy
production 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.
11/8/2016 3

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.
11/8/2016 4

Why WBG semiconductors ?
Some of the advantages of WBG semiconductors 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 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.
11/8/2016 5

Crystal structure and polytypism of SiC
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
11/8/2016 6

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
11/8/2016 7

Ec
Eg
Forbidden Band
Ev
.
Fig-Simplified energy band diagram of a semiconductor
7
Valence Band
Conduction Band
Why SiC operate at higher temperature?
11/8/2016 8

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.
As thermal conductivity of SiC is larger than Si thus
junction resistance for SiC is smaller than Si.
R th-jc =
d
λA
11/8/2016 9

Intrinsic carriers
The concentration of intrinsic carriers (ni in cm-3) is exponentially
dependent upon the temperature of the semiconductor
ni = Nc Nv exp( -Eg/2KT)
fig.: semiconductor intrinsic carrier conc(ni)
vs temperature11/8/2016 10

p-n junction leakage
The p-n junction leakage current (I) when reverse voltage is greater than
few tenths of a volt at a temperature bellow 1000 is
I ≈ -q A ni [ ni / ND DP/ τ + W/2 τ]
where , A = Area of the p -n junction (cm2),
ND⁼ n-type doping density (cm -3),
W=Width of the junction depletion region (cm),
DP = hole diffusion constant (cm2 /s),
and τ = effective minority carrier lifetime (seconds).
11/8/2016 11

Thermionic leakage
The current due to carrier emission (leakage current) as a function of
temperature and applied voltage is approximated by
I ≈ AK*T exp(-qφB/kT) [exp( qVA/kT ) -1]
where,
φB =Effective potential barrier height (i.e., Schottky barrier
height) of the junction (in eV)
K*=The effective Richardson constant of the semiconductor.
For appreciable reverse biases (VA<-0.2V), the reverse bias leakage current
calculated from (3) simplifies to
I ≈ - AK*T exp(-qφB/kT)
2
2
11/8/2016 12

R IIDUT
+
Vdc V F
-
Diode Forward Voltage, V
Fig.-Experimental I-V characteristics of the Si and SiC diodes in an
operating temperature range of 27°C to 250°C.
F
Current
Probe
DUT
oven
Conduction loss
Fig.-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
DiodeForward
Current,A
Arrows point at the
increasing temperature
27-250C
Si SiC
11/8/2016 13

iL
Probe
L1
Isolator
-
Vdc R1
Fig- Reverse recovery loss measurement circuit.
Fig.-Typical reverse recovery waveforms of the Si pn and SiC Schottky diode
Q
id
oven
Current
+
vd
Voltage
D=DUT
DUT
i
Switching loss
11/8/2016 14

6
5
Si4
3
2
1
0
1 1.5 2 2.5 3 3.5 4 4.5
Pe ak F o rw a rd Current, A
Fig.-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
Fig.-Diode switching loss of Si and SiC diodes at different operating temperatures.
Peak
Reverse
Recovery
Current,A
Diode
Switching
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
11/8/2016 15

System level benefits
• The use of SiC high temperature power electronics devices
instead of Si based 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.
11/8/2016 16

Applications of SiC
1)Microelectronic applications
2)High voltage devices
3)RF power devices
4)Optoelectronics
5)Sensors
11/8/2016 17

Commercial availability
• As of October 2003, only 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.
• Some companies have advertised controlled SiC switches, but
none of these are commercially available yet.
11/8/2016 18

Forecasting the future
• WBG semiconductors have the opportunity to meet
demanding power converter requirements. While
diamond has the best electrical properties but due to
processing and doping problem it is not used. Due to
unavailable of wafer and doping problem GaN is not
used. Thus only possible material is SiC. More
research is going on SiC for reduction the switching
loss.
11/8/2016 19

References
1) https://en.wikipedia.org/wiki/Wide-bandgap_semiconductor
2) http://web.eecs.utk.edu/~tolbert/publications/iasted_2003_wid
e_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
11/8/2016 20

SiC FOR HIGH TEMPERATURE APPLICATIONS

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