designfeature
Siddarth Sundaresan, Director-Device Design & Fabrication, Michael Digangi, Chief Business Development Officer, and
Ranbir Singh, President, GeneSiC Semiconductor, Inc.
SiC “Super” Junction Transistors Offer
Breakthrough High Temp Performance
P
ower electronics fabricated on the Silicon Carbide (SiC)
platform can operate at higher temperatures, higher frequencies, and can delive higher circuit efficiencies as compared
to traditional Si-based technologies [1]. The SiC transistors
are especially attractive at 1.2 kV-10 kV ratings, particularly for
medium and high-frequency applications [2]. Though SiC-based
Schottky diodes were readily available since 2001[3], the commercialization of SiC power transistors has lagged behind. [4-5].
GeneSiC is developing an innovative SiC power switch, a “Super” Junction
Transistor (SJT) in 1.2 kV to 10 kV voltage ratings for high efficiency power conversion in Switched-Mode Power Supply (SMPS), Uninterruptible Power Supply
(UPS), aerospace, defense, down-hole oil drilling, geothermal, Hybrid Electric
Vehicle (HEV) and inverter applications.
The Gate-oxide free, normally-off, current driven, quasi-majority device, SJT
is a “Super-High” current gain SiC-based BJT that features a square reverse biased
safe operating area (RBSOA), high temperature (> 300 °C) operation capability,
low VDS(on) and faster switching capability (10’s of MHz) than any other competitor
SiC switch. The MOS interface reliability-related issues and high channel resistance
of SiC MOSFETs have limited their temperature capability to 150 °C where as the
Gate-oxide and channel free SiC SJTs deliver high temperature performance (>
300 °C). Unlike SiC SJT, SiC MOSFET requires a customized gate driver with +20
V VGS capability due to their transconductance resulting from poor MOS channel
mobility . characteristics. On the other hand, the commercially available normallyoff SiC JFET displays a very high positive temperature coefficient of VDS(on) and
lower temperature capability as compared to the SiC SJT.
GeneSiC’s 1200
80
V/220 mΩ SiC SJTs
25°C
are packaged in stan175°C
dard TO-220 and high
225°C
275°C
60
temperature TO-257
325°C
packages (Fig. 1.) The
following three best40
in-class Si IGBT copacks with internally
integrated anti-parallel
20
Si FREDs were chosen
for comparing their
electrical performance
with that of 1200
0
0
200
400
600
800
1,000 1,200 1,400
V/220 mΩ SiC SJT:
Blocking Voltage, VDSO (V)
• NPT1: 125 °C/1200
V rated Si Non-Punch- Fig. 1: Temperature variant blocking performance of a 1200 V/220 mΩ SJT
Drain Leakage Current, Id (μA)
GeneSiC’s SiC-based 1200
V/220 mΩ Super Junction
Transistors (SJTs) feature
high temperature (> 300 °C)
operation capability, ultrafast switching transitions
(< 15 ns), extremely low
losses, and a large shortcircuit withstand time of 22
µs. Integrating SiC SJTs with
GeneSiC’s freewheeling SiC
Schottky rectifiers provides
a 64% power loss reduction
over best-in-class silicon
counterparts.
www.powerelectronics.com November 2011 | Power Electronics Technology
DI MANGI-Fig. 1.
21
Siliconcarbidetransistors
Through IGBT
• NPT2: 150 °C/1200 V rated Si Non-Punch-Through
IGBT
•TFS: 175 °C/1200 V rated Si Trench Field Stop IGBT
On-State and blocking Performance
Open-gate, blocking voltage performance of a 1200 V/220
mΩ SJT at temperatures as high as 325 °C is shown in Fig.
1. The leakage current in the SJT at VDS = 1200 V is
below 5 µA up to temperatures as high as 225 °C. Leakage
currents of < 100 µA were measured even at 325 °C, validating the high-temperature operation of the SJT. Fig. 2.
compares the temperature-dependent leakage currents of
the three Si IGBT co-packs with the 1200 V/220 mΩ SiC
SJT. The leakage current in the SJT shows a smaller positive temperature coefficient as compared to the Si IGBTs.
It must be nosted that the 325 °C temperature capability
of the SJT is only limited by the current packaging.
The on-state characteristics of a 1200 V/220 mΩ SJT
were generated using a curve tracer for operating temperatures up to 250 °C (Fig. 3.). The distinct lack of quasi-saturation region and merging of the on state curves for various
Gate currents in the saturation region of a SJT indicate
the absence of the minority carrier injection and clearly
distinguishes it from a Si “BJT”. Appropriate metallization
schemes and an optimized device design yield low Drain
Source saturation voltages. The on-state voltage values of
SJT are relatively smaller than the existing same current/
voltage rated Si IGBTs with VDS(on) values of 1.5 V at 25
o C and 2.6 V at 125 o C at 7 A of drain current. The SJTs
display a positive temperature coefficient of VDS(on) that
facilitates paralleling of multiple devices for high current
configurations. Common source current gains as high as
88 was measured on this batch of SJTs.
20
101
Drain Current , ID (A)
Leakage Current IC/D (μA)
102
100
VCE/DS = 1200 v
SJT
TFS
NPT1
NPT2
10–1
10–2
0
50
100
150
200
250
300
Junction Temperature, Tj (°C)
10
4
2
IG
0
0
50
100
Time, t (ns)
150
Fig. 4: Turn-Off switching transients of a 1200 V/220 mΩ SJT
–2
200
10
12
DI MANGI-Fig. 3.
ID
10
800
8
600
400
IG
Rg (on) = 22 Ω
Tj = 250°C
VGS = -8/15 V
0
6
4
200
2
0
200
400
Time, t (ns)
600
0
800
Fig. 5: Turn-On switching transients of a 1200 V/220 mΩ SJT
22 Power Electronics Technology | November 2011
DI MANGI-Fig. 4.
8
1,000
Drain Source Voltage VDS (V)
Drain Source Voltage VDS (V)
6
400
0
4
6
Drain Source Voltage VDS (V)
Drain/Gate Current , ID/IG (A)
Rg (on) = 22 Ω
Tj = 250°C
VGS = -8/15 V
Drain/Source Current , ID/IG (A)
VDS
200
2
VDS
8
600
0
Fig. 3: Temperature variant output characteristics of a 1200 V/220 mΩ SJT.
ID
1,000
800
5
0
350
10
DI MANGI-Fig. 2.
15
100 mA Gate current steps
Fig. 2: Leakage current comparison of Si IGBTs and 1200 V/220 mΩ SJT as a
function of temperature.
1,200
25°C
125°C
150°C
www.powerelectronics.com
DI MANGI-Fig. 5.
temperature of 250 o C, resulting in extremely low switching energies when compared to the Si IGBT co-packs, een
though the Si devices could be measured only up to 150
oC. A comparison of the overall power losses measured on
the SJT and Si IGBT co-packs is shown in Fig. 6. Si TFS
+ SiC FWD represents Si TFS IGBT as the DUT and SiC
Schottky diode as FWD respectively where as Si TFS + Si
TFS represents Si TFS IGBT as DUT and Si TFS IGBT copack as FWD respectively. The calculated gate drive, conduction and switching losses of SJT are 5.25 W, 26.65 W
and 20 W respectively at 250 o C operating temperature.
Though the gate driver losses of SJT are higher than Si
IGBTs, their contribution to the overall losses is insignificant. The higher conduction losses of SJT when compared
to the Si IGBT co-packs in Fig. 6 is due to the fact that
the SJT was evaluated at a higher temperature (250 o C),
as compared to the IGBT’s 125 to 175°C. In spite of its
higher temperature operation, an all-SiC solution reduces
the overall losses by about 64% when compared to the
best Si IGBT solution investigated in this report.
Modeling the z-transform
difference equations in the
z-domain requires the insertion
of zero order holds at the A/D
and D/A interface.
Switching Performance
An inductively loaded chopper circuit configuration and
a standard double-pulse scheme were used for comparing the switching performance of SiC SJT and Si IGBTs
comprises of an inductively loaded chopper circuit configuration. A GeneSiC 1200 V/ 7A SiC Schottky diode [6]
and Si IGBT co-packs were used as Free Wheeling Diodes
(FWDs) in the switching test circuit. The Gate and Source
terminals of Si IGBTs were tied together (VGS = 0 V) to
avoid spurious IGBT conduction during the dynamic testing. A 1 µF charging capacitor, a 150 µH inductor, 22 Ω
Gate resistor and a supply voltage of 800 V were used in
the testing process.
A commercial standard IGBT Gate driver with an
output voltage swing from -8 V to 15 V is used for driving the Si IGBTs as well as the Si SJT. A 100 nF dynamic
capacitor connected in parallel with the Gate resistor generated an initial large dynamic Gate currents of 4 A and
-1 A (Figs. 4 and 5) during turn-on and turn-off switching
respectively, while maintaining a constant Gate current of
0.52 A during its turn-on pulse. The initial dynamic gate
currents charge/discharge the device capacitance rapidly,
yielding a superior switching performance.
A drain current rise time of about 12 ns and a fall time
of 14 ns were obtained for 7 A, 800 V SJT switching at a
short-circuit performance
When the SJT is turned on to a short circuit at a drain
voltage of 800 V with 0.2 A of gate current, a short-circuit
current (ISC) of 22 µs are obtained (Fig. 7), which is considerably higher than the 10 µs reported {5} on 1200 V/20
A SiC MOSFSETs. The factor of 2 improvement in tsc for
the SiC SJT is due to its low ISC of 13 A in comparison
with a rated current of 7 A. Short-channel effects inherent
to the SiC MOSFET reported in {5} result in an undesirably
high ISC of 180 A, as compared to its rated 20 A, which
limits the tsc to 10 µs.
Conclusions
Power Loss Comparison (W)
200
C
5°
17
C
5°
12
150
5
17
100
C
0°
25
136.4
90
50
0
C
0°
15
°C
107.02
83.386
12.74
0.105
19.6
0.0675
20
26.65
5.25
8.38
0.1125
SJT + SiC FWD
Si TFS + SiC
FWD
Driver Loss
8.33
0.1125
Si TFS + Si TFS
Conduction Loss
Si NPT1 + Si
NPT1
Si NPT2 + Si
NPT2
Switching Loss
Fig. 6: Power loss comparison of SiC SJT and Si IGBT co-packs at their maximum operating temperature.
www.powerelectronics.com DI MANGI-Fig. 6.
The SiC SJTs developed by GeneSiC
offer significant benefits over the Si
IGBTs, SiC MOSFETs and JFETs by
reducing the power losses tremendously and delivering high temperature performance respectively. These
benefits result in improving the system
efficiencies, and reducing its cost and
size. As SiC SJTs are a direct replacement to the Si IGBTs, they can be driven using the standard IGBT/MOSFET
gate drivers, unlike SiC MOSFETs and
JFETs which require specialized gate
driver architectures.
GeneSiC’s silicon carbide products
have the largest range of SiC Schottky
Diodes to date – 10 Products in 1200
V and above voltage ratings. GeneSiC
also has the largest range of packages
November 2011 | Power Electronics Technology
23
14
800
10
600
8
400
6
tsc = 22 μs
4
Drain Bias (V)
Drain Current (A)
12
200
2
0
0
0
5
10
15
Time (μs)
20
25
30
introduced the industry’s first 1200 V/ 3 A SiC Schottky
product for cost-conscious customers who want save
money utilizing this product instead of an off-the-shelf 5
Amp 1,200 Volt that is a standard in SiC industry.
The company’s Silicon Product Portfolio is easily crossreferenced to major manufacturers, with short leadtimes
of 6 to 8 weeks. All of GeneSiC’s silicon products are
RoHS and UL approved, and boost the largest range of
Schottky Diode modules. GeneSiC’s products can be purchased through its global distribution channel, or through
manufacturer representatives listed on the company’s site
at www.genesicsemi.com.
References
Fig. 7: Current and Voltage waveforms of a 1200 V/220 mΩ SJT under a shortcircuit load condition..
in 1200 V and above voltage ratings – TO-220, TO-247,
DI MANGI-Fig.
7. co-packaged IGBT
TO-263 and SOT-89. The
company’s
with SiC Diodes is currently in 100-A and 35 A/ 1200 V,
ISO-Top and TO-247 packages which are best-in-class
Hybrid Si-SiC IGBT Co packs. The company has also
[1] B.J. Baliga, “Trends in power semiconductor devices”, IEEE Trans.
Electron Devices, vol 43, pp. 1717-31, 1996.
[2] A. Hefner et al, “Recent Advances in High-Voltage, High-Frequency
Silicon-Carbide Power Devices”, in IEEE 2006 Industry Applications
Conference, 2006, pp. 330-337.
[3] I. Zverev, M. Treu, H. Kapels, O. Hellmund, R. Rupp, “SiC Schottky
Rectifiers: Performance, Reliability and Key Applications”, Proceedings of
EPE 2001 Conference, August 2001.
[4] GeneSiC Semiconductor, Inc. Available: http://genesicsemi.com/index.
php/silicon-carbide-products/schottky-rectifiers/discrete-rectifiers
[5] S.H. Ryu et. al. “Performance, Reliability, and Robustness of 4H-SiC
DMOSFETS”, Materials Science Forum (645-648), pp. 969-974 (2010) .
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