IEEE Power Electronics Magazine - March 2015 - 12

Happenings

by J. Christopher Dries

SiC Research and Development at United
Silicon Carbide Inc.-Looking Beyond
650-1,200-V Diodes and Transistors

or over a decade now, silicon
carbide (SiC) Schottky diodes
have been used in myriad highperformance power-conversion applications. Currently, the largest among
these applications is power factor correction utilizing 650-V SiC Schottky diodes. The introduction of SiC transistor
offerings at 1,200 V, beginning in 2010
with junction field-effect transistors
(JFETs) and followed by SiC metal-oxide-semiconductor field-effect transistors (MOSFETs) and bipolar junction
transistors in subsequent years, is simply the beginning of a wave of commercial devices of increasingly high voltage
and current ratings. In this column, we
describe the direction that United Silicon Carbide Inc. (USCi) is taking in advancing the state of the art of SiC devices following the introduction of our
suite of 650- and 1,200-V diodes and
JFETs to the marketplace in early 2014.
First and foremost is the development of normally OFF switch solutions
utilizing a low-voltage MOSFET cascoded with 1,200-V normally ON JFETs,
which are currently in production. The
vertical-trench JFET technology presently provides the lowest drain source
resistance-area product (Rds*A) in the
world and results in an optimal cost
solution switch for this voltage node.
The specially designed low-voltage
MOSFET eliminates the need for an additional SiC antiparallel diode. CustomDigital Object Identifier 10.1109/MPEL.2014.2381995
Date of publication: 3 March 2015

IEEE POWER ELECTRONICS MAGAZINE

ers are familiar with various cascode
solutions, including Infineon's Cascode
Light, which works equally well with
USCi's JFETs as well as cascode solutions from a number of gallium nitride
(GaN) companies. As voltage ratings
go up, however, the channel contribution to the overall device resistance
becomes negligible, so we have primarily implemented normally OFF designs
at higher voltages, as discussed in the
next section. The lone exception to this
approach is the supercascode, which
will be discussed in its own section.

Higher Voltages
The 650-V node is often described by
those in the wide-bandgap community as
the battleground between GaN and SiC.
Very few in this same community argue
the advantages of SiC at voltages greater
than 1,200 V. The performance most
enjoyed by SiC, particularly against very
high-voltage silicon insulated-gate bipolar transistors (IGBTs), such as those at
6.5 kV, is simply too great to ignore. To
that end, USCi has spent the last several
years extending our diode and JFET
technologies up to 6.5 kV and beyond.
USCi recently developed enhancement-mode 6.5-kV JFETs rated for
15 A with matching Schottky diodes.
The dies are 6 mm # 6 mm and packaged in a 60-A half-bridge module in
collaboration with Powerex. Smaller,
15-A modules have been developed in
collaboration with Arkansas Power
Electronics International. Both solutions operate at temperatures up to

 March 2015

200 °C. The half-bridge is designed to
work at 20 kHz, a switching frequency
far beyond that at which silicon IGBTs
could be expected to operate with reasonable switching losses. Applications
for these devices range from traction
drives to direct grid-tie inverter solutions. Figure 1 shows the 6.5-kV JFET
alongside the 60-A half-bridge module.
USCi is currently in the process
of expanding the voltage range of the
Schottky diode and JFET product
lines at the 1.7-, 3.3-, and 4.5-kV nodes,
having successfully demonstrated
these devices at 6.5 kV. Our current
maximum die size is 8 mm # 8 mm,
but as substrate and epitaxy improve
over time, we expect reasonable process yields can be achieved on devices >1 cm2 in the not too distant future.

Supercascode for High Voltages
As SiC devices push further up in voltage, particularly higher than 6.5 kV, epitaxy costs become a significant driver
of the total costs (Figure 2). Several
other groups [1] have explored solutions to this problem that stacks multiple 1,200-V normally ON JFETs in series
with a low-voltage MOSFET to form
the normally OFF supercascode. While
care must be taken to ensure a proper
voltage sharing among the JFETs in the
stack, it is a resolved problem. We are
currently exploring these devices as
lower-cost alternatives to monolithic
high-voltage chips. Figure 3 sheds
some light on the performance of the
supercascode alternative.
2329-9207/15©2015IEEE

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