Graphene-on-SiC - ISOM

TECHNOLOGY GaN POWER ELECTRONICS
Figure 3. Material
theoretical limits and a
plot of R on
verses reverse
breakdown voltage
for various field plate
geometries
Figure 4. Lateral ACFP
GaN Schottky diode
reverse breakdown as
a function of anode to
cathode spacing
another opportunity for GaN-on-silicon is in
electric cars, including hybrid, plug-in, and fuel
cell designs. These vehicles have roadmaps
though 2014 to push to a 2 kv operating bus,
a move that will reduce on-state and switching
losses and boost efficiency. The latter gain is
highly beneficial, because it eliminates the need
for a cooling loop for inverter circuitry, it increases
battery life, and it delivers savings in terms of
weight, volume and cost. Efficiency gains resulting
from higher voltage-handling capabilities also
make it attractive to deploy GaN-on-silicon devices
in industrial drives, electric trains, and military and
merchant fleet applications. our low cost GaNon-silicon
technology has the potential to target
all of these applications. It is based on the use of
commercial GaN-on-silicon epitaxy, and involves
HEMTs with an alGaN buffer and barrier layer (see
Figure 1).
These structures can be grown by either MoCvD
or MBE, using a process that begins with the
deposition of a nucleation layer that is unique to
the starting substrate material (either silicon, SiC,
or sapphire high-resistivity substrates). a relatively
thick alGaN buffering layer or super-lattice
structure is then deposited onto the nucleation
layer. This serves two purposes: It mitigates lattice
mismatch and induced strain associated with
the substrate/epitaxy transition; and it provides
electrical isolation of the active device region from
the substrate and substrate/epitaxy interface.
Leakage from this buffer, which ultimately sets the
limit for the overall device reverse breakdown, is
determined by the composition and total thickness
of the buffering layer.
after the buffer has been deposited, growth
continues with the addition of the active GaN and
alGaN Schottky barrier layers. The aluminium
fractional content in this barrier is generally between
22 percent and 32 percent, with the precise value
chosen to deliver the best compromise between
on-resistance and leakage characteristics. a
GaN cap often completes the structure. This
reduces oxidation of the underlying alGaN film and
improves the device’s contact resistance.
We are by no means the only developers of
GaN-on-silicon technology, but the efforts of most
groups in this field have been limited to 600 v
applications. This focus has led many within the
power electronics industry to incorrectly believe
that GaN-on-silicon devices are limited to this
operating range, and GaN single-ended devices
must be built on SiC if they are to deliver voltage
stand-off capabilities above 1 kv. We have no
doubts that GaN-on-SiC devices can operate at 1
kv and more, but they are prohibitively expensive
– and we can’t see that changing. our mission is
to address the misconceptions regarding the limits
of GaN-on-silicon technology, and show that it is
capable of creating high-blocking-voltage devices
capable of carrying high currents.
Thanks to recent advances in commercial GaNon-silicon
epitaxy – especially in the construction
of advanced buffer layers coupled with field plate
design technology – HEMTs and Schottky diodes
can now deliver standoff voltages in excess of
1.5 kv. Devices made by us can even hit much
higher values than this, with GaN-on-silicon
HEMTs producing blocking voltages in excess
of 2.5 kv, and Schottky diodes incorporating
the same materials technology delivering standoff
voltages of more than 3 kv. These levels of
performance are rebuffing the accepted wisdom
that GaN devices must be built on SiC, if they are
to serve power electronics applications requiring
enhanced voltage operation.
our results include a portfolio of devices
combining 1.5 kv blocking voltages with current
handling capabilities in excess of 10 a. These
44 www.compoundsemiconductor.net April/May 2013