IEEE Electrification Magazine - December 2013 - 62

Source
n+
p-

Gate
SiO2

Source
n+
p-

Source

SiO2
GaN

Gate

Drain

AlGaN

n-
GaN
n + SiC Substrate

Transition Layer

Drain

Silicon Substrate

(a)

(b)

Figure 3. The different structures for semiconductor switches: (a) vertical and (b) lateral.

reverse conduction. The reverse conduction process in
these switches starts when the gate-drain reaches its
threshold voltage. To maintain the reverse conduction, a
voltage drop equivalent to the threshold voltage is
required. Therefore, the reverse conduction efficiency is
poor. This can be solved by actively controlling the switch
for reverse conduction. This process is illustrated in
Figure 4. In this test, the upper GaN FET in a bidirectional
dc-dc converter is turned off. The current of the output
inductor forces the lower switch to maintain the current.
Therefore, the drain voltage of the lower switch is dropped
below zero to reach a gate-drain threshold voltage. Hence,
the e-GaN switch is reverse conducting with a voltage drop
of 2 V. To improve the efficiency of the converter, following
by a dead time, the lower switch is turned on. Hence, the
switch is conducting in the reverse direction with a nearzero drain-source voltage drop. This process is a common
practice to increase the efficiency of dc-dc converters and
can be applied to motor drives as well.
Although no commercial GaN schottky barrier diode
(sbd) is available, research in this field is in progress. siC
sbds have been used in switching applications for the
past decade. These diodes bring the superior performances of sbds to higher voltage levels.

Packaging
For the transportation industry to fully benefit from the fast
switching times provided by the wide-bandgap
technologies, new types of transistor packages are of paramount importance. Conventional packages are not
designed to minimize the parasitic inductance and resistances associated with the lead wires. Therefore, the parasitic impedances of many conventional packages [such as
transistor outline (TO) packages] can result in voltage spikes
over the junction during switching periods. Although some
commercially available siC mOsFETs are using conventional TO packages, the majority of GaN-on-si switches have
been introduced with new packages. These switches are
mostly flip-chip. In this structure, the si layer is isolated and
can be directly connected to the heat-sink for improved

I E E E E l e c t r i f i c ati o n M agaz ine / december 2013

thermal performances. Land grid array (LGA) packages
minimize the stray inductances for lateral GaN FETs. These
packages are optimized for surface mount applications.
Although LGA introduces low stray inductances in lowvoltage power converters, further improvements for delivering the minimum required clearance between the drain
and source in higher-voltage applications such as automotive motor drives is demanded.
On the other hand, in a high-power traction drive system, semiconductor modules are of interest. This is due to
the large currents required for driving the motors, which
cannot be passed through a printed circuit board. Introducing conventional semiconductor module packages to the
wide-bandgap technologies will significantly reduce their
performance. The added stray inductances will not only
introduce large voltage overshoot over the junction during
the switching periods but also reduce the switching times
due to the added common source impedances. These challenges must be addressed before these technologies are
really beneficial to high-power motor drive applications.
To study the performance of wide-bandgap semiconductors in automotive applications, three switched
reluctance machine drives using si, siC, and GaN-on-si
technologies have been developed, which are shown in
Figure 5. This setup includes two 5-kW water-cooled converters using 1,200-V si and siC switches as well as a
2-kW water-cooled drive using 200-V GaN e-FETs. Accurate coolant temperature measurements provide an
estimation of the power loss over the semiconductor
switches. The GaN-based converter delivers the highest
efficiency. This is expected, due to the lower breakdown
voltage of the GaN switches, but the si and siC-based converters have comparable ratings. The siC-based converter
has three times lower power losses in compared to the
si-based converter.

conclusion
It appears that replacing si technology with siC or GaN
will create a less expensive power electronic system,
even though the cost of the switches is higher. This is

Table of Contents for the Digital Edition of IEEE Electrification Magazine - December 2013