IEEE Electrification Magazine - December 2013 - 61

drivers are mandatory for the full utilization of wide-bandgap technologies.

The thermal conductance of siC is superior to the other
technologies. These factors all serve to further relax the
package design and thermal requirements.

Voltage Limitations

101
100

m
it
m
it

Li
C

10-2

10-4
101

Li
aN

10-3

In addition to improved thermal requirements due to
efficiency improvements, generally, wide-bandgap technologies demonstrate a much lower intrinsic carrier concentration compared to si at temperatures above 300 °C. If
operation under harsh thermal conditions is of interest, siC
JFETs are good candidates. These switches are capable of
operating at temperatures twice as high as the thermal limits of si technology. Unfortunately, because of the intrinsic
characteristics of their oxide interface, siC-mOsFETs are not
capable of operating at temperatures higher than that of si.

10-1

Thermal response

Li
m
it

A highly efficient motor drive system will obviously create
a highly efficient product, but it also reduces the price of
the electric vehicle as well by relaxing the battery storage
requirements and thermal design requirements.
GaN-on-siC technologies demonstrate superior figures
for efficiency and maximum switching frequency among
the available technologies. However, these switches are
not cost effective for automotive motor drive applications
as they are more expensive to manufacture than both siC
and GaN-on-si.
siC and GaN-on-si switches benefit from low conduction
resistance Rds(on) as well as lower capacitive charges. since
wide-bandgap technologies have high critical fields, GaN and
siC switches can handle higher voltages than conventional
si for the same dimensions. by reducing the thickness of
these transistors, much better Rds(on) characteristics are
achieved as compared to si. To define a figure of merit for
comparison of various switch generations, QR:= Rds(on) × QGd
has been introduced. QR is constant for different generations
of each switch technology. In essence, this number demonstrates the efficiency of a switch technology by eliminating
the size of the die. If the number of paralleled mOsFETs in a
package is increased, Rds(on) is improved but the required
gate charge is increased. Although si technology is reaching
its limits based on QR, the young wide-bandgap technologies
demonstrate much better figures. based on the QR figures of
GaN switches, this technology can reach a conversion ratio of
50:1, which is far superior to the si ratio. It should be noted
that in soft-switching applications, QGs can provide more
accurate results. The comparison between on-resistance and
the breakdown voltage of different technologies are shown
in Figure 2.

since automotive motor drives require high currents, the
power switches must be multiplexed. Conventional si
power mOsFETs are designed in a vertical structure to maximize the utilization of the volume of the die and simplify
the internal wiring structure. This structure is shown in
Figure 3(a). A vertical structure enables paralleling a large
number of switches on a single substrate. meshing has
been a common practice in maximizing the utilization of
the die. Currently, siC power mOsFETs are being delivered
in this structure. A drawback to this structure is the formation of the parasitic body diode. This diode is of low quality
and imposes reverse recovery losses to the operation of the
siC mOsFET. Hence, methods to eliminate forward conduction for this diode are of interest. To increase the switching
performances, a lateral structure has been used.
Currently, the GaN switches are offered in a lateral
structure. This structure is demonstrated in Figure 3(b),
where the dotted line represents the electron gas formation as the result of the AlGaN layer. The lateral structure
reduces the gate-drain parasitic capacitances, which
results in faster switching times. A high gate-source to
gate-drain capacitance ratio provides better performances
in high-frequency applications. Unfortunately, this structure also introduces challenges in motor drive applications. Lateral switches cannot be paralleled in a mesh
structure such as conventional vertical si mOsFET and
IGbT switches. Also, in high-voltage applications, large
electric fields must be sustained across the surface of the
switch, which imposes technical challenges for manufacturing high-voltage GaN devices.
On the other hand, the lateral structure of GaN switches
eliminates the formation of a body diode. Hence, no
reverse recovery charges are detected in switching from

efficiency

Scalability/Internal diode

Ron (Ω mm2)

In low-voltage applications, GaN-on-si provides good efficiency and switching characteristics at a reasonable cost.
However, these switches are not commercially available at
higher voltage levels (currently, 650 V is the maximum).
On the other hand, siC mOsFETs are available in mediumvoltage levels (1,200+ V).

102
103
Breakdown Voltage (V)

104

Figure 2. The on-resistance versus blocking voltage of different technologies.

IEEE Electrific ation Magazine / d ec em be r 2 0 1 3

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