IEEE Power Electronics Magazine - March 2017 - 39

Predicting Converter Performance-Sort Of
WBG and UWBG materials for power switching devices are
typically characterized by their bandgap, mobility, dielectric
constant, critical electric field, and thermal conductivity.
These characteristics can be combined to compute various
FOMs that correspond to material and device performance,
and as FOM values improve, power densities tend to
increase. However, FOMs are not designed to quantifiably
predict converter performance. For example, the relative
value of the vUFOM for GaN, as shown in Figure 2, is 1,480
times that of Si; yet, a GaN converter is not 1/1,480th the size
of an Si converter. Likewise, UWBG devices promise unprecedented leaps in FOMs. The vUFOM of AlN is an astonishing
43,650 times that of Si, but the landscape of future converter
performance remains difficult to predict. In other words, it is
not well known how material characteristics and FOMs map
to achievable converter power density, but it is tempting to
try to find an FOM that fits well the observed rise in power
density and use it to predict future performance.
One of the earliest FOMs identified for use in power electronics was the Baliga FOM (BFOM, 1983), which represents
a relative measure of conduction loss [14]. The BFOM defined
in [15] scales with the vUFOM previously defined. Other
FOMs include the Baliga high-frequency FOM (1989) and
the new high-frequency FOM (1995) for describing switching
loss [15], but these depend on more than material properties
(e.g., the specifics of device design that result in a particular
input or output capacitance). More recently, the Huang thermal FOM (2004), which measures thermal performance, and
the Huang material FOM (HMFOM, 2004), which takes into

account switching and conduction loss, were developed [15].
Herein, the HMFOM will be considered as a potential empirical measure for achievable power density.
Table 1 provides a comparison of vUFOM and HMFOM
values for different material types. The parameter values
for Si and WBG materials are taken from [12], [16], and [17].
It is noted that published values vary and, further, that
these quantities are not simple constants for a given material. Rather, they depend on factors such as film thickness,
electric field profile, doping type and concentration, and
defect density. In fact, many of these properties are continuously being reevaluated, so the specific values listed in
the table should be regarded as approximate. Critical field
values marked with an asterisk were calculated using the
method in [18]. The critical field value cited for Al 0.3 Ga 0.7 N
is a measured value [12]. However, this value was also calculated in [12] (using the method in [18]) and was shown to
be approximately consistent with the measured value. How
these values should be determined is likewise a subject of
debate, and refined methods for evaluating critical field are
the subject of a future work.
After surveying published converter power densities, one notices a correlation between the HMFOM and the
volumetric power density achieved. Figure 4 illustrates the

Converter Power
Density (W/in3)

high source and drain contact resistance, and ongoing work is
focused on the fabrication of low-resistance ohmic contacts.
As this and other challenges are overcome and these devices
are matured, the impact on converter SWaP is expected to
be considerable. However, predicting this improvement is
challenging, and the demands (higher voltage, frequency, and
temperature) put on the converter BoS will require greater
attention to device packaging and passive component design.

600
400
200
0

AIN
0

10
15
20
25
HMFOM (Relative to Si)

dc-dc Converters
1-Φ Inverters
Trend

3-Φ Inverters
3-Φ Rectifier

FIG 4 The volumetric power density as a function of HMFOM
for several converters, extrapolated out to HMFOM for AlN.

Table 1. A comparison of material properties and FOM values [12], [16]-[18].
conventional
Properties

FOMs

Property

Silicon

wBg
6H-SiC

4H-SiC

UwBg
GaN

Al 0.3 Ga 0.7 N

Al 0.85 Ga 0.15

AIN

Bandgap (eV)

1.1

3.0

3.3

3.4

4.1

5.7

6.2

n ^cm 2 / Vsh

1,400

500

800

1,000

150

425

Diel constant

11.9

9.7

10.1

10.4

10.3

10.2

10.1

E C ^MV/cm h

0.3

2.5

2.2

4.0

5.9

13.4*

16.6*

vth (W/cmK)

1.5

4.9

1.4

0.4

0.5

2.9

vUFOM (rel)

168

191

1,480

705

8,100

43,650

HMFOM (rel)

5.0

5.5

11.3

6.4

14.6

30.5

*Calculated using the method in [18].

March 2017

z IEEE PowEr ElEctronIcs MagazInE

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