IEEE Power Electronics Magazine - March 2017 - 38

Note that different expressions apply
to vertical and lateral devices. For
2
the vertical UFOM (vUFOM), f is the
10
vUFOM
GaN
material
permittivity and n n is the bulk
= VB2/R
Ron,sp = εµnEC3 /4
Lateral Devices Fairly Mature
electron
mobility (vertical device drift
101
Vertical Devices Emerging
Improving
regions are typically n-type, due to the
Performance
higher mobility of electrons compared
100
to holes). For the lateral UFOM, q is the
N
AIN
15
0.
a
electron charge, n ch is the channel elecG
Unexplored Space
5
.8
I
0
10-1
Unprecedented
Performance
tron mobility (which may not equal the
A
bulk electron mobility n n, especially
-2
for
heterostructure-based devices),
10 2
10
103
104
and n s is the channel sheet charge
Breakdown Voltage (V)
AIGaN
density. While the vUFOM scales as
Unexplored Space
SiC
the cube of E C and the lateral UFOM
Unprecedented Performance
Many Commercial
Heterostructures Are Available
scales
only as the square of E C, n s in
Devices Now Available
the lateral UFOM may be quite large
( +10 13 cm -2 for III-N heterostructures),
FIG 2 The vUFOM for several semiconductor materials.
resulting in comparable vertical and
lateral UFOMs for a given material. For
both device types, it is desirable to simultaneously maximize
V B and minimize R on,sp. The FOM is thus maximized and proSiNx
Pd/Au
vides a convenient performance metric. Figure 2 illustrates
p-Al0.05Ga0.95N/GaN
the vUFOM for several semiconductor materials, showing the
benefits of UWBG materials. It should be noted that UWBG
p-Al0.3Ga0.7N
materials are likely better suited for realizing devices in
the approximate range of 1-15 kV and will yield the most
n-Al0.3Ga0.7N
benefit in higher-voltage and higher-power applications
(4.3 µm)
(i.e., kilovolt/kilowatt).
Ti/Al/Mo/Au
Specific On-Resistance (mΩ cm2)

Si-Today's Standard

n+ Al0.3Ga0.7N

UWBG Devices

AIN
Sapphire
(a)

50 µm
(b)
FIG 3 (a) A cross-sectional schematic of an AlGaN p-i-n diode
and (b) the processed diode.

divided by R on,sp, in turn depends on a power of the critical
electric field:
V B2 R on,sp = fn n E 3C 4,
V B2 R on,sp = qn ch n s E 2C

IEEE PowEr ElEctronIcs MagazInE

^ vertical devices h
^ lateral devices h .
z March 2017

Many of the materials challenges associated with AlGaNbased devices are being addressed, resulting in the recent
development of a +1,600-V Al 0.3 Ga 0.7 N p-i-n diode [12] and
an +800-V AlN/Al 0.85 Ga 0.15 N high-electron-mobility transistor (HEMT) [13]. The p-i-n diode is based on a low-doped
(mid-10 16 cm −3), + 4 - nm -thick Al 0.3 Ga 0.7 N n-type drift
region grown on a sapphire substrate. Figure 3 shows the
diode cross-sectional schematic and processed device.
This is a quasi-vertical device, so named because the electrically insulating substrate requires that the n-contacts be
placed on the front surface of the wafer. Combining the
breakdown voltage of +1, 600 V with the differential specific on-resistance R on,sp of +16 mX $ cm 2 (which is believed
to be limited by lateral current spreading in the n-type contact layer) yields a vUFOM of +150 MW/cm 2, which is to
our knowledge the highest reported for a power device
based on AlGaN.
The HEMT is based on an AlN/Al 0.85 Ga 0.15 N heterostructure. In addition to a breakdown voltage in excess of
800 V, this device has good gate control, a high I on /I off ratio
^ 210 7 h, a low gate leakage current ^ 110 -7 A h, and an excellent subthreshold slope (75 mV/decade). To our knowledge,
this is the highest-bandgap material (channel E G +5.7 eV )
from which a working transistor has been demonstrated to
date. Currently, the performance of the HEMT is limited by

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - March 2017