One of the suggested ways to counter the low base conductivity is to employ a
lower gap material such as InGaN. InGaN is also more resistant against the
damage introduced by the dry etching step used to expose the surface of the base
layer [97]. Thus, the potential of an InGaN/GaN HBT was quickly
realized, first in theoretical studies [98,79,99] and
subsequently in experiments [97]. The latter is the first reported
InGaN/GaN (D)HBT. It achieved a of 20 but also a high offset voltage
(5 V). Further studies reported breakdown voltages as high as 100 V, mainly due
to the low damage of the InGaN base layer [100]. Ongoing
improvement of the base ohmic characteristics resulted in a decrease of the
offset voltage (from 5 V down to 1 V) in addition to a record
=2000
[101]. The same group reported a collector current density of
6.7 kA/cm
corresponding to a power density of 270 kW/cm
[102].
Several works employed a theoretical approach in exploring and
optimizing the device performance. Ensemble Monte Carlo simulations were
used to determine the temperature and doping concentration dependence of
the low-field mobility [99]. Based on this calculations the
temperature dependence of the cut-off frequency was studied. The impact
of non-uniform base doping on was studied in [103].
The implementation of a graded emitter layer was experimentally
studied by Keogh et al. [104]. A low offset voltage
(2-3 V) and =27 with breakdown voltage greater than 40 V was
reported. The current gain decreased to 10 at 300
C, but no
device degradation was observed. The same technique was used by other
groups
[105,106]. A state-of-the-art device with
=42,
a current density of 5.2 kA/cm
, and a breakdown voltage larger than
75 V was obtained in [106]. A record current gain of 2450 was reported
in [107] at room temperature. At 40 K it reaches
5000 due to a reduction of the recombination current in the base layer. By
introducing an AlGaN collector Kumakura et al. [108]
were able to achieve a breakdown voltage of 190 V albeit the current
gain was only 3 due to the low quality AlGaN.