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In the end of the 1990s III-Nitride technology advanced rapidly. The
interest was mainly driven by the progress in lasers and LEDs but also
in HEMTs. The latter soon reached record microwave performance and
thus prompted the development of heterojunction bipolar transistors
(HBTs). They offer traditional advantages over field effect
transistors, such as linearity, threshold uniformity, and current
handling. One of the first working samples was demonstrated
in [78], but was plagued by poor contacts and the quality
of the material. Subsequently a current gain (
) of only 3 was
reached with a very high V
offset of 5 V. Other
groups [79] faced similar problems. However, while the
technology was still lacking, various theoretical studies pointed out
the large potential of the devices. Using a two-dimensional model and
based on material properties reported in literature, [80]
predicted a maximum of 1130, collector saturation current of
3.5 kA/cm
, breakdown voltage of 55 V, and
of 18 GHz. Even
higher theoretical values (reaching =2000 and
=30 GHz)
were reported by employing compact models
[81]. Other studies focused on a comparison between npn- and
pnp-structures [82] and the worse high-frequency
characteristics of the latter. Using models and material parameters
verified by modeling experimental device characteristics, an
optimization was performed in [83] and a theoretical
of 44 GHz was predicted. Based on improved fabrication
processes developed previously for other compound systems, HBTs with a
similar performance (
3 at room temperature) were
fabricated on MBE and MOCVD grown material [84]. The device
performance was again limited by the base resistance. Better results
were achieved in [85] by using MOCVD grown material, with =80, but
high forward resistance at the base-emitter junction due to possible
diffusion of Magnesium into the emitter. The same group also
introduced selective area growth resulting into high crystalline
quality, but a still leaky base-emitter diode [86]. The
same technique was used by McCarthy et al. [87],
while they also employed emitter mesa regrowth to avoid etch damage,
and material grown using lateral epitaxial overgrowth technique to
achieve low dislocation density. Through optimization of the width and
grading of the base, operation at 70 V with a =6 was possible.
In the following years work on AlGaN/GaN based HBTs continued
[88,89,90,91,92,93,94]
(Fig. 2.9). A possible issue with the measured values of the
extrinsic current gain, was however pointed out [92]. Due
to the low quality ohmic contacts and the leaky base-collector
junction, the anomalous current gain at low current levels can be
erroneously attributed to the intrinsic device performance. This was
also observed by Hsueh et al. [95], who proposed that
a common-emitter I-V under high current bias is the best way to
evaluate the transistor performance, instead of the Gummel plot. Some
of the experimental studies focused on high-temperature
performance [90,96]. As the hole concentration
increases by thermal activation, the current gain of pnp structures is
enhanced [90]. Device operation at temperatures up to
400
C was demonstrated, although the device performance shows
a degradation after prolonged operation at this
temperature [96]. Despite the progress in the last years
the technology faces still several major problems:
![\includegraphics[height=0.44\textheight]{figures/state/BetaT.eps}](img137.png)