The evolution of MBE growth technique and modulation doping together with a vivid interest in the behavior of quantum well structures in the late 70s made the demonstration of the first AlGaAs/GaAs HEMT by Mimura et al. [3] possible. The potential of the technology was quickly realized and several designs (AlGaAs/InGaAs PHEMT and AlInAs/InGaAs/InP HEMT) were proposed in order to counter various problems. The first AlGaN/GaN based HEMTs were demonstrated in the early 90s [8] after methods for deposition of GaN on sapphire by MOCVD were developed.
However, the main driving force for the continuous improvements in the material growth technology [55] were Nitride-based light-emitting diodes (LEDs). There is a strongly growing demand for the latter for home electronics but also lighting and back-lighting, where the market in 2010 amounts to over 10 billions USD. For comparison the market for GaN RF transistors in 2010 is only 100 millions USD.
The very high values for the electron sheet charge density were the
reason for a shift of the research interest from AlGaAs/GaAs to
AlGaN/GaN based devices. Consequently the characteristics of GaN based
HEMTs have been improved steadily in the last decade. While the first
HEMTs exhibited a cut-off frequency
and a maximum oscillation
frequency
of 11 GHz and 35 GHz
(
=0.25
m) [56] later devices reached 50 GHz and
120 GHz, respectively, in 2002
[57]. Currently the highest reported
and
values are 190 GHz and 240 GHz, respectively, for
devices with high Al-composition and thin barrier layers
[58]. Another notable achievement is reported by
Shinohara et al., who measured a cut-off frequency of 153 GHz of a
DH-HEMT with
[59], and also Chung et al.,
who produced a device with the same gate length and a maximum frequency
of 300 GHz [60]. However, such a performance is near the limit
of the AlGaN/GaN technology, imposed by the limited polarization-induced
electric fields and current collapse. Fig. 2.3 shows a steady
increase of the measured
over the years, however such a
illustration does not account only for the technology improvement, but
also for the down-scaling of the gate lengths.
This is avoided in
Fig. 2.4, where the product
is
depicted. There is a clear limit of roughly 20 GHz
m, which
was rarely exceeded. This value was also recently reached with AlGaN/GaN
HEMTs [61]. First proposed by Kuzmik in 2001
[62], the InAlN/GaN interface posseses a higher
polarization-induced sheet charge density as AlGaN/GaN. As the InAlN
layer can be grown lattice-matched to GaN, possible strain relaxation
problems are significantly reduced. Consequently, this potential was
quickly realized and the focused research of such structures is
yielding excellent results: e.g. cut-off frequencies of 144 GHz for
a
device [63].
Another optimization goal is the maximum power density (Fig. 2.5). The first HEMTs exhibited barely 1.1 W/mm at 2 GHz [64] (Fig. 2.5). Employing multiple field-plates the power density was raised up to 40 W/mm at 4 GHz [65]. By using internally matched amplifier technology the limits have been pushed up to 550 W at 3.5 GHz [66]. Because of the large band gap, GaN based HEMTs are also considered for high power operations.
Early samples demonstrated impressive breakdown voltages in the range
of 230 V, but also poor subthreshold behavior [67]. Those
issues were addressed and breakdown voltages
=570 V were reached
by gate geometry optimization [68]. By using a
``slant-field-plate'' technology a
=1900 V could be achieved at
the cost of drain current degradation [69]. Another way of
increasing
is by using a thick buffer layer, leading to a
maximum value of
1800 V [70]. While the
breakdown voltage increases steadily over the years
(Fig. 2.6), a more significant characteristic is the product
. Some of the achieved very high voltages are due
to exceptionally large devices. The cut-off frequency of the latter is
quite low, which translates in a low
(Fig. 2.7).