A PHEMT technology is very suitable for the Ka-band
since it can cope with both possible restrictions, firstly to
supply the necessary gain and secondly to provide the breakdown
hardness. In Fig. 6.18 the power handling capabilities in
class A operation at
= 40% of
can be observed for
a standard HEMT used e.g. for medium power amplifier design. For
the frequency range between 30 GHz and 40 GHz large-signal gain,
, and
are obtained for a
=
4
60
m pseudomorphic AlGaAs/InGaAs/GaAs HEMT.
Fig. 6.19-6.21 show measurements of
several HEMTs with
= 6
60
m gate width. For
high-power applications Fig. 6.19 and Fig. 6.20
show a comparison of two different variations of Technology B at
f= 30 GHz. The variations differ by slight changes in the
epitaxial structure. The most significant change is a higher
-doping concentration in variation B. All other device
and process parameters, such a gate length
, were kept
constant. A better output power performance of
Variation B can be observed already at 30 GHz with higher power
added efficiency of 38%. For Variation A, the gain is higher for
low input power levels, but the onset of gain compression occurs
earlier, so that
is significantly higher for
Variation B.
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For applications at 38 GHz and 40 GHz a pseudomporphic double recess HEMT is limited
even more by gain than by breakdown voltage. Fig. 6.21 shows the critical impact of the
lattice temperature increase for another 660
m device of Technology B at f= 40 GHz.
The relative reduction of the saturated output power as a function of temperature goes along with
the gain reduction.
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For the development of high-power amplifiers on coplanar wave-guide technology
[37], power measurements were performed varying the substrate temperature
.
Fig. 6.22 shows the dependence of the saturated output power and the gain on temperature at
f= 35 GHz for a two stage high-power amplifier. Over the wide range of the substrate
temperature
between 278 K and 423 K we observe a linear dependence of both gain and output
power, represented by P
, P
, and
.