For a completely different technology Fig. 7.31 shows the output characteristics of a
pseudomorphic AlGa
As/InGa
As/GaAs HEMT at
= 300 K with
=
150 nm. The device is a high current (
900 mA/mm) and high gain device and therefore
serves from the Ka-band (26.5-40 GHz) to the W-band (75-110 GHz). The maximum DC- transconductance
amounts to
800 mS/mm for a gate length
= 150 nm.
Such devices are very suitable for high-power
amplifiers in the Q-band, as shown by Bessemoulin et al. in
[37], and in the V-band, as shown in [169]. For
gate lengths of about
= 150 nm-200 nm the output power of a
pseudomorphic device is limited by the gain for these
frequencies, as shown in the previous section. Applications
beyond f= 42 GHz require more gain than typically available in
double recess devices. Thus, the frequency of about 42 GHz
represents the critical point of the usefulness of the double
recess pseudomorphic AlGaAs/InGaAs/GaAs. The output power
limitation switches from a breakdown voltage limitation to a gain
limitation.
For the comparison of a pseudomorphic AlGaAs/InGaAs/GaAs HEMT to a GaAs MESFET of similar
gate length
, Fig. 7.32 shows the result of a simulation experiment to obtain the
maximum transconductance. Using the model reported and information from [185,198] a
simulation study is performed to obtain maximum transconductance for a MESFET with
= 100 nm.
The measured data shown in Fig. 7.32 are taken from [185].
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These findings agree very well with the published general experience, e.g. from Fig. 2.1-Fig. 2.4 in Chapter 2 and [196] who further state: