A metamorphic technology for analog applications was
investigated. Fig. 7.41 shows the simulated and measured
transfer characteristic of a metamorphic
In
Al
As/In
Ga
As/GaAs HEMT for
two different temperatures
. A decrease of the current
with rising
for low
= 1 V can be observed.
![\includegraphics[width=10 cm]{D:/Userquay/Promotion/HtmlDiss/fig66.eps}](img891.gif)
|
![\includegraphics[width=10 cm]{D:/Userquay/Promotion/HtmlDiss/fig67.eps}](img892.gif)
|
Fig. 7.42 shows the transfer curves of a 2
60 
m metamorphic depletion
type HEMT and comparison with measurements for
= 400 nm. The large gain for this relatively
large gate length
makes this materials system well suitable for high gain applications.
value versus
channel In content 
for a device structure is found for = 0.43 for the metamorphic HEMTs.
Using the given parameter set from Chapter 3 this idea is not confirmed, as shown in
Fig. 7.43.
of three devices available for
= 150 nm and the simulation results
are compared in Fig. 7.43. For the 
0.25 composition range the values are taken from
typical pseudomorphic AlGaAs/In
Ga
As/GaAs HEMTs. The values suggest a linear
dependence of
. Between = 0.25 and x= 0.53, no devices are experimentally available for
this work. The simulation results obtained for = 0.43 and = 0.3 support the measured values
and the monotonous decreasing behavior for decreasing .
Devices with = 0.3 were demonstrated by Bollaert et al. in [47] for power
applications, where the further reduction of the In content beyond = 0.43 to = 0.3 leads to a
further decrease of
. For power applications Zaknoune et al. in [325] reported a
value
= 125 GHz for
= 150 nm using an extremely high 
-doping of
110
cm
.
Thus, the In content in metamorphic devices serves
mainly to adjust the device in the trade-off between speed and
breakdown considerations. The InAlAs/InGaAs HEMTs are limited by
the on-state breakdown, as argued in Chapter 6. In [235]
no significant improvement of the device burnout was found using
a reduced In content to = 0.41 for the device in the channel
in comparison with the = 0.53. This suggests when using the
In
Al
As/In
Ga
As in high power applications
in order to make use of the higher gain performance, that the In
content needs to be further reduced than = 0.41. However,
such a reduction also decreases the possible improved speed
performance in comparison with the PHEMT on GaAs.
Hence, in order to make use of the higher gain in the InAlAs/InGaAs materials system to
compensate the gain shortage of the pseudomorphic HEMTs on GaAs, the In content remaining to be
investigated for reliable high-power application is 0.25 
0.41.
Comparing AlGaAs/InGaAs HEMT and AlGaAs/InGaAs HEMT lattice matched to InP del Alamo
and Sommerville
determined the frequency 
= 94 GHz at which better high-power
performance was achieved for InAlAs/InGaAs devices for
= 100 nm devices while PAE was not
considered an issue.
An In content 0.4 is one choice to lower this frequency. Finding the
break-even point in terms of the In composition for the change of sign of the temperature
coefficient of impact ionization is necessary.
Once this is achieved the concepts of electric field relaxation found useful for the PHEMT on GaAs
can be used, as introduced in the last sections.
= 150 nm.
versus
of a 
as a function of material
composition.![\includegraphics[width=10 cm]{D:/Userquay/Promotion/HtmlDiss/fig74.eps}](img893.gif)