For this comparison the same InGaN cap device is used as described in
the previos section. The gate recess structure and its fabrication is
reported by Palacios et al. [49]. The 11 nm thick
GaN channel is grown on-top of a 1 nm thick InGa
N
back-barrier. A 1 nm thick AlN layer between the channel and the 25 nm
Al
Ga
N is grown in order to improve the electron
mobility. After the AlGaN surface treatment a 12 nm gate recess is
performed, resulting in a gate-to-channel distance of 13 nm. The gate
length
is 160 nm, source-gate distance is 0.6
m, and
gate-drain distance is 0.9
m.
Fig. 5.61 shows the results for the transfer characteristics of both devices. After the calibration of the sheet charges a good agreement is achieved. The InGaN/GaN device exhibits lower current. However, a higher threshold voltage is achievable without recessing the InGaN cap layer [18]. The threshold voltage of the recess device can be increased, too, (Fig. 5.62) by increasing the recess depth.
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Fig. 5.63 compares the DC transconductance
for both
devices. The decrease in the measured
of the InGaN/AlGaN/GaN
transistor at higher gate bias might be due to non-idealities in the
source and drain ohmic contacts, which are not considered in the
simulation. As expected, the recessed gate device exhibits a higher
due to the much shorter gate length
and the reduced
gate-to-channel separation.
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AC analysis of the transistors is also performed. The recessed gate
structure (
) exhibits a cut-off frequency
=85 GHz,
compared to
=10 GHz for the InGaN/AlGaN/GaN device (
). Note
that the product
=19 GHz
m is higher
than 14.4 GHz
m of the recessed-gate device. The simulation of
an InGaN cap structure with
shows that
=30 GHz can be
achieved, which even gives
=24 GHz
m.