Effects of Multigate-Feeding Structure on the Gate Resistance and RF Characteristics of 0.1-$\mu{\hbox{m}}$ Metamorphic High Electron-Mobility Transistors

We investigate the effects of a multigate-feeding structure on the gate resistance (R
_g
) and RF characteristics of the high electron-mobility transistors (HEMTs). In this structure, the increase of R
_g
with the gatewidth (W) is minimized; therefore, high maximum frequency of oscillation (f
_max
) is achieved. Various numbers of gate feedings (N
_gf
) using the air-bridge interconnections are adopted for fabricating the 0.1-mum depletion-mode metamorphic HEMTs. From these structures, we observe great reduction in R
_g
with the increase of N
_gf
, and their relationship is given by R
_g
prop 1/[2middot(N
_gf
-1)]
²
, where N
_gf
=2,3,4,...; on the other hand, the effects of N
_gf
on other small-signal parameters are negligible. Calculated cutoff frequency (f
_T
) and f
_max
from the extracted small-signal parameters all show good agreement with the measurement results. f
_T
is slightly decreased with the increase of N
_gf
due to the increase of gate-to-source capacitance. f
_max
is, however, greatly increased with N
_gf
, and this effect becomes greater at longer total gatewidth (W times number of gate fingers) . This is due to the smaller R
_g
at greater N
_gf
in the multigate-feeding structure. We propose that this gate-feeding structure provides a very effective way to suppress R
_g
and maximize f
_max
for the applications of the HEMTs with long W.