Quaternary Barrier InAlGaN HEMTs With $f_{T}/f_{\max}$ of 230/300 GHz

Depletion-mode quaternary barrier
$\hbox{In}_{0.13}\hbox{Al}_{0.83}\break \hbox{Ga}_{0.04}\hbox{N}$
high-electron-mobility transistors (HEMTs) with regrown ohmic contacts and T-gates on a SiC substrate have been fabricated. Devices with 40-nm-long footprints show a maximum output current density of 1.8 A/mm, an extrinsic dc transconductance of 770 mS/mm, and cutoff frequencies
$f_{T}/f_{\max}$
of 230/300 GHz at the same bias, which give a record-high value of
$\surd f_{T} \cdot f_{\max} = \hbox{263}\ \hbox{GHz}$
among all reported InAl(Ga)N barrier HEMTs. The device speed shows good scalability with gate length despite the onset of short-channel effects due to the lack of a back barrier. An effective electron velocity of
$\hbox{1.36} \times \hbox{10}^{7}\ \hbox{cm/s}$
, which is comparable with that in the state-of-the-art deeply scaled AlN/GaN HEMTs, has been extracted from the gate-length dependence of
$f_{T}$
for gate lengths from 100 to 40 nm.