220-GHz Quaternary Barrier InAlGaN/AlN/GaN HEMTs

Depletion-mode high-electron mobility transistors (HEMTs) based on a quaternary barrier
$\hbox{In}_{0.13}\hbox{Al}_{0.83}\hbox{Ga}_{0.04}\break\hbox{N/AlN/GaN}$
heterostructure on SiC substrate were fabricated. The 66-nm-long gate device shows a dc drain current density of 2.1 A/mm, a peak extrinsic transconductance of 548 mS/mm, and a record current gain cutoff frequency
$f_{T}$
of 220 GHz for quaternary barrier GaN-based HEMTs, which is also among the highest
$f_{T}$
for all GaN-based HEMTs. The large
$L_{g} \cdot f_{T}$
product of 14.5
$\hbox{GHz} \cdot \mu\hbox{m}$
with a gate-length-to-barrier-thickness aspect ratio of 5.8 indicates a high effective electron velocity of
$\hbox{0.9} \times \hbox{10}^{7}\ \hbox{cm/s}$
, attributed to a high electron Hall mobility (1790
$\hbox{cm}^{2}/\hbox{V} \cdot \hbox{s}$
at an
$n_{s}$
of
$ \hbox{1.8} \times \hbox{10}^{13}\ \hbox{cm}^{-2}$
)—the highest reported in GaN-channel HEMTs with In-containing barriers. An intrinsic electron velocity of
$\hbox{1.7} \times \hbox{10}^{7}\ \hbox{cm/s}$
, extracted from conventional Moll delay-time analysis, is comparable to that reported in the state-of-art AlGaN/GaN HEMTs.