${\rm SiN}_{x}$ /InAlN/AlN/GaN MIS-HEMTs With 10.8 ${\rm THz}\cdot{\rm V}$ Johnson Figure of Merit

A high combination of three-terminal breakdown voltage (VBK) and current gain cutoff frequency (f
_T
) was achieved with SiN
_x
/InAlN/AlN/GaN metal-insulator- semiconductor high-electron mobility transistors (MIS-HEMTs). A 1-nm SiN
_x
gate dielectric was deposited ex situ in a molecular beam epitaxy system and used to increase the carrier density of the 2-D electron gas under an ultrathin InAlN/AlN (2.3 nm/1 nm) barrier. Passivated MIS-HEMTs with a gate length of 80 nm exhibited a drain current density greater than 1.1 A/mm, a peak intrinsic transconductance g
_m,max
of 800 mS/mm, and a maximum frequency of oscillation f
_max
of 230 GHz. The combination of f T of 114 GHz and VBK of 95 V provides a Johnson figure of merit of 10.8 THz · V, which is among the highest reported values for fully passivated GaN HEMTs. A peak power-added efficiency of 37.5% with an output power of 1.25 W/mm and an associated gain of 9.7 dB was obtained by load-pull measurements at 40 GHz.