Enhancement-Mode AlN/GaN/AlGaN DHFET With 700-mS/mm $g_{m}$ and 112-GHz $f_{T}$

An enhancement-mode (E-mode) AlN/GaN/AlGaN double-heterojunction field-effect transistor (DHFET) with record high-frequency performance is reported. E-mode operation was achieved through vertical scaling of the AlN barrier layer. Parasitic resistances were reduced through ohmic contact recess etching followed by regrowth of n+ GaN by molecular-beam epitaxy and SiN deposition to increase the sheet charge density in the access regions of the device, resulting in an extremely low on-resistance of 1.06 Ω · mm. A DHFET with an 80-nm gate length had a threshold voltage of 0.21 V, an extrinsic transconductance (g_m) of 0.70 S/mm, a current-gain cutoff frequency (f_T) of 112 GHz, and a maximum oscillation frequency (f_max) of 215 GHz. To our knowledge, these are the highest g_m , f_T, and f_max values reported to date for an E-mode GaN HFET.