Enhancement-Mode AlN/GaN/AlGaN DHFET With 700-mS/mm gm and 112-GHz fT

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 ohm . 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.