1.0 THz fmax InP DHBTs in a refractory emitter and self-aligned base process for reduced base access resistance

We report 220 nm InP double heterojunction bipolar transistors (DHBTs) demonstrating f_t = 480 GHz and f_max = 1.0 THz. Improvements in the emitter and base processes have made it possible to achieve a 1.0 THz f_max even at 220 nm wide emitter-base junction with a 1.1 μm wide base-collector mesa. A vertical emitter metal etch profile, wet-etched thin InP emitter semiconductor with less than 10 nm undercut and self-aligned base contact deposition reduces the emitter semiconductor-base metal gap (W_gap) to ~ 10 nm, thereby significantly reducing the gap resistance term (R_gap) in the total base access resistance (R_bb), enabling a high f_max device. Reduction in the total collector base capacitance (C_cb) through undercut in the base mesa below base post further improved f_max. These devices employ a Mo/W/TiW refractory emitter metal contact which allows biasing the transistors at high emitter current densities (J_e) without problems of electromigration or contact diffusion under electrical stress.