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<inf>τ</inf> = 480 GHz and f<inf>max</inf> = 1.0 THz. Improvements in the emitter and base processes have made it possible to achieve a 1.0 THz f<inf>max</inf> 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<inf>gap</inf>) to ∼ 10 nm, thereby significantly reducing the gap resistance term (R<inf>gap</inf>) in the total base access resistance (R<inf>bb</inf>), enabling a high f<inf>max</inf> device. Reduction in the total collector base capacitance (C<inf>cb</inf>) through undercut in the base mesa below base post further improved f<inf>max</inf>. These devices employ a Mo/W/TiW refractory emitter metal contact which allows biasing the transistors at high emitter current densities (J<inf>e</inf>) without problems of electromigration or contact diffusion under electrical stress [1].