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