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
τ
= 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 [1].