1.0 THz f<inf>max</inf> InP DHBTs in a refractory emitter and self-aligned base process for reduced base access resistance

Abstract

We report 220 nm InP double heterojunction bipolar transistors (DHBTs) demonstrating f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">τ</inf> = 480 GHz and f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> = 1.0 THz. Improvements in the emitter and base processes have made it possible to achieve a 1.0 THz f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">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 xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gap</inf> ) to ∼ 10 nm, thereby significantly reducing the gap resistance term (R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gap</inf> ) in the total base access resistance (R <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">bb</inf> ), enabling a high f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> device. Reduction in the total collector base capacitance (C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cb</inf> ) through undercut in the base mesa below base post further improved f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">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 xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> ) without problems of electromigration or contact diffusion under electrical stress [1].