Fabrication of high-speed InP/InGaAs/InP DHBT with a new self-aligned metallization technique for reduced base resistance

A new self-aligned emitter–base metallization (SAEBM) technique with wet etch is developed for high-speed heterojunction bipolar transistors (HBTs) by reducing extrinsic base resistance. After mesa etch of the base layer using a photo-resist mask, the base and emitter metals are evaporated simultaneously to reduce the emitter–base gap (SEB) and base gap resistance (RGAP). The InP/InGaAs/InP double heterojunction bipolar transistor (DHBT) fabricated using the technique has a reduced RGAP, from 16.48 Ω to 4.62 Ω comparing with the DHBT fabricated by conventional self-aligned base metallization (SABM) process. Furthermore, we adopt a novel collector undercut technique using selective etching nature of InP and InGaAs to reduce collector–base capacitance (CCB). Due to the reduced RGAP, the maximum oscillation frequency (fmax) for a 0.5 μm-emitter HBT is improved from 205 GHz to 295 GHz, while the cutoff frequency (fT) is maintained at around 300 GHz.