High-performance self-aligned InP/InGaAs DHBTs with a passivation ledge utilizing a thin etching stop layer

The relation between effective emitter resistance, r_e ^eff, and collector current, I_C, was investigated for InGaP/GaAs HBTs and InP/InGaAs DHBTs. While the relation between r_e ^eff and I_C ^-1 of InGaP/GaAs HBT is linear over a wide range of I_C, that of InP/InGaAs DHBT is nonlinear as if r_e ^eff in the low collector current region increased more than the dynamic emitter resistance component that is proportional to I_C ^-1. Although this phenomenon is enhanced in InP/InGaAs DHBTs with a thin InGaAs etching stop layer inserted in the emitter to make it controllable to form a ledge passivation structure, the etching stop layer is acceptable in high-performance high-reliability HBT applications. Using an emitter structure with an etching stop layer, we developed self-aligned InP/InGaAs DHBTs with a passivation ledge structure that attained an f_T of 302 GHz, f_max of 388 GHz, and BV_CEO of 6.2 V.