Fabrication of novel self-aligned InP/InGaAs HBTs using dummy emitter

A novel self-aligned process for InP/InGaAs heterojunction bipolar transistors (HBTs) was developed based on the crystallographic wet etching characteristics of an InP dummy emitter layer. The desired contact spacing between the emitter and the base electrodes was obtained by utilizing consistent undercut profiles of InP layers according to the crystal orientations. The new self-aligned emitter-base technology was found to be very effective not only in maintaining high current gain down to small emitter-size devices, but also in showing good overall device performance at high frequencies. The maximum f_T and f_max of the device were measured to be 77 GHz and 115 GHz, respectively.