I. Introduction
It IS well known that using a depleted surface passivation ledge over the extrinsic base surface can effectively improve current gain [1], [2] and noise performance [3] for heterojunction bipolar transistors (HBTs). The emitter passivation ledges have also been proven to be important and critical to HBTs reliability [4]. Both realigned and self-aligned approaches have been used for ledge fabrications. The realigned approach relies upon critical alignment steps with an additional photomask and tends to have larger spacing between emitter and base contacts than fully self-aligned HBTs, which may degrads the device microwave performance. On the other hand, the reported self-aligned ledge processes normally require complicate process steps such as an additional dielectric layer ( or SiN) with a critical etch back process to form a dielectric sidewall [3], base metal penetration annealing which requires a stringent diffusive base metal and a carefully controlled annealing process to allow the base metal penetrating through the thin emitter ledge layer without punching through the base layer, or a modification of HBT layer structure implementing dual etch-stop layers in the emitter [5], which complicates the material growth and device processing.