Patent ID: 6982442

Claim:
A heterojunction bipolar transistor (HBT), comprising: a collector; an intrinsic base overlying said collector, said intrinsic base including a layer of a single-crystal semiconductor alloy; a raised extrinsic base including a first semiconductive layer overlying said intrinsic base and a second semiconductive layer formed on said first semiconductive layer, said first semiconductive layer being etch distinguishable from said second semiconductive layer; and an emitter overlying said intrinsic base, said emitter disposed in an opening of said first and second semiconductive layers, such that said raised extrinsic base is self-aligned to said emitter, wherein said first semiconductive layer has a first composition Si x1 Ge y1 , where x 1 and y 1 represent precentages of silicon and germanium of said first composition, respectively, and said second semiconductive layer has a second composition Si x2 Ge y2 , where x 2 and y 2 represent precentages of silicon and germanuim of said second composition, respectively, said precentages y 1 and y 2 of germanium being sufficiently different to make said semiconductive layer etch distinguishable from said second semiconductive layer and at least one of said precentages y 1 and y 2 of germanium varies as a function of vertical position over a thickness of said first semiconductive layer or said second semiconductive layer, respectively.