HBT with emitter electrode having planar side walls

A method for fabricating a heterojunction bipolar transistor (HBT), comprising:

• providing a structure comprising sequentially a substrate, a collector layer for the HBT, a base layer for the HBT, an epitaxial layer, and an emitter metal layer for the HBT;
• etching the emitter metal layer and the epitaxial layer to form an emitter electrode comprised of an emitter epitaxy pedestal underlying an emitter metal cap, the emitter epitaxy pedestal and the emitter metal cap having horizontal dimensions that are substantially equal and having contiguous and co-planar vertical walls;
• depositing a base metal layer over the entire structure;
• wherein depositing a base metal layer over the entire structure comprises depositing the base metal layer over emitter metal cap and on the vertical walls of the emitter metal cap and the emitter epitaxy pedestal;
• depositing a photoresist layer over the base metal layer;
• irradiating the photoresist layer through a mask;
• removing a portion of the photoresist layer so that only a portion of the base metal layer that is spaced a predetermined distance away from the emitter metal cap and the emitter epitaxy pedestal remains covered by photoresist;
• removing the portion of the base metal layer that is not covered by photoresist so that after removal the remaining base metal layer forms a base electrode;
• and
• removing the remainder of the photoresist;
• wherein the predetermined distance between the base electrode and the emitter metal cap and between the base electrode and the emitter epitaxy pedestal is between 0.05 μm and 0.2 μm;
• and
• wherein one or more of the emitter epitaxy and base layer comprise one or more of InGaAs, InAlAs, InAs, InSb, AlInGaAs, InGaSb, InP, Si and SiGe.