High speed double heterojunction InP based HBT´s

InP based HBT´s for circuit applications were fabricated on MOMBE grown HBT layers. The layer structure included an InP emitter, InGaAs base, and InP collector. A C-doped base was employed for excellent device stability. The emitter-base and base-collector heterojunctions were both graded with InGaAsP layers for better electrical characteristics. An emitter-base self-aligned process was employed with dry etch and wet chemical etch mesa isolations. For base contacts, Pd/Zn/Pt/Au ohmic contacts were placed on top of the InGaAsP graded layer between the emitter and base, and made to diffuse into the base layer using rapid thermal annealing. This method produces an emitter-base junction buried under the InGaAsP graded layer and helps increase device reliability and life time. The device showed about 3% degradation in DC characteristics when stressed at a collector current density (J_c) of 100 kA/cm² and collector bias of 2 V at room temperature for 360 hours. Typical common emitter current gain was 40. f_t of 78 GHz and f_max of 129 GHz were achieved for 3×5 μm² emitter size devices. Maximum f _t was achieved at 70 kA/cm² collector current density and 2.25 V collector bias. f_t decreased to 70 GHz at 130 kA/cm² collector current density. F_max decreased for longer emitter size devices (3×10 and 3×15 μm²) while f_t maintained comparable values