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/sub c/) of 100 kA/cm/sup 2/ and collector bias of 2 V at room temperature for 360 hours. Typical common emitter current gain was 40. f/sub t/ of 78 GHz and f/sub max/ of 129 GHz were achieved for 3/spl times/5 /spl mu/m/sup 2/ emitter size devices. Maximum f/sub t/ was achieved at 70 kA/cm/sup 2/ collector current density and 2.25 V collector bias. f/sub t/ decreased to 70 GHz at 130 kA/cm/sup 2/ collector current density. F/sub max/ decreased for longer emitter size devices (3/spl times/10 and 3/spl times/15 /spl mu/m/sup 2/) while f/sub t/ maintained comparable values.