Base-collector leakage currents in InP/InGaAs double heterojunction bipolar transistors

For InP/InGaAs HBT's, base-collector leakage current can restrict their operation to a narrow emitter-collector voltage range. We studied several factors that can degrade the leakage current: the base-collector junction design, the base mesa etching technique, and the base metallization process. A step-graded base-collector heterojunction offered the best results. A leakage free multiple step etching process, combining wet and dry etching, has been developed. Ti/Pt/Au is a suitable base metallization, provided that the platinum layer is not too thick, and that the contact annealing temperature is not too high. Finally, very low leakage current HBT's were fabricated.