Base-collector leakage currents in circular geometry InGaP/GaAs double heterojunction bipolar transistors

Circular geometry InGaP/GaAs double heterojunction bipolar transistors with spacings of 94.5, 44.5, 19, 10.5 and 5.5 mum between the base metal and emitter mesa were fabricated and characterized. The measured Gummel plots showed a current gain of 50 for the 94.5 mum device. As the spacing decreases to 5.5 mum, the base current increases due to the extrinsic base surface recombination current at V-BE = 0.9 V thus reducing the current gain to a value of 10. It was found that the use of Ti/Au over Au/Zn/Au base metallization schemes eliminates this recombination current component of the base thereby improving the current gain significantly to 100. With a novel scheme employed here termed the delta(+)-layer composite collector structure, the collector leakage current was investigated.