Effect of surface passivation with SiN on the electrical properties of InP/InGaAs heterojunction bipolar transistors

The effects of the SiN layer normally used to passivate and protect the exposed junction surfaces in InP/InGaAs heterojunction bipolar transistors have been studied and shown to degrade the transistor properties. These effects are ascribed primarily to surface damage associated with the high SiN deposition temperature (350 °C). A degradation of the emitter-base properties was observed through the nonideal behavior of the base current and the measured short minority-carrier lifetime in the base, extracted by using the base width modulation method. Degradation in the current gain and emitter injection efficiency was also observed. A clear recovery of the transistor was observed after removing the SiN passivation layer indicating that the high SiN deposition temperature results in a high-surface-state density which increases the surface recombination velocity and degrades the junction properties. It is concluded that a low-temperature deposition and good quality dielectric are necessary to exploit the excellent electrical properties of InP-based heterojunction bipolar transistors.