Improved performance of carbon-doped GaAs base heterojunction bipolar transistors through the use of InGaP

Abstract

Carbon-doped GaAs/AlGaAs heterojunction bipolar transistors (HBTs) typically exhibit severe leakage at the base-emitter interface which limits their utility for low-current applications. Furthermore, the device breakdown voltage, and hence power handling capability, is limited due to the band gap of the GaAs collector material. In this letter we will demonstrate for the first time that both of these limitations can be overcome through the use of InGaP. Since InGaP is not readily doped with carbon, it does not suffer from compensation due to carryover of carbon from the GaAs base. Hence, the ideality factor of the base-emitter junction improves from 1.3 to 1.09 when the conventional n-AlGaAs emitter layer is replaced with n-InGaP. Moreover, InGaP eliminates the crossover of the base and collector currents typically observed in heavily carbon doped GaAs HBTs. This results in the maintenance of gain even at very low collector currents. As a collector material, we have found that InGaP produces significantly higher breakdown voltage than GaAs (19 V vs 12 V) of the same thickness and doping, due to its larger band gap. As in the emitter, InGaP collectors exhibit excellent ideality factors of ∼1.05.