Abstract
The responsivity characteristics of heterojunction phototransistors (HPTs) with a strained InAs/InGaAs multiquantum well (MQW) absorption layer inserted in the base or collector are investigated. It is shown that although the hetero-emitter injects hot electrons into the base, the effective electron diffusion length in the base with MQWs becomes five times lower than that of a base without MQWs. This results in higher current gain for HPTs with MQWs in the collector. In addition, enhanced absorption coefficient due to excitonic absorption is observed only on HPT with MQW in the collector. Due to these two factors, a high responsivity of more than 10 A/W is realized at a wavelength around 2.35 µm for the device with MQWs in the collector.