In this paper, we present a monolithic integration of a microlens and an InP/InGaAs heterojunction bipolar phototransistor (HPT). Three types of HPTs were fabricated and their optical characteristics were investigated. The optical gain of the proposed HPT was improved 6-fold compared to the front type HPT. The,HPT with a back-side microlens showed 0.61 A/W de responsivity (PD mode) and that of an HPT without a lens showed 0.18 A/W (PD mode) at 1.55 mu m illumination. Quantum efficiency was increased by 34.8%. The fiber alignment tolerance has also been improved. The HPT with a lens showed a 40 mu m tolerance, while the HPT without. a lens exhibited a 22-mu m tolerance at 3 dB below the maximum values.