Two terminal InP/InGaAs heterojunction phototransistor with lateral photodiode as sensing section

A heterojunction bipolar transistor (HBT) is known as a high-speed transistor. However, a speed of a two terminal heterojunction phototransistor (2T-HPT) is generally very low due to so small self-bias-current generated by an weak incident optical signal. In this paper, we describe the fabrication and characterization of a lateral InGaAs photodiode and (utilizing it) 2T-InP/InGaAs HPT. The responsivity of the photodiode is 0.2 A/W at a wavelength of 1.55 μm. The current gain of the amplifier section is 25. The cut-off frequency, under a low incident power of 10 μW, was improved 9 times by narrowing the emitter area from 5 μm×100 μm to 3 μm× 15 μm.