The Effect on Base Bias for InGaP/GaAs Heterojunction Phototransistors

We reported on characterization of 2-terminal and 3-terminal InGaP/GaAs heterojunction phototransistors (HPTs). For a current-bias 3T-HPT, an independent current flowing into or out of base electrode is employed. Addition of biased current pushes HBT's operating point to a higher current level where the current gain is larger. It is found that the optical gain increases from 28.4 for a 2T-HPT to 34 for a 3T-HPT with a biased current of 10 μA. Although an independent voltage source also pushes HBT's operating point to a higher current level, p-i-n photocurrent generated within the B-C region contributes very few to final collector photocurrent for a voltage-bias 3T-HPT. The optical gain obtained from voltage-bias 3T-HPT is as small as 0.8 ∼ 1.6.