The Characteristics of Functional Phototransistors

Experiments and modeling of functional heterojunction phototransistors (FPTs) are reported for comparing to HPTs without base electrode and conventional HPTs with a base electrode using the same epi-layers. Functional phototransistors having double emitters with different area ratio (A₁:A₂) but a fixed total area together with collector form a three-terminal device. As a voltage-bias emitter instead of a current-bias base is used, all kinds of FPTs exhibit an enhanced collector photocurrent as compared with a HPT with the same total emitter area. Experimental results reveal that 2:1 and 1:1 FPTs exhibit 1.85- and 1.5-fold optical gain of that from a HPT, respectively. Moreover, a new circuit model with three sets of parallel diodes was proposed to demonstrate the performance enhancement and polarity-dependent behaviors. Theoretical results are in very good agreement with experimental ones and indicate that more than 3-fold enhancement is expected