Low optical power characterization of a base current-biased four-terminal dual-emitter heterojunction phototransistor

This paper reports on a four-terminal dual-emitter heterojunction phototransistor (4T-DEPT) with a base biased by a current source in comparison with a three-terminal dual-emitter heterojunction phototransistor (3T-DEPT). While only voltage can be used to tune the optical performance of the 3T-DEPT, voltage- and current-control modes are considered for the 4T-DEPT. One emitter (E2) serves as the voltage (VE21)-controlled terminal at a fixed base biasing current while the other one (E1) is at the ground state. On the other hand, the base terminal controls the operation by applying an additional current (IBdc) bias when the VE21 is constant. Therefore, voltage- and current-dependent characteristics will be detailed. Four operating regions exist in the voltage-dependent characteristics for both 3T- and 4T-DEPTs: negative-saturation, negative-tuning, positive-tuning, and positive-saturation regions. When the 4T-DEPT operates at the bias condition of VE21 = 0.4 V, IBdc = 0.001 μA, and the incident optical power (Pin) of 0.423 μW, it shows the maximum current-dependent current-tuning efficiency and gain-tuning efficiency of 21.40 and 73.87 μA−1, respectively. In addition to the power- and voltage-tunable optical gains, a current-tunable one is also available in the 4T-DEPT.