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 (V-E21)-controlled terminal at a fixed base biasing current while the other one (El) is at the ground state. On the other hand, the base terminal controls the operation by applying an additional current (I-Bdc) 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 V-E21 = 0.4 V, I-Bdc = 0.001 mu A, and the incident optical power (P-in) of 0.423 mu W, it shows the maximum current-dependent current-tuning efficiency and gain-tuning efficiency of 21.40 and 73.87 mu A(-1), respectively. In addition to the power-and voltage-tunable optical gains, a current-tunable one is also available in the 4T-DEPT. (c) 2006 Elsevier Ltd. All rights reserved.