In this work, we give the first report on the physical and functional characteristics of a three-terminal InGaAsP–InP-based double heterostructure optoelectronic switch (DOES) device. In this device, electrical contact is made to the emitter, collector, and active layers while optical input is made via a window in the emitter contact. The DC current–voltage characteristic of a 100 μm × 80 μm device exhibits a switching voltage of 6.6 V with an OFF-state resistance of 200 kΩ and an ON-state resistance of <50 Ω. In conjunction with the electrical outputs, optical emission is generated with a ratio of >10 in the light emission between the ON (2 mA) and OFF states. We show that injection of current and (or) injection of light into the active region reduces the switching voltage. This is used to switch the device from the OFF state to the ON state without having to alter the output circuit bias (emitter–collector bias). The ability to switch the three-terminal InGaAsP–InP DOES with both electrical and optical inputs demonstrate its functionality, flexibility, and suitability for use in optoelectronic switching applications and for InP-based optoelectronic integrated circuits.