A photonic switching device with very large gain and high sensitivity has been developed by the vertical and direct integration of a heterojunction phototransistor and a laser diode. The device switches on with very low input power of ~10 nW and emits output power of ~ 4 mW under continuous-wave condition at room temperature. The minimum energy for switching on is estimated to be as low as 80 fJ. The internal optical feedback of the device is quantitatively discussed to interpret the low-power operation for the switch-on. © 1992 IEEE