The sensitivity of the photodiode may be raised to a value greater than about 100 times by adding a junction resulting in an NPN transistor. The phototransistor is normally connected in a common emitter configuration with the base open. The illumination (radiation) is concentrated in the region near the base junction of the emitter. A DC voltage V is applied between the emitter and the manifold in such a way that the manifold connection is reverse polarized and the applied emitter voltage appears through the inverter manifold connection.
Initially there is no radiant excitation (illumination) in this condition, the minority carriers are thermally generated and the electrons that cross from the base to the manifold as well as the holes that pass through the manifold to the base constitute the inverse saturation current Ico. With Ib = 0 the collector is given by IC = (1 + BETA) ICO.