A photodetector that consists of a bipolar junction transistor operated with the base region floating. The potential of the base region is determined by the number of charge carriers stored in it. The electromagnetic (usually ultraviolet) radiation to be detected is applied to the base of the transistor and produces the base photocurrent; the transistor is operated essentially in common-emitter connection.

The collector current is essentially equal to the current generated in a p-n junction photodiode multiplied by β, the beta-current gain factor. β is measured when the same structure is used as a simple transistor with a base contact. A typical structure can have a very large value of β (about 100) and therefore a greatly increased sensitivity is possible compared to the p-n junction photodiode. The speed of operation of the phototransistor however is comparatively less due to the time required to charge the base region to a sufficient potential to realize the transistor action.

During the response time the emitter current rises from zero to a steady-state value determined by the rate of generation of excess minority carriers in the base. If the intensity of illumination reaches a sufficiently high value the collector current becomes limited by the external circuit components and the transistor saturates.