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;A phototransistor is an optoelectronic semiconductor device, a variant of a bipolar transistor. It differs from a conventional bipolar transistor in that the semiconductor base layer of the device is accessible for exposure to external optical radiation, due to which the current through the device depends on the intensity of this radiation.
It differs from a photodiode in that it has an internal amplification of the photocurrent and therefore is more sensitive to optical radiation fluxes. The phototransistor can have a semiconductor structure of both an n-p-n and a p-n-p transistor.
Most commercial types of phototransistors do not have an electrical base lead, but some models do have such a lead and usually serve to shift the starting operating point of the device by supplying some current to the base.
A bipolar phototransistor is a semiconductor device with two p-n junctions and three layers of an alternating type of semiconductor - an analogue of a conventional bipolar transistor with a base current control. But in a phototransistor, the base current is the photocurrent. When the base layer of the phototransistor is illuminated, electron-hole pairs are generated in its base due to the internal photoelectric effect, generating a photocurrent. This process lowers the potential barrier from the contact potential difference in the emitter-base junction, which increases the diffusion of minority carriers (for the base) from the emitter to the base, that is, we can assume that the photocurrent in this device is the base current of a conventional transistor. We can say that the phototransistor is similar to a conventional bipolar transistor, with a reverse-biased photodiode connected between the collector and base terminals.
A bipolar phototransistor is a semiconductor device with two p-n junctions and three layers of an alternating type of semiconductor - an analogue of a conventional bipolar transistor with a base current control. But in a phototransistor, the base current is the photocurrent. When the base layer of the phototransistor is illuminated, electron-hole pairs are generated in its base due to the internal photoelectric effect, generating a photocurrent. This process lowers the potential barrier from the contact potential difference in the emitter-base junction, which increases the diffusion of minority carriers (for the base) from the emitter to the base, that is, we can assume that the photocurrent in this device is the base current of a conventional transistor. We can say that the phototransistor is similar to a conventional bipolar transistor, with a reverse-biased photodiode connected between the collector and base terminals.
