Phototransistor

a transistor, usually bipolar, in which minority carriers are injected on the basis of an internal photoelectric effect. Phototransistors are used to convert light signals into amplified electric signals.

A phototransistor consists of a single-crystal Ge or Si semiconductor wafer in which three regions are produced by means of special technological processes. As in a conventional transistor, the regions are called the emitter, collector, and base; as a rule, the base has no lead. The crystal is placed in a housing with a transparent window. A phototransistor is connected to an external circuit in the same way as a bipolar transistor with a common-emitter connection and a zero base current. When light is incident on the base or collector, charge-carrier pairs (electrons and holes) are generated in that region; the carrier pairs are separated by the electric field in the collector junction. As a result, the carriers accumulate in the base region, causing a reduction of the potential barrier in the emitter junction and an increase, or amplification, of the current across the phototransistor in comparison with the current that is due only to the migration of carriers generated directly by the action of the light.

As with other photoelectric devices, such as photocells and photodiodes, the main parameters and characteristics of photo-transistors are the luminous sensitivity, spectral response, and time constant. The luminous sensitivity is the ratio of the photoelectric current to the incident luminous flux. For the best specimens of phototransistors—for example, diffused planar devices—the luminous sensitivity may be as high as 10 amperes per lumen. The spectral response, which is the sensitivity to monochromatic radiation as a function of wavelength, defines the long-wavelength limit for the use of a particular phototransistor; this limit, which depends primarily on the width of the forbidden band of the semiconductor material, is 1.7 micrometers for germanium and 1.1 micrometers for silicon. The time constant characterizes the inertia of a phototransistor and does not exceed several hundred microseconds. In addition, a phototransistor is characterized by the photoelectric gain, which may be as high as 10²–10³.

The high reliability, sensitivity, and temporal stability of phototransistors, as well as their small size and relatively simple design, have led to their extensive use in control and automation systems, for example, as light detectors and as components of optoisolators (seeRADIATION DETECTOR, OPTICAL DETECTOR, and OPTRON). Field-effect phototransistors, which are similar to field-effect transistors, were developed in the 1970’s.