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A junction transistor that may have only collector and emitter leads or also a base lead, with the base exposed to light through a tiny lens in the housing; collector current increases with light intensity, as a result of amplification of base current by the transistor structure.



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.


Ambroziak, A. Konstruktsiia i tekhnologiia poluprovodnikovykh fotoelektricheskikh priborov. Moscow, 1970. (Translated from Polish.)



A transistor that uses light rather than electricity to cause an electrical current to flow from one side to the other. It is used in a variety of sensors that detect the presence of light. Phototransistors combine a photodiode and transistor together to generate more output current than a photodiode by itself. See photoelectric, photodiode and transistor.
References in periodicals archive ?
This feat was never achieved in current phototransistors.
In Fairchild's OptoHiT[TM] Series, the FODM8801 is a first-of-its kind phototransistor, utilizing Fairchild's leading-edge proprietary process technology to achieve high operating temperature characteristics, up to 125[degrees]C.
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Infrared systems also are susceptible to water, dust and dirt, which can obstruct the phototransistors, leading to either false input or "dead" touch zones.
While many phototransistors are fabricated on inflexible surfaces and, as a result, are flat, the new phototransistors are bendable, meaning they can easily imitate the conduct of mammalian eyes.
Koppens at ICFO, have demonstrated a QD/2-D(graphene) phototransistor with a photoresponse up to 5 orders of magnitude higher than phototransistors based on single graphene or MoS2 atomic layers without QDs, showing the potential of QD/2-D hybrid devices for photovoltaics.
Light-sensing technology, in the form of photodiodes and phototransistors, was invented around the 1950s.
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Devices on display will consist of 5 MBd high-speed optocouplers, matched infrared emitters and phototransistors, high-brightness 0603 ChipLEDs, 2.
As shown in Figure 1, phototransistors have a wide bandwidth but with a peak sensitivity at around 800 nm.
This book presents the electrical models for the different elements of a photonic microwave link like lasers, external modulators, optical fibers, photodiodes and phototransistors.