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Photo is detectors
      Semiconductor transducers , intended for measuring of changes of parameters of light radiation, are named a photo is detectors. Photo-electric transducers, being the simplest type ofa photo is detectors , and there is a semiconductor diode. There are a few types of such transducers. One of basic among them is a photo is a diode in which the effect of irradiation р-п- transition  light (visible or other lengths of waves) is used with negative displacement. A current, flowing through a transition, changes at presence of irradiation. Time of response of such photo of diode makes only a few nanoseconds.
      For providing of more rapid reaction on the change of parameters of radiation PIN is worked out - diodes, in which between the layers of р- and n -type there is a layer of unalloyed semiconductor. It promotes a sensitiveness to the light radiation and simultaneously diminishes a transition capacity, due to what a diode quicker reacts on the change of level of measurand.
      In a number of devices a photo is a diode is used together with strengtheners for a sensitisation. Certainly, in an ordinary transistor (three-layered semiconductor device of п-р- n - or р-п-р-type) there is р- n - transition with negative displacement, and a device is able to strengthen a current, i.e. he possesses all necessary properties of fotodiode and strengthener. And all this in one device.
      A phototransistor differs from an ordinary semiconductor triode that he is executed in a transparent corps which skips a light radiation. Light falling on a transition collector-base of phototransistor (р- n - transition with negative displacement), causes in a base a photoelectric which increases with an amplification of transistor factor, that results in the very large current of emitter.
The current of emitter of phototransistor is determined from next correlation:

 IE = (1+ hFE ) IF
where  hFE  amplification of transistor factor on a DC current; IF is a photoelectric of base.
      For achievement of the higher strengthening use fotodetectors of Darlingtona, containing a phototransistor and transistor with high, by a coefficient strengthening, working in the mode of pair of Darlingtona. Both transistors take place in a demihull.
      As a photo is detectors are semiconductor devices, their current of satiation depends on a temperature. Therefore in default of light radiation the so-called dark current flows in them, limiting possibilities of device on measuring of low levels of light radiation.

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See also:
Temperature sensors, thermocouple             Sensors of mechanical stress
Force, pressure, flow                                    Phototransducers
Ionization transducers                                    Hall sensors
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Датчики, преобразователи. Sensors, transducers