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.
In each converter, which converts the element is based on certain physical principle, which is associated with the electrical characteristics of the device so that a change in the quantity being measured >>>