Photodiode

A photodiode consists of a back-biased p-n junction which, under dark conditions, behaves as a normal back-biased diode. With these conditions the only current flowing through the diode ‘A’ will be the usual leakage current (typically 1 μA). Absorbed light will generate electron/hole pairs as described above, and the current through the diode will increase to a typical value of 100 μA. A photodiode has the response similar to B which shows that it can be considered as a constant current device with the current determined by the light intensity. The current/intensity relationship is quite linear, and the response is fast; typically 0.2 /μs but devices as fast as 1 ns are available. In general, photodiodes are the smallest optical sensor which, in conjunction with their high speed, makes them well suited for fibre optic data transmission and similar applications. Typical operating wavelengths are 8000 A to 11 000 A (silicon) and 13 000 A to 20 000 A (germanium). The relatively low level current can easily be converted to a high-level voltage using a DC amplifier. The light-dependent diode current flows through R to give an out put voltage IR which is directly related to the light intensity.

A phototransistor is a variant of the photodiode. A photodiode is connected between collector and base so that the light-dependent diode current becomes the transistor base current. The transistor multiplies this by its current gain, β, to give a far larger collector/emitter current change. Unfortunately the dark current is also multiplied by β. A typical device has the response, with a change from dark to light of 0.1 mA to 10 mA.

Using similar ideas it is also possible to construct photothyristors, photoFETs, and photoDarlingtons, but these are relatively rare.

Questionnaire:

• What is photodiode? Explain