A reverse biased Semiconductor PhotoDiode passes only a very small leakage current (a fraction of 1μA in silicon diodes), if the junction is exposed to light. Under illumination, however, the current rises almost in direct proportion to the light intensity. Hence, the photo-diode can be used for the same purposes as a photoconductive cell.
This device, when operated with a reverse voltage applied, functions as a photo-conductive cell. When operated without reverse voltage it operates as a photo-voltaic cell.
A photo-diode can also be arranged to change from photo-conductive to photo-voltaic mode.
The response time of a photo-diode is very fast, so that it may be used in applications where light fluctuations occur at high frequency, but a photo-diode cell is useful only at very low frequencies.
The symbol and illumination characteristics are as shown in Figs. 13.35(a) and (b).
The Photo Transistor
The sensitivity of a photo diode can be increased by as much as 100 times by adding a junction, resulting in an NPN device. A simple representation of the construction is shown in Fig. 13.36.
Illumination of the central region causes the release of electron hole pairs. This lowers the barrier potential across both junctions, causing an increase in the flow of electrons from the left region into the centre region and on to the right region.
For a given amount of illumination on a very small area, the photo-transistor provides a much larger output current than that available from a photo diode, i.e. a photo-transistor is more sensitive.
Arrays of transistors and low current photo diodes are widely used as photo detectors for such applications as punched card and tape readouts. Photo-transistors are more sensitive than photo-diodes, but the latter have a faster switching time.
One application of a photo transistor is shown in Fig. 13.37.
The light incident on the photo-transistor causes its current to increase and therefore increases both the voltage drop across 50 kΩ and the input to the transistor which drives the relay. This raises the current to the operational level.
Photo Voltaic Cell
The photo-voltaic or solar cell, produces an electrical current when connected to a load. Both silicon (Si) and selenium (Se) types are known for these purposes.
Multiple unit silicon photo-voltaic devices may be used for sensing light in applications such as reading punched cards in the data processing industry.
Gold-doped germanium cells with controlled spectral response characteristics act as photo-voltaic devices in the infra-red region of the spectrum and may be used as infra-red detectors.
The silicon solar cell converts the radiant energy of the sun into electrical power. The solar cell consists of a thin slice of single crystal P-type silicon, up to 2 cm2 into which a very thin (0 5 micron) layer of N-type material is diffused. The conversion efficiency depends on the spectral content and intensity of illumination.