Working Principle of Photoresistor

Working Principle of Photoresistor

The working principle of photoresistor is based on the internal photoelectric effect. Electrode leads are installed at both ends of the semiconductor photosensitive material, and it is encapsulated in a tube shell with a transparent window to form a photoresistor as shown in the figure. In order to increase the sensitivity, the two electrodes are often made into a comb shape. The materials that constitute the photoresistor include semiconductors such as metal sulfides, selenides, and tellurides. The conductivity of the semiconductor depends on the number of carriers in the semiconductor conduction band. When the photoresistor is exposed to light, the electrons in the valence band absorb the energy of photons and transition to the conduction band to become free electrons. At the same time, holes are generated, and the appearance of electron-hole pairs makes the resistivity smaller. The stronger the light, the more photogenerated electron-hole pairs there are, and the lower the resistance. When a voltage is applied to both ends of the photoresistor, the current flowing through the photoresistor increases with the increase of light. When the incident light disappears, the electron-hole pairs gradually recombine, the resistance gradually returns to its original value, and the current gradually decreases.