Virtually all the P-N junctions exhibit characteristics which change when electromagnetic rays strike them. The reason that the conventional diodes are not affected by these rays is that most of the diodes are enclosed in opaque packages. Some of the photosensitive diodes have variable resistance which depends on light intensity. Others actually generate direct current voltages in presence of electromagnetic radiation.
A silicon diode, kept in the transparent case and constructed in such a way that visible light can strike barrier between P-type and N-type materials, forms photodiode.
A reverse bias is applied to the device. At the time when the light falls on junction, current flows. The current is directly proportional to the intensity of light, within particular limits. Silicon photodiodes are more sensitive at some wavelengths than at others. The greatest sensitivity is in near infrared part of spectrum, at wavelengths a little longer than visible red light.
When light of variable brightness falls on the P-N junction of a reverse-biased silicon photodiode, output current follows light intensity variations. This makes the silicon photodiodes quite useful for receiving modulated light signals of kind used in fiberoptic systems.