Photodiodes are sensors that convert light into electrical energy. They are components of optoelectronics, which are operated as germanium or silicon diodes in reverse direction and are based on the photoelectric effect. Between the two doped semiconductor layers (P and N) there is an undoped region.
The electron emission of photodiodes is based on the photoelectric effect. This effect causes free electrons to be pulled out of the atomic structure when light is incident, the number of which depends on the light intensity. The reverse current depends on the illumination intensity( lx): the higher the illumination intensity, the higher the reverse current, which is largely independent of the applied reverse voltage.
In the case of photodiodes, a distinction is made between reverse operation, in which the voltage drop across the working resistor is proportional to the illuminance, and elementary operation, which is characterized by a high degree of temperature independence.
Characteristic values of photodiodes
In contrast to the photoresistor, photodiodes have a much lower inertia and can switch signals in the nanosecond and microsecond range. The switching speed depends on the reverse voltage, the higher it is, the shorter the switching times become. This is due to the fact that when the reverse voltage is increased, the capacitance of the junction decreases.
The spectral sensitivity of photodiodes depends on the semiconductor material used. In the case of selenium( Se), the highest spectral sensitivity is between 500 nm and 600 nm, in the case of gallium arsenide( GaAs) between 700 nm and 800 nm, in the case of silicon( Si) between 800 nm and 900 nm, and in the case of germanium at about 1,400 nm, i.e. infrared. Therefore, these devices are used specifically in infrared technology. However, germanium photodiodes have higher inherent noise compared to silicon-based photodiodes.
Examples of photodiodes are the low-cost PIN d iodes and the more sensitive APD diodes. Photodiodes are used in photoelectric sensors, remote controls and in light measurement devices; they are also used in optical transmission systems and in solar cells.