Photodiodes are semiconductor components that convert light into voltage (photoelectric effect) or photocurrent. P-N junctions in silicon semiconductors are the physical basis for this process. Photons with sufficient energy are absorbed by the detector to form photogenerated carriers (electron-hole pairs). The photocurrent is generated after the photogenerated carriers are separated in the space charge region.

Photogenerated carriers can also be separated without applying an external voltage. The process can be accelerated with a reverse voltage. If the diode is not operated in saturation, the photocurrent and absorbance values can maintain a linear relationship over many orders of magnitude.

Depending on the external connection, we divide the operating states into two categories: element operation and diode operation. During component operation, the diodes are connected directly to the electrical terminals, eliminating the need for an external voltage source. In this working state, no dark current is generated in the system, which facilitates the detection of minimum intensity changes.

In the diode operating state, an external voltage source is connected in series with the electrical terminal to apply a reverse voltage to the diode. This mode of operation is ideal for applications requiring a fast signal response. But its main disadvantage is that the dark current increases exponentially with increasing temperature.

A PIN diode adds an intrinsic semiconductor layer between the P-type semiconductor and the N-type substrate to form an intrinsic semiconductor region (usually a space charge region). However, the term can also be used for components with reverse conductivity, provided that the component does not involve other nonlinear effects.