It is a three region reverse biased junction diode. A layer of intrinsic silicon is sandwiched between two heavily doped P and N type silicon materials. This has the effect of reducing the transit time of photo-induced electron-hole pairs. It is so because carries generated by light photons incident on the middle of the I-layer have less distance to travel than if generated at one or the other side of the layer. Hence, such diodes have faster response than even the P-N photodiode. Moreover, the relatively thick I- layer ensures the absorption of most of the incident light. The reverse current increases linearly with the level of illumination.
PIN photodiode are ultrafast having a switching speed of nanosecond or so. They have broad spectral response and generate very low noise. They are capable of processing very weak signals. Hence, such devices are widely used for detecting laser pulses and in ultrafast switching and logic circuit.
