PINdiodes

A PIN diode is a semiconductor device consisting of a layer of intrinsic (undoped) semiconductor sandwiched between a p-type and an n-type region. The intrinsic layer widens the depletion region that forms under bias, giving the device a combination of high resistance in reverse operation and controllable conduction in forward operation. The structure is typically fabricated from silicon or compound semiconductors such as GaAs, InP, or GaN, with the intrinsic layer being lightly doped or nearly undoped to maximize its effect on the electric field and carrier dynamics.

Operation and characteristics. Under reverse bias, the depletion region expands through the intrinsic layer, resulting in very low capacitance and a high impedance, making the PIN diode suitable for high-frequency and high-voltage applications. Under forward bias, the intrinsic region becomes populated with carriers, and the device conducts, acting as a resis­tive element whose value is controlled by the forward current. The presence of the intrinsic layer allows the device to function effectively at microwave frequencies and to handle large signals with relatively fast switching times, though switching speed depends on carrier lifetime and circuit RC constants. The diode is often modeled by a series resistance and a parallel capacitance, reflecting its on-state resistance and off-state capacitance.

Applications. PIN diodes are widely used as RF switches and attenuators in microwave and optical systems, where their bias-dependent impedance provides fast, controllable switching with low capacitance in the off state. They serve as photodiodes in fiber-optic receivers, converting light into current in the intrinsic region. Other uses include limiter devices for overload protection, high-voltage rectifiers in power electronics, and various optoelectronic and microwave circuits.

See also: photodiode, Schottky diode, meaning of intrinsic region, RF switch.