Photodiode Technology

A photodiode is a semiconductor diode designed to convert light into an electrical signal. Like other diodes it has a PN junction, but its junction is exposed to light, and when photons are absorbed there they free charge carriers that produce a current. This makes the photodiode the reverse of a light-emitting diode: where an LED turns current into light, a photodiode turns light into current. The current it produces is proportional to the intensity of the light striking it over a wide range, which makes it useful for measurement as well as detection.

Photodiodes are usually operated under reverse bias, which widens the junction’s depletion region, lowers its capacitance and speeds its response. The PIN photodiode adds an undoped (intrinsic) layer between the p and n regions, enlarging the light-sensitive volume and allowing very fast operation; this makes it well suited to high-speed optical communication. An avalanche photodiode is operated at a high reverse voltage so that the carriers it generates multiply through impact ionisation, giving internal gain and high sensitivity for detecting very weak light.

The spectral response of a photodiode — the range of wavelengths to which it reacts — depends on its semiconductor material, so devices are chosen to match the light they must detect, whether visible, infrared or other. Photodiodes are found in light meters, optical fibre receivers, barcode readers, remote-control receivers and many sensing systems where light must be measured quickly and accurately.

Frequently asked questions

How does a photodiode work?

Light absorbed at its PN junction frees charge carriers, producing a current proportional to the light intensity. It is effectively the opposite of an LED, turning light into current rather than current into light.

What is a PIN photodiode?

It is a photodiode with an undoped intrinsic layer between the p and n regions. This enlarges the light-sensitive area and lowers capacitance, giving a fast response suited to optical communication.

What is an avalanche photodiode?

It is operated at a high reverse voltage so that generated carriers multiply through impact ionisation, providing internal gain and high sensitivity for detecting very weak light.