PHOTODIODE

Photodiodes are semiconductor light sensors that

generate a current or voltage when the P-N junction

in the semiconductor is illuminated by light.

When a photon of sufficient energy strikes the

diode, it excites an electron, thereby creating a free

electron(and a positively charged electron hole).

This mechanism is also known as the inner

photoelectric effect.

This device can be used in three modes:

photovoltaic as a solar cell, reversed–biased as a

photo detector, and forward–biased as an LED.

A photodiode is a type Of photo

detector capable of converting light energy

into electrical energy.

Photodiodes are similar to

regular semiconductor diode except that they

may be either exposed (to detect UV or X-rays)

or packaged with a window or optical fiber to

allow light to reach the sensitive part of the

device

A photodiode is designed to operate in reverse

bias.

Features of photodiode

Excellent linearity with respect to incident light

Low noise Wide spectral response Mechanically rugged Compact and lightweight Long life

Materials commonly used to produce photodiodes include:

MATERIALS ELCTROMAGNETIC SPECTRUM WAVELENGTH RANGE (NM)

SILICON 190-1110

GERMANIUM 400-1700

INDIUM GALLIUM ARSENIDE

800-2600

LEAD SUFIDE 100-3500

PHOTODIODE CONSTRUCTION

CROSS SECTION OF THE SILICON PHOTODIODE

N type silicon is the starting material. A thin "p" layer is formed on the front surface of the device by thermal diffusion or ion implantation of the appropriate doping material (usually boron).

The interface between the "p" layer and the "n" silicon is known as a pn junction. Small metal contacts are applied to the front surface of the device and the entire back is coated with a contact metal.

The back contact is the cathode, the front contact is the anode. The active area is coated with either silicon nitride, silicon monoxide or silicon dioxide for protection and to serve as an anti-reflection coating.

The thickness of this coating is optimized for particular irradiation wavelengths. As an example, a Centro Vision Series 5-T photodiode has a coating which enhances its response to the blue part of the spectrum.

At the PN junction there will a concentration gradient that causes electrons to diffuse into the p-layer and holes to diffuse into the n-layer. This diffusion results in an opposing electrical potential, often referred to as an internal bias (depletion region).

In a generic p-n photodiode, light enters the device through the thin p-type layer. Absorption causes light intensity to drop exponentially with penetration depth.

Any photons absorbed in the depletion region produce charge carriers that are immediately separated and swept across the junction by the natural internal bias. Eventually there will be the movement of the charge carriers.

This movement of charge carriers across the junction upsets the electrical balance and produces a small photocurrent, which can be detected at the electrodes.

In many applications it is desirable to maximize the thickness of the depletion region. For example, device response is faster when most of the charge carriers are created in the depletion region.

This also increases the quantum efficiency of the device, since most charge carriers will not have the opportunity to recombine. The quantum efficiency is defined as the ratio of the photocurrent in electrons to incident light intensity in photons.

APPLICATIONS Photodiodes are used in consumer electronics devices such

as CD players, smoke detectors, and the receivers for infrared

remote control devices  used to control equipment from

televisions  to air conditioners.

Photodiodes are used as a light sensors.

Photodiodes are often used for accurate measurement of light

intensity in science and industry.

They are also widely used in various medical applications, such

as detectors for computer tomography, instruments to analyze

samples, and pulse oximeters.