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.
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.