2.  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.

3.  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.

4. Features of photodiode
 Excellent linearity with respect to incident light
 Low noise
 Wide spectral response
 Mechanically rugged
 Compact and lightweight
 Long life

5.  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

6. PHOTODIODE CONSTRUCTION
CROSS SECTION OF THE SILICON PHOTODIODE

7.  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.

8.  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.

9.  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.

10.  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.