biosensors and transducers

1. PHOTODIODE
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Mukesh Sundararajan

2. Photodiodes
▶ Photodiodes are semiconductor light sensors that
generate a current or voltage when the P-N junction in
the semiconductor is illuminated by light. It is a type of
photodetector capable of converting light energy into
electrical energy in either current or voltage, depending
upon the mode of operation. Solar cell is an example of
it.

3. ▶ 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.

4. Structure

6. Operation
▶ A photodiode is designed to operate in reverse bias.
▶ Photoelectric effect
The photoelectric effect is the observation that many
metals emit electrons when light shines upon them.
Electrons emitted in this manner can be called
photoelectrons.
▶ Movement of those electrons give rise to photocurrent.

7. Characteristics
▶ 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.
Many diodes designed for use specifically as a
photodiode use a PIN junction rather than a p-n
junction, to increase the speed of response.

8. Characteristics

9. Types of photodiode
▶ Pin photodiode
▶ P-N photodiode
▶ Avalanche photodiode

10. Features of Photodiode
❑ Excellent linearity with respect to
incident light
❑ Low noise
❑ Wide spectral response
❑ Mechanically rugged
❑ Compact and lightweight
❑ Long life

11. ▶ 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

12. PHOTODIODE CONSTRUCTION
CROSS SECTION OF THE SILICON PHOTODIODE

13. ▶ 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 p-n 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.

14. ▶ 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.

15. ▶ 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.

16. 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.