Department of Physics,
Swami Shraddhanand College
(University of Delhi) Delhi, INDIA
Brijender Dahiya and Vinod Prasad
Department of Physics, Swami Shraddhanand College
(University of Delhi) Delhi, INDIA
Layout
Introduction
 Principle of Operation
 Photovoltaic Mode
 Photoconductive Mode
 Material/ Construction
 Applications
 Conclusion

INTRODUCTION
 A photodiode
is a type of photo- detector
capable of converting light into either
current or voltage, depending upon the
mode of operation.
 The common, traditional solar cell used
to generate electric solar power is a large
area photodiode.
 Silicon
photodiodes are semiconductor
devices responsive to high energy
particles and photons.
 Photodiodes
operate by absorption of
photons or charged particles and
generate a flow of current in an
external circuit, proportional to the
incident power.
Photodiodes are similar to regular
semiconductor diodes except that they
may be either exposed (to detect
vacuum UV or X-rays) or packaged with
a window or optical fiber connection to
allow light to reach the sensitive part of
the device.
Symbol of a Photodiode
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. A photodiode is
designed to operate in reverse
bias.
PIN Junction or PIN Diode

A PIN diode is a diode with a wide, lightly
doped 'near' intrinsic semiconductor region
between a p-type semiconductor and an ntype semiconductor region. The p-type and
n-type regions are typically heavily doped
because they are used for ohmic contacts.
Other Importance of PIN Junction
 The wide intrinsic region is in contrast
to an ordinary PN diode. The wide
intrinsic region makes the PIN diode
an inferior rectifier (one typical function
of a diode), but it makes the PIN diode
suitable for attenuators, fast switches,
photo- detectors, and high voltage
power electronics applications
Principle of Operation
There are three major parts of
the principle of operation
 Photovoltaic
Mode
 Photoconductive Mode
 Other Modes of Operation
Principle of Mechanism



A photodiode is a p-n junction or PIN structure. When a
photon of sufficient energy strikes the diode, it excites an
electron, thereby creating a free electron and hole.
(photoelectric effect).
If the absorption occurs in the junction's depletion region,
or one diffusion length away from it, these carriers are
swept from the junction by the built-in field of the depletion
region. (Holes
anode, Electrons
cathode)
The photocurrent is the sum of both the dark current
(without light) and the light current, so the dark current
must be minimized to enhance the sensitivity of the
device.
 Photovoltaic Mode
When used in zero bias or
photovoltaic mode, the flow of
photocurrent out of the device is
restricted and a voltage builds up.
This mode exploits the
photovoltaic effect, which is the
basis for solar cells.
 Photoconductive mode
In this mode the diode is often reverse
biased, dramatically reducing the
response time at the expense of
increased noise. This increases the
depletion layer, decreasing the junction's
capacitance resulting in faster response
times.
 For a given spectral distribution, the
photocurrent is linearly proportional to
the illuminance.

The Two modes
Other modes of operation

Avalanche photodiodes have a
similar structure to regular
photodiodes, but they are operated
with much higher reverse bias. This
allows each photo-generated carrier
to be multiplied by avalanche
breakdown, resulting in internal gain
within the photodiode, which
increases the effective responsivity
of the device.
 A phototransistor is in essence a
bipolar transistor encased in a
transparent case so that light can reach
the base-collector junction. The electrons
that are generated by photons in the
base-collector junction are injected into
the base, and this photodiode current is
amplified by the transistor's current gain
β. If the emitter is left unconnected, the
phototransistor becomes a photodiode.
Phototransistors also have significantly
longer response times.
Material/ Construction
The material used to make a photodiode is
critical to defining its properties, because only
photons with sufficient energy to excite
electrons across the material's bandgap will
produce significant photocurrents.
 Materials commonly used to produce
photodiodes include

Material
λ Range (nm)
Material
λ Range (nm)
Silicon
190-1700
Germanium
400-1700
InGaAs
800-2600
Pb(II)Sulfide
<1000-3500
Because of their greater bandgap, Si-based photodiodes generate less
noise than Ge-based photodiodes.
Applications
Photodiodes have wide range of
applications
 Consumer Electronics
 Auto- circuits
 Light measurements
 Medicals
 Optical Communication
 Scientific Instruments
Consumer Electronics
CD Player
 Smoke Detector
 Receivers for IR remote control Devices
 Camera light meters
 Street Light Switches etc.

Auto-circuits
Combined with a light source photodiode is
used for auto circuits such as : Various types of Safety alarms
 Counters
 Slotted optical Switch
 Auto electric switches for low power
consumption, etc.
animation
Light measurements
 Photodiodes
are often used for
accurate measurement of light
intensity in science and industry.
 They generally have a more linear
response than photoconductors.
Medicals
 They
are also widely used in
various medical applications,
such as detectors for computed
tomography (coupled with
scintillators), instruments to
analyze samples (immunoassay),
and pulse oximeters.
Optical Communication
 PIN
diodes are much faster
and more sensitive than p-n
junction diodes, and hence
are often used for optical
communications and in
lighting regulation.
Scientific Instruments

P-N photodiodes are not used to
measure extremely low light intensities.
Instead, if high sensitivity is needed,
avalanche photodiodes, intensified
charge-coupled devices or
photomultiplier tubes are used for
applications such as astronomy,
spectroscopy, night vision equipment
and laser rangefinding.
Conclusion
Silicon photodiodes are semiconductor devices
responsive to high energy particles and photons.
Photodiodes can be used to detect minute quantities
of light and can be calibrated for extremely accurate
measurements. Silicon photodiodes are utilized in
such diverse applications as spectroscopy,
photography, analytical instrumentation, optical
position sensors, beam alignment, surface
characterization, laser range finders, optical
communications, daily life consumer devices and
medical imaging instruments. Hence making them
the Multipurpose Device.