Photodiodes
• A photodiode is a PN-junction diode that
consumes light energy to produce an electric
current. They are also called a photo-detector, a
light detector, and a photo-sensor. Photodiodes
are designed to work in reverse bias condition.
• Typical photodiode materials are Silicon,
Germanium and Indium gallium arsenide.
• Symbol :-
Working Principle
• A photodiode is subjected to photons in the form of light
which affects the generation of electron-hole pairs. If the
energy of the falling photons (hv) is greater than the
energy gap (Eg) of the semiconductor material, electronhole pairs are created near the depletion region of the
diode. The electron-hole pairs created are separated
from each other before recombining due to the electric
field of the junction. The direction of the electric field in
the diode forces the electrons to move towards the nside and consequently the holes move towards the pside. As a result of the increase in the number of
electrons on the n-side and holes on the p-side, a rise in
the electromotive force is observed. Now when an
external load is connected to the system, a current flow
is observed through it.
• The more the electromotive force created, the greater
the current flow. The magnitude of the electromotive
force created depends directly upon the intensity of the
incident light. This effect of the proportional change in
photocurrent with the change in light intensity can be
easily observed by applying a reverse bias.
• Under normal condition, it carries small reverse current
due to minority charge carriers.
• When light is incident through glass window on the p-n
junction, photons in the light bombard the p-n junction
and some energy is imparted to the valence electrons.
• Due to this, valence electrons are dislodged from the
covalent bonds and become free electrons. Thus more
electron-hole pairs are generated. Thus total number of
minority charge carriers increases and hence the reverse
current increases. This is the basic principle of operation
of photodiode.
• The reverse current without light in diode is in the range
of micro amperes. The change in this current due to the
light is also in the range of micro amperes. Thus such a
change can be significance observed in the reverse
current.
• If the photodiode is forward biased, the current flow
through it is in mA. The applied forward biased voltage
takes the control of the current instead of the light. The
change in forward current due to light is negligible and
can not be noticed. The resistance of forward biased
diode is not affected by the light.
• Hence to have significant effect of light on the current
and to operate photodiode as a variable resistance
device, is always connected in reverse biased condition.
The wavelengths at which the detector responds to light
depend on the detector’s material composition. The
following graph shows the detector response curve for
different materials.
Performance Specification
• Response (speed/time) of the photodiode is determined
by the capacitance of the P-N junction. It is the time
needed for charge carriers to cross the P-N junction.
This is directly affected by the width of the depletion
region.
• Responsivity is the ratio of photocurrent generated from
incident light, to that incident light power. This is usually
expressed in units of A/W (current over power).
• Dark current is the current in the photodiode when there
is no incident light. This can be one of the main sources
of noise in the photodiode system. Photocurrent from
background radiation can also be included in this
measurement.
• Breakdown Voltage is the largest reverse voltage that
can be applied to the photodiode before there is an
exponential increase in leakage current or dark current.
Photodiodes should be operated below this maximum
applied reverse bias or damage to the photodiode may
occur. Breakdown voltage decreases with an increase in
temperature.
• Other important parameters include material, size of the
photodiode, active area and cost. Careful consideration
is needed when browsing for photodiodes for your
research or application.
• Photodiodes made from different materials (silicon,
germanium, indium gallium arsenide phosphide, or
indium gallium arsenide) have varying levels of
sensitivity as well as differing speeds and dark current.
Advantages & Disadvantages
Adavantages
• Can be used as variable resistance device.
• Highly sensitive to the light.
• The speed of operation is very high.
Disadvantages
• The dark current is temperature dependent.
• The overall photodiode characteristics are
temperature
dependent
hence
have
poor
temperature stability.
• The current and change in current may not be
sufficient to drive other circuits. Hence amplification
is necessary.
Applications
• Photodiodes with the help of optocouplers provide
electric isolation. When two isolated circuits are
illuminated by light, optocouplers are used to couple the
circuit optically.
• Used in safety electronics such as fire and smoke
detectors.
• Used in numerous medical applications as in instruments
that analyze samples, detectors for computed
tomography and also used in blood gas monitors.
• Used in solar cell panels, logic circuits, detection circuits,
character recognition circuits.
• Used for the exact measurement of the intensity of light
in science and industry.
• Used for lighting regulation and optical communication.
PIN Photodiodes
• PIN photodiode has an intrinsic (very lightly doped)
semiconductor region sandwiched between a p-doped
and an n-doped region
• Since the intrinsic (i) region has no free charges, its
resistance is high, so that most of the reverse-biased
voltage is applied to this i region.
• The i region is usually wide so that incoming photons
have a greater probability of absorption in the i region
rather than in the p or n regions.
• Since the electric field is high in the i region, any
electron-hole pairs generated in this region are
immediately swept away by the field. e-h pairs generated
in the p and n regions have to first diffuse into the
depletion region before being swept away. Also, these eh pairs may suffer recombination, resulting in a reduced
current.
• There are many benefits to the added intrinsic layer
because the depletion region is greatly increased. The
capacitance of the junction is decreased, and so the
speed of the photodiode increased. The increased layer
also allows for a larger volume of photon to electron-hole
conversion and higher Quantum Efficiency.
• PIN photodiodes also have high frequency response.
The major advantage of the PIN photodiode, compared
to the P-N junction, is the high response speed from the
increased depletion region.
Avalanche photodiodes (APD)
• Avalanche photodiodes (APD) use
impact ionization (avalanche effect)
to create an internal gain in the
material. APDs require high
reverse bias operation (near
reverse breakdown voltage).
• Each
photo-generated
carrier
creates more pairs and so is
multiplied
by
avalanche
breakdown. This creates internal
gain within the photodiode, which
in turn increases the effective
responsivity
(larger
current
generated per photon).
• The construction of both the PIN photodiode and
Avalanche photodiode is similar. This diode includes two
heavily doped & two lightly doped regions. Here, heavily
doped regions are P+ & N+ whereas lightly doped
regions are I & P.
• In the intrinsic region, the depletion layer width is fairly
thinner in this diode as compared to the PIN photodiode.
Here, the p+ region works like the anode whereas the n+
region acts as the cathode.
• As compared to other photodiodes, this diode works in a
high reverse bias condition. So this allows avalanche
multiplication of the charge carriers formed through the
light impact or photon. The avalanche action allows the
gain of the photodiode to be enhanced several times to
provide a high range of sensitivity.
Working
• Avalanche
breakdown
occurs
mainly
once
the photodiode is subjected to maximum reverse
voltage. This voltage enhances the electric field beyond
the depletion layer. When incident light penetrates the
p+ region then it gets absorbed within the extremely
resistive p region then electron-hole pairs are generated.
• Charge carriers drift including their saturation velocity to
the pn+ region wherever a high electric field exists.
When the velocity is highest, then charge carriers will
collide through other atoms & produce new electron-hole
pairs. A huge charge carrier’s pair will result in high
photocurrent.
• Avalanche photodiodes are highly sensitive, high-speedbased diodes which use an internal gain method that
works through applying a reverse voltage.
Advantages
•
•
•
•
The sensitivity range is high.
High performance.
Quick response time.
These diodes are applicable here the gain level
is very important as the high voltage required,
through lower reliability means that they are
frequently less convenient to utilize.
• It detects low-intensity light.
• A single-photon generates a huge number of
charge carrier pairs.
Disadvantages
•
•
•
•
The required operating voltage is high
The output of this diode is not linear
High range of noise
It is not used regularly because of the low
reliability
• It uses high reverse bias for its proper
operation
Applications
•
•
•
•
•
•
•
•
LASER scanner
Analyzer bridge of antenn,
PET scanner
Barcode reader
Laser microscopy
Laser Rangefinders
Speed gun.
APDs are used in receivers of OFC (optical fiber
communications), imaging, finding the range, laser
microscopy, laser scanners & OTDR (optical-time
domain reflectometers).
Avalanche Diode
PIN Diode
Includes four layers like P+, I, P &
Includes four layers like P+, I & N+.
N+.
Response time is very high.
Response time is very low.
Internal gain is 200 dB.
Internal gain is insignificant.
Sensitivity is high.
Sensitivity is low.
High noise.
Low noise.
The reverse bias voltage is very
high.
The reverse bias voltage is very
low.
High-temperature stability.
Low-temperature stability.
The amplifier is not necessary
because of the available gain.
The amplifier is mandatory
Characteristics
Quantum Efficiency
• η measures the fraction of incoming photons that
generate electrons at the detector. It is defined as
the ratio of the number of electron-hole (e-h) pairs
generated to the number of incident photons.
Responsivity
• Responsivity is the ratio of electrical output from
the detector to the input optical power. If the
output current varies proportionally to the input,
this is measured as amps per watt (A/W). Since
in fiber optic communication systems, input
powers are usually in microwatt level,
responsivity is often expressed as uA/uW.
• ρ is the photo current generated per unit optical
power.
Speed of response and Bandwidth
Depend on three factors.
• The transit time of the photo-generated carriers through
the depletion region
• The electrical frequency response as determined by the
RC time constant, which depends on the diode’s
capacitance
• Time taken by the carrier to perform the avalanche
multiplication process.
Dark current
• Dark current is the current through the photodiode in the
absence of light, when it is operated in photoconductive
mode. The dark current includes photocurrent generated
by background radiation and the saturation current of the
semiconductor junction.
• Dark current sets a floor on the minimum detectable
signal, because a signal must produce more current than
the dark current in order to be detected. Dark current
depends on operating temperature, bias voltage, and the
type of detector.
• Dark current must be accounted for by calibration if a
photodiode is used to make an accurate optical power
measurement, and it is also a source of noise when a
photodiode is used in an optical communication system.
Noise-Equivalent Power (NEP)
• Noise-Equivalent Power (NEP) is the minimum
input optical power to generate photocurrent,
equal to the rms noise current in a 1 hertz
bandwidth.
• This more directly measures the minimum
detectable signal because it compares noise
directly to optical power.
• NEP depends on the frequency of the modulated
signal, the bandwidth over which noise is
measured, the detector area, and the operating
temperature.
Quantum Noise
• The quantum behavior of electromagnetic radiation must
be taken into account at optical frequencies since hf > KT
and quantum fluctuations dominate over thermal
fluctuations.
• The detection of light by a photodiode is a discrete process
since the creation of an electron–hole pair results from the
absorption of a photon, and the signal emerging from the
detector is dictated by the statistics of photon arrivals.
• Hence the statistics for monochromatic coherent radiation
arriving at a detector follow a discrete probability
distribution which is independent of the number of photons
previously detected.
• It is found that the probability P(z) of detecting z photons
in time period τ when it is expected on average to detect
zm photons obeys the Poisson distribution
where zm is equal to the variance of the
probability distribution.
• This equality of the mean and the variance is typical of
the Poisson distribution.
• Now, the electron rate re generated by incident photons
is re = ηPo/hf. The number of electrons generated in time
τ is equal to the average number of photons detected
over this time period zm. Therefore:
Quantum Limit
• For digital optical fiber systems it is possible to calculate
a fundamental lower limit to the energy that a pulse of
light must contain in order to be detected with a given
probability of error. The premise on which this analysis is
based is that the ideal receiver has a sufficiently low
amplifier noise to detect the displacement current of a
single electron–hole pair generated within the detector
(i.e. an individual photon may be detected).
• Thus in the absence of light, and neglecting dark current,
no current will flow. Therefore the only way an error can
occur is if a light pulse is present and no electron–hole
pairs are generated.
• The probability of no pairs being generated when a light
pulse is present is given by:
• Thus in the receiver described P(0|1) represents the
system error probability P(e) and therefore:
• Represents an absolute receiver sensitivity and allows
the determination of a fundamental limit in digital optical
communications.
• This is the minimum pulse energy Emin required to
maintain a given bit-error-rate (BER) which any practical
receiver must satisfy and is known as the quantum limit.
SONET/SDH
SONET
• SONET stands for Synchronous Optical Network.
• SONET is a communication protocol, developed by
Bellcore that is used to transmit a large amount of data
over relatively large distances using optical fibre.
• With SONET, multiple digital data streams are
transferred at the same time over the optical fibre.
• A single clock (Primary Reference Clock, PRC) handles
the timing of transmission of signals & equipments
across the entire network, that is why called
synchronous.
Synchronous Network Elements
• STS Multiplexer: Performs multiplexing of signals
Converts electrical signal to optical signal
• STS Demultiplexer: Performs demultiplexing of signals
Converts optical signal to electrical signal
• Regenerator: It is a repeater, that takes an optical signal
and regenerates (increases the strength) it.
• Add/Drop Multiplexer: It allows to add signals coming
from different sources into a given path or remove a
signal.
Connections:
• Section: Portion of network connecting two
neighbouring devices.
• Line: Portion of network connecting two neighbouring
multiplexers.
• Path: End-to-end portion of the network.
SONET Layers
• The SONET standard standard includes includes four
functional functional layers: the photonic, the section, the
line, and the path layer. They correspond correspond to
both the physical physical and the data link layers.
• Path Layer: It is responsible for the movement of
signals from its optical source to its optical destination.
STS Mux/Demux provides path layer functions.
• Line Layer: It is responsible for the movement of
signal across a physical line. STS Mux/Demux and
Add/Drop Mux provides Line layer functions.
• Section Layer: It is responsible for the movement of
signal across a physical section. Each device of
network provides section layer functions.
• Photonic Layer: It corresponds to the physical layer of
the OSI model. It includes physical specifications for
the optical fibre channel (presence of light = 1 and
absence of light = 0).
SONET Frame
• Each synchronous transfer signal STS-n is
composed of 8000 frames. Each frame is
a two-dimensional matrix of bytes with 9
rows by 90 × n columns.
Advantages
•
•
•
•
Transmits data to large distances
Low electromagnetic interference
High data rates
Large Bandwidth