A photodetector is a device used to detect light or other electromagnetic Requirements for optical detectors can vary depending on the

pending on the specific

radiation and convert it into an electrical signal. These devices are essential in application
various applications, including photography, telecommunications, astronomy, Sensitivity: Optical detectors should be sensitive enough to detect the desired level
and many scientific instruments. Photodetectors can operate across a wide range of optical signal within the operating environment. This sensitivity is often expressed
of wavelengths, from ultraviolet to infrared, and they come in various types such as the minimum detectable signal level or the device's responsivity.
as photodiodes, phototransistors, photomultiplier tubes, and CCD (charge- Wavelength range: Different applications may require detectors that are sensitive to
coupled device) sensors. Each type has its own characteristics and is suitable for specific wavelengths or a broad range of wavelengths. The detector's spectral
different applications. For instance, photodiodes are commonly used in optical response should match the wavelengths of interest.
communication systems, while CCD sensors are prevalent in digital cameras and Response time: The response time of the detector refers to how quickly it can react
astronomical imaging devices. to changes in the incident optical signal. Fast response times are crucial in
applications such as optical communications and sensing.
P-N Principle of Working: Linearity: Linearity describes how accurately the detector's output corresponds to
A p-n photodiode consists of a semiconductor material with a p-type region the intensity of the incident light over its operating range. Linear detectors provide a
(with an excess of positively charged carriers, or "holes") and an n-type region consistent response to changes in light intensity.
(with an excess of negatively charged carriers, or electrons) forming a junction. Noise performance: Low noise levels are essential for achieving high signal-to-
When light, composed of photons, strikes the semiconductor material, it noise ratios (SNR) in optical detection systems. Sources of noise include thermal
generates electron-hole pairs through the absorption of photons. noise, shot noise, and dark current noise.
Dynamic range: The dynamic range of a detector refers to the range of light
intensities it can accurately detect without saturation or loss of sensitivity. A wide
dynamic range enables the detection of both weak and strong optical signals
This process is governed by the photoelectric effect. Each photon with energy
greater than the bandgap energy of the semiconductor can excite an electron from
the valence band to the conduction band, leaving behind a hole in the valence
band. This creates an electron-hole pair, contributing to a net flow of charge
carriers across the p-n junction, resulting in a photocurrent.

Key Properties of Photodiodes:

The most important properties of photodiodes are:
• the responsivity, i.e., the photocurrent per unit optical power – related to
the quantum efficiency, dependent on the wavelength
• the active area, i.e., the light-sensitive area
• the breakdown voltage, setting a limit to the usable bias voltage
• the maximum allowed photocurrent (usually limited by saturation,
possibly lower for high bias voltages)
• the dark current (in photoconductive mode, dependent on the bias voltage,
important for the detection of low light levels)
• the speed, i.e. the bandwidth (see below), related to the rise and fall time,
often influenced by the electric capacitance
The detection process in a p-n photodiode involves the conversion of incident Avalanche Photodiodes (APDs):
photons into an electrical signal. Definition: An Avalanche Photodiode (APD) is a highly sensitive type of photodiode
that exploits the photoelectric effect to convert light into electricity. Unlike solar cells
1. Photon Absorption: When photons from incident light strike the (which generate electricity), APDs are optimized for detecting incoming photons.
semiconductor material of the p-n photodiode, they interact with the atoms in Working Principle:APDs operate by impact ionization:
the material. If the energy of a photon is greater than or equal to the bandgap • Incident photons generate electron–hole pairs in the semiconductor material.
energy of the semiconductor material, it can be absorbed. • Applying a high reverse bias voltage causes an avalanche breakdown,
2. Generation of Electron-Hole Pairs: Upon absorption, the energy from the multiplying the charge carriers.
incident photon promotes an electron from the valence band to the conduction • The induced photocurrent is amplified due to this high gain effect.
band, leaving behind a positively charged hole in the valence band. This • APDs are analogous to photomultiplier tubes but operate at lower voltages.
process generates an electron-hole pair. • Applications:
3. Electron and Hole Movement: The newly generated electron and hole are • Laser rangefinders
now free to move within the semiconductor material due to the presence of • Long-range fiber-optic telecommunication
an electric field created by the built-in potential across the p-n junction. • Positron emission tomography (PET)
4. Electric Field Drift: The electric field within the depletion region of the p-n • Particle physics research.
junction causes the electron and hole to separate. The electron moves toward Key Characteristics of APDs:
the n-type region, while the hole moves toward the p-type region. 1. Quantum Efficiency (QE):
5. Collection of Charge Carriers: Electrons are collected at the n-type region, o Measures how efficiently incident optical photons are absorbed and
while holes are collected at the p-type region. This movement of charge generate primary charge carriers.
carriers contributes to the generation of a photocurrent. o Although APDs have high sensitivity, their QE may not be 100% due to
6. External Circuit Connection: The photocurrent generated by the movement factors like reflection and other losses3.
of charge carriers is conducted through an external circuit connected to the 2. Responsivity (η):
terminals of the photodiode. o The ratio of output current to input optical power.
7. Output Signal: The photocurrent flowing through the external circuit o Indicates the efficiency of the device in converting light to electrical
produces an output voltage or current signal proportional to the intensity of signals.
the incident light. This signal can be further amplified and processed for o Responsivity is highest when the photon energy is slightly above the
various applications, such as optical communications, imaging, and sensing. bandgap energy of the material4.
3. Long Cutoff Wavelength:
Phototransistors are either tri-terminal (emitter, base and collector) or bi- o The upper wavelength limit where a specific semiconductor material
terminal (emitter and collector) semiconductor devices which have a light- efficiently detects light.
sensitive base region. Although all transistors exhibit light-sensitive nature, these o Determined by the material’s bandgap energy.
are specially designed and optimized for photo applications. These are made of o Longer cutoff wavelengths allow detection of lower-energy photons1.
diffusion or ion-implantation and have much larger collector and base regions in 4. Detector Response Time:
comparison with the ordinary transistors. o The time it takes for the detector to respond to changes in incident light
power.
In the case of homojunction phototransistors, the entire device will be made of a o Faster response times are desirable for high-speed applications
single material-type; either silicon or germanium. However to increase their
efficiency, the phototransistors can be made of non-identical materials (Group
III-V materials like GaAs) on either side of the pn junction leading to
heterojunction devices. Nevertheless, homojunction devices are more often used
in comparison with the hetero junction devices as they are economical.
 Advantages of Phototransistor
The advantages of phototransistors include:
• Simple, compact and less expensive.
• Higher current, higher gain and faster response times in comparison with
photodiodes.
• Results in output voltage unlike photo resistors.
• Sensitive to a wide range of wavelengths ranging from ultraviolet (UV) to
infrared (IR) through visible radiation.
Applications of Phototransistor
The applications of phototransistors include:
• Object detection
• Encoder sensing
• Automatic electric control systems such as in light detectors
• Security systems
• Punch-card readers
• Relays
Photo Transistor - Principle of Working:
Characteristic Photodiode Avalanche Phototransistor
A phototransistor is a bipolar transistor that utilizes light to control the flow of
Photodiode (APD)
current between its collector and emitter terminals.
Principle of Photoelectric Avalanche Transistor action
Working effect multiplication
Base-Emitter Junction: Incident photons create electron-hole pairs in the base
Quantum Moderate to High High Moderate to High
region of the phototransistor, generating a photocurrent.
Efficiency
Transistor Action: The photocurrent modulates the base current, which in turn
controls the collector-emitter current according to the transistor's amplification Responsivity Moderate to High High Moderate to High
characteristics. Long Cutoff Visible to NIR NIR to SWIR Visible to NIR
Amplification: Phototransistors offer amplification of the incident light signal Wavelength
due to the transistor's inherent gain. Detector Fast Very fast Moderate to Fast
Response Time
Applications Optical Lidar, optical Light sensing,
communications, communications, optical switches
imaging, sensing low-light imaging
Thermal Noise: A photodiode has a quantum efficiency of 65% when photons of energy 1.5 x 10-
Definition: Thermal noise, also known as Johnson-Nyquist noise, arises due to 19 J are incident-upon-it
the random motion of charge carriers (electrons and holes) within a conductor or i) at what wavelength is the photodiode operating?
semiconductor. It is proportional to temperature and manifests as fluctuations in ii) Calculate the incident opt photocurrent 2.5 ua?
voltage or current. Vrms=4kTRΔf Where:
• 𝑘k is Boltzmann's constant (1.38×10−231.38×10−23 J/K).
• 𝑇T is the temperature in Kelvin.
• 𝑅R is the resistance in ohms.
• Δ𝑓Δf is the bandwidth in hertz.
Effect on Detectors: Thermal noise can limit the detection sensitivity of
photodetectors by introducing additional noise that obscures the desired signal.
Lowering the operating temperature of the detector can reduce thermal noise.
Dark Current Noise:
Definition: Dark current is the electric current that flows through a photodetector
in the absence of light. It originates from thermally generated charge carriers and
other leakage mechanisms within the detector. Idark=Vdark/Req Where:
• 𝑉darkVdark is the voltage equivalent of the dark current.
• 𝑅eqReq is the equivalent noise resistance.
Effect on Detectors: Dark current noise can contribute to the baseline noise level
of a detector, reducing its sensitivity, especially in low-light conditions. Cooling
the detector can reduce dark current and mitigate its impact.
Quantum Noise:
Definition: Quantum noise arises from the discrete nature of light, characterized A p-n photodiode has a quantum efficiency of 50% at a wavelength of
by fluctuations in the number of photons detected by a photodetector. It is 0.9 m. Calculate:
inherent to all optical detection systems and follows Poisson statistics. Iquantum i) its responsivity at 0.9 m;
=2qIoptical Where: ii) the received optical power if the mean photocurrent is 10–6 A;
• 𝑞q is the elementary charge (1.6×10−191.6×10−19 C). iii) the corresponding number of received photons at the wavelength .
• 𝐼opticalIoptical is the optical signal current.

Effect on Detectors: Quantum noise sets a fundamental limit on the minimum

achievable noise level in optical detection systems. It becomes significant in low-
light conditions and can affect the signal-to-noise ratio (SNR) of the detected
signal.
Receiver Sensitivity:
Definition: Receiver sensitivity quantifies the minimum optical power required
at the input of a receiver for a specified bit error rate (BER). It is a measure of
the receiver's ability to detect weak optical signals reliably. Pmin=SNRBER
×Roptical1×B Where: 𝑅opticalRoptical is the responsivity of the photodetector.]

Effect on Detectors: Higher receiver sensitivity allows for the detection of

weaker optical signals with a lower probability of errors. It depends on factors
such as detector noise, optical losses, and the signal-to-noise ratio (SNR) required
for the specific application.
Bit Error Rate (BER):
Definition: Bit error rate (BER) is the ratio of the number of bits received
incorrectly to the total number of bits transmitted in a digital communication
system. It quantifies the reliability of data transmission and is typically expressed
as a fraction or percentage. BER=Ntotal/Nerror

Effect on Detectors: BER is a key performance metric for evaluating the

effectiveness of optical communication systems.