Photodetector : Circuit, Working, Types & Its Applications

The photodetector is an essential component in an optical receiver that converts the incoming optical signal into an electrical signal. Semiconductor photodetectors are usually called photodiodes because these are the main types of photodetectors utilized in optical communication systems due to their quick detection speed, high detection efficiency & small size. At present, photodetectors are widely used in industrial electronics, electronic communications, medicine & healthcare, analytical equipment, automotive & transport, and many more. These are also known as photosensors and sensors of light. So, this article discusses an overview of a photodetector – working with applications.

What is Photodetector?

A photodetector definition is; an optoelectronic device that is used to detect the incident light or optical power to convert it into an electrical signal is known as a photodetector. Usually, this o/p signal is proportional to the incident optical power. These sensors are absolutely needed for different scientific implementations like process control, fiber optic communication systems, safety, environmental sensing & also in defense applications. Examples of photodetectors are phototransistors and photodiodes.

How Photodetector Works?

Photodetector simply works by detecting light or other electromagnetic radiation or devices may by receiving the transmitted optical signals. Photodetectors that use semiconductors operate on the electron-hole pair creation upon the light irradiation principle.

Once a semiconductor material is illuminated through photons that have high or equivalent energies to its bandgap, then absorbed photons encourage valence band electrons to move into the conduction band, so leaving behind holes within the valence band. The electrons in the conduction band perform as free electrons (holes) that can disperse under the power of an intrinsic or externally applied electric field.

The photo-generated electron-hole pairs because of optical absorption may recombine & re-emit light unless subjected to an electric field-mediated separation to give an increase to a photocurrent, which is a fraction of the photo-generated free charge carriers received at the electrodes of the photodetector arrangement. The photocurrent magnitude at a specified wavelength is directly proportional to the intensity of incident light.

Properties

The properties of photodetectors are discussed below.

Spectral Response – It is the photodetector’s response as a photon frequency function.

Quantum Efficiency – The number of charge carriers generated for each photon

Responsivity – It is the output current separated by the total power of light dropping on the detector.

Noise-equivalent Power – It is the required amount of light power to generate a signal that is equivalent in size to the noise of the device.

Detectivity – The square root of the area of the detector separated by the noise equivalent power.

Gain – It is the photo detector’s output current which is divided by the directly produced current by the incident photons on the detectors.

Dark Current- The flow of current throughout a detector even in the deficiency of light.

Response Time – It is the required time for a detector to go from 10 – 90% of the final output.

Noise Spectrum – The intrinsic noise current or voltage is a function of frequency that can be signified in a noise spectral density form.

Nonlinearity – The photo detector’s nonlinearity limits the RF output.

Photodetector Types

The photodetectors are classified based on the detection mechanism of light like the photoelectric or photoemission effect, polarisation effect, thermal effect, weak interaction, or photochemical effect. The different types of photodetectors mainly include a photodiode, MSM photodetector, phototransistor, photoconductive detector, phototubes & Photomultipliers.

Photodiodes

These are semiconductor devices with a PIN or PN junction structure where light is absorbed within a depletion region & produces a photocurrent. These devices are fast, highly linear, very compact and generate a high quantum efficiency which means it generates almost one electron for each incident photon & a high dynamic range. Please refer to this link to know more about Photodiodes.

MSM Photodetectors

MSM (Metal–semiconductor–metal) photodetectors include two Schottky contacts rather than a PN junction. These detectors are potentially faster as compared to photodiodes with up to hundreds of GHz bandwidths. MSM detectors allow very large area detectors to make easy coupling with optical fibers without bandwidth degrading.

Phototransistor

The phototransistor is one type of photodiode which uses internal amplification of the photocurrent. But these are not frequently used as compared to photodiodes. These are mainly used for detecting light signals & change them into digital electrical signals. These components are simply operated through light rather than electric current. Phototransistors are low-cost and provide a large amount of gain, so they are used in various applications. Please refer to this link to know more about phototransistors.

Photoconductive Detectors

Photoconductive detectors are also known as photoresistors, photocells & light-dependent resistors. These detectors are made with certain semiconductors like CdS (cadmium sulfide). So this detector includes a semiconductor material with two connected metallic electrodes for detecting the resistance. Compared to photodiodes, these are not expensive but they are quite slow, not extremely sensitive & exhibit a nonlinear response. Alternatively, they can react to long-wavelength IR light. Photoconductive detectors are separated into different types based on the function of spectral responsivities like as the visible wavelength range, near-infrared wavelength range, and IR wavelength range.

Phototubes

The gas-filled tubes or vacuum tubes that are used as photodetectors are known as phototubes. A phototube is a photoemissive detector that uses an external photoelectric effect or photoemissive effect. These tubes are frequently evacuated or filled sometimes with gas at low pressure.

Photomultiplier

A photomultiplier is one type of phototube that changes incident photons into an electrical signal. These detectors use an electron multiplication process to obtain a much-increased responsivity. They have a large active area & high speed. There are different types of photomultipliers available like Photomultiplier tube, Magnetic photomultiplier, Electrostatic photomultiplier, and Silicon photomultiplier.

Photodetector Circuit Diagram

The light sensor circuit using a photodetector is shown below. In this circuit, the photodiode is used as a photodetector to detect the existence or nonexistence of light. This sensor’s sensitivity can be simply adjusted by using the preset.

The required components of this light sensor circuit mainly include a photodiode, LED, LM339 IC, Resistor, Preset, etc. Connect the circuit as per the circuit diagram shown below.

Working

A photodiode is used as a photodetector to generate current within the circuit once light falls on it. In this circuit, the photodiode is used in reverse bias mode through the R1 resistor. So this R1 resistor does not permit too much current to supply throughout the photodiode in case a huge amount of light drops on the photodiode.

When no light falls on the photodiode, then it results in high potential at the pin6 of an LM339 comparator (inverting input). Once light falls on this diode, then it allows current to supply throughout the diode & thus voltage will drop across it. The pin7 (non-inverting input) of the comparator is connected to a VR2 (variable resistor) to set the comparator’s reference voltage.

Here, a comparator works when the non-inverting input of the comparator is high as compared to inverting input then its output remains high. So the output pin of IC like pin-1 is connected to a light emitting diode. Here, the reference voltage is set throughout a VR1 preset to correspond to a threshold illumination. At the output, the LED will be turned ON once light falls on the photodiode. So, the inverting input drops to a lower value as compared to the reference set at the non-inverting input. So, the output goes supplying the required forwards bias to the light-emitting diode.

Photodetector vs Photodiode

The difference between photodetector and photodiode includes the following.

Photodetector	Photodiode
Photodetector is a photosensor.	It is a light-sensitive semiconductor diode.
The photodetector is not used with an amplifier to detect the light.	The photodiode uses an amplifier for detecting low levels of light as they permit a leakage current that changes with the light that falls on them.
A photodetector is simply made with a compound semiconductor with a 0.73 eV band gap.	The photodiode is simply made with two P-type and N-type semiconductors.
These are slower than photodiodes.	These are faster than photodetectors.
The photodetector response is not faster as compared to the photodiode.	The photodiode response is much faster as compared to the photodetector.
It is more sensitive.	It is less sensitive.
The photodetector converts the light’s photon energy into an electrical signal.	Photodiodes convert light energy and also detect light brightness.
The temperature range of the photodetector ranges from 8K – 420 K.	The photodiode temperature ranges from 27°C to 550 °C.

Quantum Efficiency of Photodetector

The photodetector’s quantum efficiency can be defined as the fraction of the incident photons that are absorbed through the photoconductor to the electrons produced are collected at the detector terminal.

The quantum efficiency can be denoted with ‘η’

Quantum Efficiency (η) = Generated electrons/Total number of incident photons

Thus,

η = (Current/ Charge of an electron)/(Total incident photon’s optical power/ Photon energy)

So mathematically, it will become like

η = (Iph/ e)/(PD/ hc/λ)

Advantages and Disadvantages

The advantages of photodetector include the following.

• Photodetectors are small in size.
• Its detection speed is fast.
• Its detection efficiency is high.
• They generate less noise.
• These are not expensive, compact & lightweight.
• They have a long lifetime.
• They have high quantum efficiency.
• It doesn’t require high voltage.

The disadvantages of photodetector include the following.

• They have very low sensitivity.
• They have no internal gain.
• The response time is very slow.
• The active area of this detector is small.
• Change within current is extremely small, so may not be adequate to drive the circuit.
• It requires offset voltage.

Applications of Photodetectors

The applications of photodetector include the following.

• Photodetectors are used in different applications which range from automatic doors in supermarkets to TV remote controllers within your home.
• These are essential significant components used in optical communications, security, night-vision, video imaging, biomedical imaging, motion detection & gas sensing which have the capability to change light into electrical signals exactly.
• These are used for measuring optical power & luminous flux
• These are mainly used in different kinds of microscope & optical-sensor designs.
• These are significant for laser rangefinders.
• These are normally used in frequency metrology, optical-fiber communication, etc.
• Photodetectors in photometry & radiometry are used to measure different properties such as optical power, optical intensity, irradiance & luminous flux.
• These are used for measuring optical power within spectrometers, optical data storage devices, light barriers, beam profilers, fluorescence microscopes, autocorrelators, interferometers & different kinds of optical sensors.
• These are used for LIDAR, laser rangefinders, night vision devices & quantum optics experiments.
• These are applicable in optical frequency metrology, optical fiber communications & also for the classification of laser noise or pulsed lasers.
• The two-dimensional arrays with several identical photo detectors are mainly used as focal plane arrays & frequently for imaging applications.

What is a photodetector used for?

Photodetectors are used for converting light’s photon energy into an electrical signal.

What are the characteristics of a photodetector?

The characteristics of photodetectors are photosensitivity, spectral response, quantum efficiency, forward-biased noise, dark current, noise equivalent power, timing response, terminal capacitance, cutoff frequency & frequency bandwidth.

What are the requirements of a photodetector?

The requirements of photodetectors are; short response times, the least noise contribution, reliability, high sensitivity, linear response over a broad range of light intensities, low bias voltage, low cost & stability of performance characteristics.

What is used in the specification of optical detectors?

The noise equivalent power is used in the specification of optical detectors because it is the optical input power that generates an extra output power that is equal to that noise power for a specified bandwidth.

Is quantum yield & quantum efficiency the same?

The quantum yield and quantum efficiency are not the same because the probability of a photon emitting once one photon has been absorbed is the quantum yield whereas the quantum efficiency is the probability that a photon is emitted once the system has been energized to its emitting condition.

Thus, this is an overview of a photodetector – working with applications. These devices are based on the internal and external photoelectric effect, so mainly used for the detection of light. Here is a question for you, what are optical detectors?