Topic of Discussion : PIN Photodiode
What is PIN Photodiode ?
A Photodiode is a PN junction diode that operates in reverse bias. As the name suggests, PIN photodiode is a particular type of photodiode in which an intrinsic layer is placed in between a heavily doped p-type and a heavily doped n-type layer. As resistivity decreases with an increase in impurity and vice-versa, p and n layers have very low resistivity , while resistivity in the I layer is very high. PIN-Photodiode has a large depletion region which is used in the reception of light.
PIN Photodiode Symbol
Symbolic representation of the PIN-photodiode is the same as that of the standard p-n junction diode except for the downward arrows over the diode , which indicate light.
PIN Photodiode Structure
PIN-Photodiode comprises three layers- p-layer, I or intrinsic layer , and n-layer. P-layer is doped with a trivalent impurity , and N-layer is doped with a pentavalent impurity. The I-layer is undoped or very lightly doped. P terminal acts like anode , and N terminal acts like cathode. Unlike the general PN junction diode, the width of the intrinsic layer in the PIN-Photodiode is larger.
It can be constructed in two ways:
- Planar Structure: In this type of structure, a thin epitaxial film is fabricated on p-layer.
- Mesa Structure: In this type of structure, already doped semiconductor layers are grown on the intrinsic layer.
PIN Photodiode Circuit Diagram
The PIN-photodiode works as a photodetector only when it is functioning in reverse bias. The Anode is connected with the negative terminal of the battery. The positive side of the battery is connected to the cathode through a resistor.
Operation of pin photodiode | Working principle of PIN Photodiode
- When reverse bias is applied to the device, the depletion region starts expanding in the intrinsic layer. The width goes on increasing until it reaches the thickness of the I layer.
- As a result, the depletion region becomes free of any mobile charge carriers. So no current flows. At this point, no electron-hole recombination takes place in the depletion region.
- When the light of sufficient energy ( h𝜈 ≥ bandgap energy of the semiconductor) enters the I region, each photon absorbed generates one electron-hole pair. These pairs experience a strong force due to the barrier electric field present in the depletion region. This force separates the pairs , and charge carriers move in opposite directions , and current is generated. Thus optical energy gets converted into electrical energy.
- As the current is generated from the light energy, it is called photocurrent and written as Ip.
PIN Photodiode Characteristics
- Resistivity: It offers low resistivity in P , and N layers ( less than 1kΩ/cm) and high resistivity in I layer ( up to 100 kΩ/cm)
- Capacitance: As capacitance is inversely proportional with the gap between P and N layers, capacitance in this photodiode is lower than the standard diode.
Where 𝜀0= dielectric value of free space
𝜀r= dielectric constant of the semiconductor
A= area of the intrinsic layer
d= width of depletion region
- Breakdown Voltage: The intrinsic layer widens the depletion region , due to which breakdown voltage is very high.
- The flow of current: The current flow is directly proportional to the amount of light incident on the detector.
- Forward bias condition: If it is operated in forward bias mode, the width of depletion layer reduces and current flows. In this case, the diode behaves like a variable resistor.
- Quantum efficiency(𝜂): It is referred to the number of electron-hole pairs generated per photon having energy h𝜈
- Responsivity: It measures the output gain per input (photon).
Modes of operation in PIN Photodiode
It has primarily two modes of operation-
- Unbiased Photovoltaic Mode
- Reverse Biased Photoconductive Mode
PIN Photodiode IV curves
Photodiode pin diagram
Photodiode pin configuration
|Name of the pin||Identification|
|Cathode||Shorter in length|
|Anode||Longer in length|
3 pin photodiode
Three-pin photodiodes are high-speed Silicon PIN-photodiodes especially designed to detect nearby infrared light. At low bias, these devices provide the facility of wideband characteristics, which makes them usable for optical communication and other photometry.
Noise in PIN Photodiode
Noise refers to any undesirable occurrence or an error in the received information signal. It is the amalgamation of disturbing energies coming from different sources.
Following are the noises that attribute to the total noise of a photodiode:
- Quantum or shot noise
- Dark current noise
- Thermal noise
While the first two types of noises are generated from the statistical nature of photon to electron conversion procedure, thermal noise is associated with the amplifier circuitry.
Quantum or shot noise:
It happens due to the proton to the electron conversion process. The Poisson process is followed here. Mean square value of Shot noise iq on photocurrent ip is,
Where, q= charge of an electron
Dark current noise:
Dark current is the current that flows through the circuit when no light is incident on the photodetector. It has two major components- bulk dark current noise and surface leakage current noise. Bulk dark current is the result of thermally generated holes and electrons in the PN junction.
Mean square value of bulk dark current noise idb on dark current id is,
Mean square value of surface leakage current noise ids on surface leakage current iL is,
It is also called Johnson noise. The thermal noise of the load resistor is much higher than the thermal noise of the amplifier as load resistance has a smaller value than amplifier resistance.
Therefore, mean square value of thermal noise ir due to the load resistance RL
Where KB= Boltzmann constant
T= absolute temperature
InGaAs PIN Photodiode
InGaAs( indium gallium arsenide) is an alloy of indium arsenide and gallium arsenide. Gallium arsenide can efficiently convert electricity into coherent light.
InGaAs PIN-Photodiode or photodetectors are optoelectronic devices capable of providing very high quantum efficiency that can range from 800 to 1700 nm. They exhibit low capacitance in extended bandwidth, high linearity, high sensitivity due to increased resistance, low dark current, and uniformity across the detector’s active area. All of these characteristics help to increase flexibility and offer a wide range of applications.
GaAs PIN Photodiode
GaAs( Gallium arsenide) is a semiconductor that has high electron mobility and high electron velocity than silicon. It can function at very high frequencies.
GaAs PIN photodiodes are used in detecting optical signals at 850 nm. It has a large activation area that ensures a stable and sensitive response. This can also be used in optical telecommunications as optical receivers, in testing machines, etc. GaAs photodiodes provide fast response, low dark current, and high reliability.
PIN Photodiode detector
The photodetector is used to convert light signal to electrical signal, their amplification, and further processing. In optical fiber systems, the photodetector is an essential element. Semiconductor photodiodes are amongst the most widely used detectors as they offer excellent performance, are small in size, and low in cost.
Example: Gallium arsenide photodiode, Indium gallium arsenide photodiode, etc
PIN Photodiode in optical communication
Photodetectors are vividly used in the automobile sector, medical purpose, Safety equipment, cameras, industry, astronomy, and most importantly, in communications. There are two distinct photoelectric mechanisms available for photodetection:
- External effect: PMT or photomultiplier tubes
- Internal effect: PN junction photodiodes, PIN-photodiodes, avalanche photodiodes
- Electron-hole pair photogeneration occurs
- The PIN junction is reverse biased
- The depletion region sees carrier drift
- Electron-hole pair moves in the opposite direction and causes photocurrent
PIN Photodiode radiation detector | PIN photodiode gamma detector
PIN photodiodes are able to detect individual photons in gamma radiation. A PIN photodiode, a comparator, and four low noise operational amplifiers are together used in this process.
In reverse bias condition, when photons strike the depletion region, they produce a small charge directly proportional to the energy of photons. The resultant signal gets amplified and filtered by the op-amps. Comparator distinguishes the signal and the noise. The final output of the comparator shows a high pulse every time a gamma photon with minimum required energy strikes the PIN photodiode.
PIN Photodiode receiver
Optical receivers are responsible for the optical to electrical energy conversion. The most crucial element of the optical receiver is the photodiode.
The receiver must detect distorted, weak signals first and then, based on the amplified version of that signal, decide which type of data was sent. Errors coming from various sources can be found associated with the signal. So signals should be controlled , and processed with utmost precision as noise consideration is a significant factor in the design of the receiver.
Silicon PIN Photodiode
Silicon or Si PIN-photodiodes can accommodate different applications. Due to the PIN structure, it produces fast response and high quantum frequency to detect photons. They are capable of light detection in the range of 250 nm to 1.1 μm. It detects high-energy radiation in high frequencies. The width of the depletion region varies from 0.5 to 0.7 mm.
Si PIN photodiode detector
In PIN photodiodes, the depletion region almost coincides with the intrinsic layer. Charge carrier generation happens due to the incident radiation.
Along with the light radiation, Gamma radiation, X radiation, particles too can generate charge carriers.
When photons meet with the metal contact of the diode, it produces electron-hole pairs in large numbers. Electrodes collect these , and the signal is generated. Electron-hole pairs that are more mobile helps in receiving easily detectable signals. Those are subsequently processed through a low noise amplifier , and the analyzer detects the amount of radiation from the pulses.
PIN photodiode array
Photodiode arrays are generally used in X-ray machines by scanning the object in the image line by line. X-rays are transformed into light through the scintillator crystal. Then the photodiode measures light intensity.
High-speed PIN Photodiode
High-speed PIN-Photodiodes are preferred for their precise triggering against signal strength, enhanced sensitivity, low operating voltage, and high bandwidth.
PIN Photodiode Amplifier
Operational amplifiers are used with a feedback resistor to convert photocurrent to measurable voltage. It is also called a trans-impedance amplifier.
Application of pin photodiode
PIN-photodiodes are one of the most popular photodiodes that have varied characteristics , making them suitable for different applications. Besides photo-detection, it is used in DVD players, CD drives, switches, medical treatment, and many more.
- High voltage rectifier: The intrinsic layer provides a greater separation between the P and N region, allowing higher reverse voltages to be tolerated.
- RF and DC-controlled microwave switches: The intrinsic layer increases the distance between the P and N layers. It also decreases the capacitance , thereby increasing the isolation in reverse biased condition.
- Photodetector and photovoltaic cells: Light to current conversation occurs in the depletion region. As the width of the intrinsic layer is more, it improves the performance of capturing light.
- RF and variable attenuators
- RF modulator circuit
- MRI machine
PIN Photodiode Advantages and Disadvantages
PIN photodiode Advantages
- It has high light sensitivity.
- The response speed is high.
- Its bandwidth is wide.
- Implementation cost is low.
- It generates low noise.
- Temperature sensitivity is low.
- It is small in size.
- Longevity better than standard diodes.
PIN photodiode Disadvantages
- It can only be operated in the reverse biased condition.
- The voltage applied must be low.
- It is sensitive to every kind of light.
- Temperature specifications have to be maintained.
What is the use of polar capacitance in PIN photodetector?
Polar capacitance means the capacitor plates are electrodes having a positive and a negative polarity. In a PIN photodetector, the P and N layers act as electrodes, and as the width of the depletion layer is vast; the capacitance value is low. Due to low capacitance, the speed improves.
What is the advantage of PIN photodiode?
It has high sensitivity, low noise, wide bandwidth, low implementation cost. the detailed explanation is in top section .
What does I in PIN Photodiode stands for?
I in PIN photodiode stands for Intrinsic layer.
What is the difference between a regular photodiode and a PIN photodiode?
The increased intrinsic layer makes PIN photodiodes capable of carrying more current and also improves frequency response. The detailed explanation is in top section .
What are the drawbacks of PIN photodiode?
It is highly light-sensitive , and it can perform well only in reverse bias.
What is photodiode and its symbol?
A photodiode is a semiconductor that converts light energy in electrical energy.
What is a photodiode array?
It is a sensor used in photodetection, spectrophotometry , etc.
What is photodiode most commonly used?
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