The photodiode is a semiconductor device used to convert solar energy (light energy) into electric current or voltage. This is one type of light detector or photosensors.
When the photons are absorbed by the photodiode, the electric current is generated. It may have a large or small surface area. The response time of the photodiode will decide by the surface area. Generally, it has a slower response time than a large surface area.
What is Photodiode?
The photodiode is a PIN junction device used to convert light energy into electric energy. Depends on the mode of operation, it can be a current source or voltage source. It contains optical fiber or built-in lenses.
Usually, photodiode operates in reverse biased mode. Therefore, the P channel of a diode is connected with the negative terminal, and the N channel of a diode is connected with the positive terminal of the battery.
Between the P-layer and N-layer, an extra intrinsic layer is preset. There are many types of diodes designed specially as a photodiode use a PIN junction alternative to the PN junction.
The material used to make photodiode is silicon, germanium, indium gallium arsenide phosphide, Lead (II) sulfide, Mercury cadmium telluride, and Indium gallium arsenide.
The symbol of a photodiode is as shown in the below figure.
Principle of Photodiode
A photodiode is a PIN structure or PN junction diode. When an adequate amount of photon energy wallop on the diode, it generates electron-hole pair.
If the photon energy absorbs by the junction’s depletion region, these charge carriers glided from the junction by a built-in electric field of the depletion region. This effect is known as the inner photoelectric effect.
Due to this effect, holes move towards the anode and electrons move towards the cathode. Therefore, movement of charge carriers, the electric current products in the circuit. Sometimes, this current is also known as photocurrent.
The total current of the diode is a sum of the dark current and the photocurrent. The dark current is the current generated in the absence of light. To increase the sensitivity of the diode, the dark current must be low.
Modes of Operation
The operation of Photodiode divided into three modes;
- Photovoltaic mode
- Photoconductive mode
- Avalanche diode mode
This mode is also known as the Zero Bias mode. In this mode of operation, the photocurrent flows from the anode through the short circuit to the cathode.
If the circuit is open (high load impedance), it will restrict the photocurrent, but the voltage builds up. This voltage is in the direction of the forward bias diode. Because anode is positive with respect to the cathode.
If the circuit is short-circuited (low load impedance), forward current flows through the circuit. This forward current will be all or some part of the photocurrent. This effect is well-known as the photovoltaic effect.
The solar cells are working on the photovoltaic effect. Regular solar panels have a large area photodiode. And for optimum power output, the cell operates at a voltage that causes a small forward current compared to the photocurrent.
In this mode of operation of a photodiode, it operates on the reverse bias. As the cathode is positive with respect to the cathode.
Due to the reverse bias, the width of the depletion layer increase. And that decrease the junction’s capacitance. So, it will reduce the response time. It also increases the region area of an electric field by which the electrons are collected quickly.
Without a change in photocurrent, the reverse bias creates a dark current. Due to dark current and avalanche effects, the photoconductive mode expresses more electrical noise. And for good PIN diode has a low leakage current.
When a bias voltage is applied to the photodiode, some leakage current will flow. This leakage current is known as dark current.
In photoconductive mode, a higher dark current is available and it varies directly with temperature. The presence of dark current also depends on the photodiode material and the size of the effective area.
The photodiode is designed for operating high reverse bias and it allows to sustain voltage up to reverse bias voltage.
Due to avalanche reverse bias, each photo-generated carrier resulting in an internal gain in the photodiode. And each photo-generated carrier produces electron-hole pair. Which causes the current to flow through the circuit.
Construction of Photodiode
As we know, the photodiode is used to convert the photon energy into electrical energy. Hence, the PN junction is enclosed in the glass material. The construction of Photodiode is as shown in the below figure.
The junction of the PN diode is placed inside of glass material in such a way that, it allows the photons to pass through it. Another portion of the diode is pained black or it may cover by a metal.
This entire construction is very small in size. It nearly about 2.5mm. And a very small amount of current will generate from one unit. This current is in terms of a microampere.
So, for higher rated current and voltage, a greater number of units are connected in series or parallel.
Types of Photodiode
There are several different types of photodiode technology is used to produce electrical energy. All of these photodiodes work on the same basic principle. But some of them are improved employing construction, functions and other effects.
The list of different types of a photodiode is as below;
- PN Photodiode
- PIN Photodiode
- Avalanche Photodiode
- Schottky Photodiode
This is the first developed form of a photodiode. It is used in less application as it is not efficient as compared to the advanced photodiode. This diode is small in size. But the sensitivity is not up to marks.
This is the most commonly used photodiodes at present time. Because of the wide intrinsic area between P and N layers, this diode can collect more light photons. And also this diode has low junction capacitance.
This diode operates in the avalanche effect. Due to high gain levels, this type of diode is used in low light areas. This diode generated more noise. Therefore, this technology is not used in such applications.
The Schottky diode is used as a photodiode. It has a small junction. Hence, the junction capacitance is small. So, the operating speed of this diode is very high. because of this functionality of the Schottky diode, it is more preferred in high bandwidth applications like fiber-optic links.
V-I Characteristics of Photodiode
In most of the conditions, photodiodes operates in the condition of reverse bias. Hence, the characteristic of a photodiode is a graph between reverse voltage and reverse current. Reverse voltage placed on X-axis and reverse current placed on Y-axis.
When there is no light illumination, there is no relation between reverse current and reverse voltage. And in this condition, the reverse current is almost zero. But still, there is a very small amount of current being present due to dark current.
As the light illumination is present at the junction of a photodiode, the reverse current starts flowing through the circuit. In this condition, the reverse current increases linearly and the characteristic or relation between reverse current and reverse voltage is as shown in the below figure.
What is the difference between Normal Diode and Photodiode?
The photodiode is not similar to the normal diode. There are many differences like; construction, operation, material, etc. Here, we will briefly explain the difference between normal diode and photodiode.
- The diode or power diode is a semiconductor device that conducts when it connected in forward bias. And the photodiode is a device that is used to convert photon energy into electrical energy.
- The operating mode of both devices is different. Like the diode operates when it is connected in forward bias. And the photodiodes operates in reverse bias.
- Both devices are made up of different materials.
- In photodiodes, an anti-reflection layer is present which is not present in a normal diode.
- The photocurrent produced in photodiode depends on the illumination. Hence, it varies with illumination. While in a normal diode, the forward current depends on the forward voltage.
Comparison between Photodiode and Phototransistor
Both of these devices are used to convert photon energy into electrical energy. Phototransistor uses transistor and photodiodes uses diode as a semiconductor device. Hence, the response of the phototransistor is good compared to the photodiodes.
The below table shows a comparison between phototransistor and photodiodes.
|It is used to convert the energy of photons into electrical energy using the transistor.||It uses a PIN junction diode to convert photon energy into electrical energy.|
|Response time is faster.||Response time is slow.|
|It works only in forward bias conditions.||It can work in both biasing conditions; forward and reverse bias.|
|The dark current in the phototransistor is high.||Low dark current.|
|Phototransistor generates current only.||The photodiode can generate current and voltage.|
|It is used to detect light.||It is used to make a solar panel.|
How to Calculate Quantum Efficiency of Photodiode?
The quantum efficiency is defined as the ratio of incident photons on the junction which is absorbed by the photoconductor and generates electrons that are collected at the detector terminal.
It means, quantum efficiency of photodiodes is defined as the ratio of incident photons which utilized to generate photocurrent. It relates to the Responsivity (R) and equates as the below equation.
Rλ = Responsivity in A/W
λ = Wavelength in nm
Responsivity is a function of the wavelength of incident radiation. And also it depends on the sensor properties like the bandgap of material by which the photodiode is made.
It is expressed in units of either amperes or volts per watt of incident radiant power. It measures the input-output gain of the photodetector system.
The equation of responsivity is;
n = quantum efficiency
q = electron charge
f = frequency of optical signal
h = plank’s constant
λ = wavelength of optical signal
How to connect a Photodiode to an External Circuit?
In most of the conditions, photodiode operates in a reverse bias condition. So, the cathode positive with respect to the anode.
For such configuration, the anode connected with the ground (0 V), and the cathode is connected with the positive voltage of the circuit.
When the light illuminates, a small amount of current will flow from cathode to anode. The basic connection diagram is as shown in the below figure.
The amount of current generated by a single unit of a photodiode is small. And it is not sufficient to drive any load. hence, for that number of photodiodes are connected in series and parallel to generate rated voltage and current.
The advantages of a photodiode are listed below.
- It used in the instruments that test the laser pulse shape.
- It has lower noise.
- Photodiodes used as variable resistance device.
- The frequency response is good.
- The dark current is less compared to the phototransistor.
- High quantum efficiency.
- The operation of a photodiode is high and quick.
- The spectrum response is good.
The disadvantages of photodiode are listed below.
- Less sensitive device.
- It has a small active area.
- The photodiode generates a low current. hence, it is necessary to amplification at low irradiance.
- It needs offset voltage.
- If the temperature increase, the dark current increase rapidly. And the temperature stability of this device is very poor.
There are many applications of a photodiode. Out of that, some applications are listed below.
- It used in solar panels to generate electric energy from solar light.
- Photodiode used in optical communications and logic circuits.
- It used in the safety electronics devices like smoke detectors, fire alarms, etc.
- In medical applications like instruments used to analyze samples, detectors for computed tomography, and also used in blood gas monitors.
- It used as photosensors in cameras.
- It used in the character reorganization circuit.
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