The photodiode is a type of semiconductor that converts the incoming light energy into electrical energy. There are various types of the photodiode. Some of them are PN junction photodiode, a PIN photodiode, avalanche photodiode, etc.
Components of Photodiode
There are two main components for making a photodiode.
- P-type semiconductor:- It consists of the holes (+ charge) as the majority charge carrier and free electrons as the minority charge carrier.
- N-type semiconductor:- It consists of the free electrons (- charge) as the majority charge carrier and holes as the minority charge carrier.
The symbol shows a type of equilateral triangle. The broader side is the positive end. That is the anode or the p-type semiconductor. Similarly, the other thinner end of the symbol is the negative end. That is the cathode or the n-type semiconductor. This is the symbol of a normal diode. Now, two arrows facing downward in the symbol indicate the photon of incident energy. Thus, the normal diode becomes a photodiode. Also, the direction of the arrow of the photodiode is exactly opposite to that of the light-emitting diode.
PN Junction Photodiode
When p and n-type semiconductors come together, the process of recombination occurs between the charged carriers. The negative and positive nature of the charge carrier cancels out each other during the recombination process. This chargeless zone is the depletion layer. The edge of the p-type semiconductor will become negatively charged as it receives more free electrons. Similarly, the edge of the n-type semiconductor becomes positively charged. This leads to the formation of an electric field whose direction is from positive to negative.
PIN photodiode consists of highly doped p and n-type extrinsic semiconductors in comparison to the normal PN junction photodiode. In between them lies the intrinsic semiconductor which acts as the depletion layer. The width of the depletion layer is wider in comparison to that of the PN junction photodiode.
2. Advantages of PIN photodiode over the PN junction photodiode
The intrinsic layer increases the width of the depletion layer.
We know that capacitance is directly proportional to the size of the electrodes and inversely proportional to the distance between them. In PIN photodiode, p and n act as electrodes and I acts as the dielectric. Thus increase in the depletion layer decreases the capacitance. A decrease in the capacitance refers to an increase in the resistance of the photodiode. This increases the response time and makes it suitable for applications related to microwaves.
The intrinsic layer also causes an increase in an internal electric field in comparison to that of the PN junction photodiode. Thus. this increases the reverse breakdown voltage. An increase in the reverse breakdown voltage increases the electron-hole formation. So, there will be an increase in the flow of photocurrent.
The light should fall on the depletion layer of its operation. Thus, a wider depletion layer means the absorption of more light energy. This again increases the flow of current and provides better quantum efficiency.
Due to these superior natures of the PIN photodiode, it has replaced the traditional form of PN junction photodiodes.
The structure looks similar to that of the PN and PIN photodiode. The major difference is that it operates at a higher reverse bias voltage. Due to this reason, it produces more noise and the output is also nonlinear. The major advantage of this type of photodiode is that it offers a higher level of sensitivity.
This type of photodiode is generally not preferred.
Modes of Operation
1. Photoconductive Mode
During the process of reverse bias, we connect the positive side of the voltage source to the n-type semiconductor. Similarly, we connect the negative side to the p-type semiconductor. This causes the attraction of the free electrons from the n side towards the battery and vice versa. The electrons then go towards the p-side. Thus, in the end, there will be an increase in the negative charge on the p-side and a decrease in the n-side. Now there will be the recombination of electrons and holes in both sides of the photodiode. Thus, this causes an increase in the width of the depletion layer. Thus it behaves like a capacitor.
At some point, the repulsive force of charge carriers on both sides will be greater than the driving force of the battery. So, the flow of charge carriers will stop.
When the photon of light falls on the transparent layer of the photodiode, it excites the neutral atom. This again causes the formation of electrons and holes. The external reverse voltage causes the free electrons to move towards the n-side and vice versa. This causes a decrease in the depletion layer and the internal electric field simultaneously. Thus, the difference between the electric field and the applied external voltage again causes the flow of current.
Thus in conclusion, whenever the photodiode gets exposed to the light, a small amount of current flows through it.
2. Photovoltaic Mode
For the photovoltaic mode, you do not supply the reverse bias voltage. So, there will not be any increase in the width of the depletion layer.
3. Comparison between photovoltaic and photoconductive mode
(Photovoltaic Mode Vs Photoconductive Mode)
- The dark current or the leakage current is responsible for the noise in the photodiode. It is directly dependent upon the supply voltage. So, in the case of photovoltaic mode, the leakage current is almost equal to zero. On the other hand, the dark current is more in the case of the photoconductive mode. So, it will have more noise in the signal.
- The response speed is greater in the photoconductive mode in comparison to the photovoltaic mode. So, you may even need to amplify the response time for the photovoltaic mode.
- Thus, in conclusion, the photovoltaic mode is suitable for those applications where precision matters. Similarly, the photoconductive mode is suitable for those applications which need higher speed or frequency.
- Response Time (speed/time):- It is the time required for the charge carriers to cross the PN junction. Response time is directly dependent upon the capacitance of the PN junction.
- Responsivity:- It is the ratio of photocurrent generated by the incident light to the power of incident light. The unit of responsivity is A/W where A is the current in ampere and W is the power in watt.
- Quantum Efficiency:- It is the ratio of the number of charge carriers to the number of photons of incident light energy.
- Dark Current:- The leakage current that flows through the photodiode in the absence of an incident light energy is the dark current. It directly depends upon the bias voltage. It is also the cause of noise in the photodiode. The output of photodiode in the presence of light is low. Thus, even a low amount of dark current can give you a fault reading.
- Breakdown Voltage:- It is the reserve bias voltage that can be applied before there is a substantial rise in the dark current.
- In medical sectors photodiodes are used in various instruments such as pulse oximeter, biochemistry analyzer, CT scanners, etc.
- Bar code scanners, cameras, fiber optic links, solar cells, etc.
Specifications of a silicon PIN photodiode
The specification of the photodiode depends upon various factors such as fabrication, the material used, etc. So the table shown here is the approximate value.
Advantages of Photodiode
- It produces low noise (dark current/leakage current)
- It has got the fastest response time in comparison to other types of photodetectors.
- Works as a variable resistance device:- We know that the resistance of a device is the ratio of voltage supplied to the flow of the current. The intensity of the reverse light current varies with the intensity of the incident light energy for the fixed reverse bias voltage. Thus, the ratio of voltage to the current varies. So, there will be a variation in the resistance of the photodiode.
- It is highly sensitive to light energy.
- It can operate at a high frequency
- The spectral range for the operation of the photodiode is high.
- The lifespan of the photodiode is indefinite if handled properly.
- Has high linearity between the incident photon and the photocurrent in comparison to other photodetectors.
Disadvantages of the Photodiode
- It is temperature-dependent. An increase in the temperature can cause an increase in the leakage current or the noise.
- It needs offset voltage
- The current produced is quite low. So, this photocurrent may not be enough to drive the circuit. Thus, you may need to amplify the output signal using an amplifier.
- It has got a small active area.
Blow away the dust particles of the photodiode by using the blower. Then, gently clean the surface of the window of the diode by using a lint-free swap or the lens tissue moistened with alcohol. After that, dry the surface by using a lint-free swab or the lens tissue. Repeat the above steps a couple of times to get rid of more stubborn contamination.
Connect the red wire from the voltage point of the multimeter to one end of the photodiode. Connect the black wire from the common point of the multimeter to the other end. Then, set the multimeter to the diode mode. You will see some readings on the display. Now focus the light source on the photodiode. The reading on the display should increase. If there is no change in the reading, then reverse the direction of the red and black wire and try again. If still there is no change in the reading, then the diode may be faulty.