What is Photodiode? Its operations and applications

What Is Photodiode?

A photodiode is a p-n junction semiconductor diode that is always operated in reverse-biased conditions. It is a special type of diode called a light detector or photodetector. It is used to convert light signals into electrical signals.

Symbol of Photodiode

The photodiode symbol is very similar to the normal p-n junction diode except that it contains arrows striking the diode. The arrows striking the diode represent light or photons. It has two terminals Anode and Cathode.

Characteristics of Photodiode

1. Photodiode should be always operated in reverse bias condition.
2. Applied reverse bias voltage should be low.
3. It generates low noise
4. High gain
5. High response speed
6. High sensitivity to light
7. Low sensitivity to temperature
8. Low cost
9. Small size
10. Long lifetime

Types of Photodiode

There are various types of photodiodes available in the market. The working of different types of photodiodes works in a slightly different way, but the basic operation of these diodes remains the same. The types of photodiodes can be classified based on their construction and functions as follows.

• PN junction Photodiode
• Schottky Photo Diode
• PIN Photodiode
• Avalanche Photodiode

Operating principle of Photodiode

When a P-N junction diode is reverse-biased, a reverse saturation current flows due to the thermally generated hole and electron being swept across the junction as the majority carriers. With the increase in temperature of the junction, more and more holes and electron pair are generated and so the Reverse Saturation Current Io increases. The same effect was had by illuminating the junction. When photon (light energy) bombards a p-n junction, it dislodged (knocks) valance electrons. The more photon (light) striking the junction the large the reverse current in a diode. It is due to the generation of more and more charge carriers with the increase in the level of illumination.

V-I Characteristics of Photodiode

The VI characteristic curve of a photodiode represents the relationship between the voltage (V) across the photodiode and the current (I) flowing through it.

Here’s a general description of the behavior of a photodiode’s VI characteristic curve:

1. Dark Current Region: When no light is incident on the photodiode, it typically exhibits a small leakage current known as the dark current. In this region, the current is relatively low, and the voltage may be close to zero or have a slight reverse bias.
2. Photocurrent Region: When light falls on the photodiode, it generates electron-hole pairs, leading to an increase in the current flow. As the incident light intensity increases, the current through the photodiode also increases, resulting in a positive slope in the VI characteristic curve.
3. Saturation Region: At higher light intensities, the current reaches a saturation point where further increases in light intensity do not significantly affect the current. The VI characteristic curve levels off in this region, indicating that the photodiode has reached its maximum response.

It’s important to note that the specific shape of the VI characteristic curve can vary depending on the characteristics of the photodiode, such as its material, size, and operating conditions. Additionally, different types of photodiodes, such as PIN photodiodes or avalanche photodiodes, may exhibit slightly different behaviors in their characteristic curves.

Photodiode Operation Modes

The photodiode can be operated in one of the two modes, namely Photovoltaic mode, Photoconductive mode.

Selection of operation mode of photodiode is depends upon the speed requirements of the application and the amount of dark current that is tolerable.

Photovoltaic Mode: 

This mode is also known as zero-bias or unbiased mode, In this mode no external voltage is applied to the photodiode. Dark current is very low in photovoltaic mode. In photovoltaic mode the speed of operation of photodiode is very low .

The photodiodes operated in photovoltaic mode are generally used for low speed applications or for detecting low light levels.

Photoconductive Mode:

 In photoconductive mode photodiode usually operated in reverse biased. The reverse voltage application will increase the depletion layer’s width, which in turn decreases the response time & the junction capacitance. This mode is too fast and displays electronic noise.

Why photodiode operated in reverse biased condition?

In reverse biased condition the only current flowing through the diode in the absence of light the reverse saturation current which is very small in magnitude. Hence the change in diode current due to the light incident on it is significant (photocurrent is in microampere). If diode is forward biased then the forward current in the absence of light would be in mA and change in the forward current due to light will not be even noticeable. Hence the photodiode operated in reverse biased condition.

Advantages of Photodiode

1. High sensitivity – This mean, a large change in the photocurrent for a small change in light intensity.
2. High speed of operation as compared to LDR (Light Dependent Resistor).
3. High gain

Disadvantages of Photodiode

1.Dark current increases with temperature.

2. Poor temperature stability.

3. External bias voltage is essential for operation.

4. Amplification is required, as the output current is small magnitude.

Applications of Photodiode

1. In cameras for sensing the light intensity.
2. In the fiber optical receiver.
3. In light intensity meters.

Types of diodes read here

wiki