Photo Current in P-N-Junction Diode [Photo Diode]

PHOTO CURRENT IN P-N-JUNCTION DIODE [PHOTO DIODE]

Principle

When a p-n junction diode is exposed to light (photons), under reverse bias, it produces electron and hole pairs. Due to the flow of these charge carriers, it produces a reverse current.

Fabrication

The photo-diode is made up of 'p' and 'n' type materials with a small glass window on the top for the light to pass through it and strike the p-n-junction as shown in Fig. 4.11. The p-n junction mounted on the insulated (Cathode) substrate is sealed inside the metal case for safety.

The symbol of the photo-diode is as shown in Fig. 4.12.

Operation

1. The photo-diode is reverse biased as shown in Fig. 4.13.

2. When no light is incident on the p-n junction of the photo-diode, then no charges are created and therefore no current (or) a very little reverse current (I_R) flows through the circuit.

This current is called Dark current.

3. Now, when light is made to incident on the p-n-junction, then each photon creates an electron-hole pair at the junction.

4. These photo-generated charge carries move towards the potential and therefore constitute a current known as photo current.

5. The photo current increases with the increase in intensity of light falling on the p-n junction.

6. The current reaches a maximum and is called as saturation current.

Modes of operation

The photo-diode operates in 3 modes viz.

1. Photo-voltaic mode.

2. Photo-conductive mode.

3. Avalanche diode mode.

Types

Based on the modes of operation, there are different types of photo-diodes viz.

1. PIN [p type - intrinsic - n type] photo diode.

2. APD [Avalanche pin photo-diode].

3. Schottky photo-diode.

4. Solar cell etc.

Advantages

1. They have long life period.

2. It is light in weight.

3. It is very compact (small in size).

4. It can be rugged mechanically.

5. Noise of the photo diode is very less.

6. The response of the photo-diode is wide spectral.

Disadvantages

1. Here the dark current is temperature dependent.

2. The thermal stability is very poor.

3. Amplification is compulsorily required for better performance.

4. Efficiency is poor in rainy/winter seasons.

Applications

1. Photo-diodes are used in charge-couple devices [CCD], photo-conductors and photo-multiplier tubes.

2. They have wide applications in clocks, radio, camera, street lights, etc.

3. They are used for lighting regulation and in optical communication systems.

4. Photo-diodes are used in electronic devices such as smoke detectors, CD players, TVs, remote controls etc.

5. In medicine they are used in computed tomography (CT) instrument.