photodiode

Photodiodes are PN junction diodes designed specifically to harness the photoelectric effect. Photodiodes are semiconductor light sensors that generate a current or voltage when the P-N junction is illuminated by light.

Photodiodes can be classified by function & construction as follows:

Types Of Photodiode:

1. PN Photodiode

2. PIN Photodiode

3.Schottky type Photodiode

4. Avalanche Photodiode

Principle Of Operation:

1. the P layer material at the active surface & the N layer at the substrate form a PN junction that operates as a photoelectric converter.

2. The thickness of a P layer is approximately 1um or less & the neutral region at the junction b/w P & N material is known as the depletion layer.

3. By controlling the thickness of the outer Player, substrate N layer & bottom N layer as well as the doping concentration, the spectral response & the frequency response can be controlled.

4. When light strikes a photodiode, the electron within the crystal structure becomes simulated.

5. if light energy is greater than the band energy Eg, the electrons are pulled up into the conduction band so leaving holes in their place in the valance band.  

6. These electron-hole pairs occur throughout the P layer, the depletion layer & the N layer materials.

7. In the depletion layer the electric field accelerates this electron towards the N layer & the holes towards the P layer.

8. Of the electron hole pairs generated in the N layer, the electrons along with electrons that have arrived from the P layer are left in the N layer in the conduction band.

9. The holes at this time are being diffused through the N layer up to the depletion layer while being accelerated and collected in the Player valence band.

10. In this manner EHP which are generated in proportion to the amount of the incident light are collected in the N & P layers.

11. This results in a positive charge in the P layer & a negative charge in the N layer.