Question

1. What is the working principle of a photodiode? Give three examples of photodiodes’ real-life applications.

2. Which of the following photodiode modes performs better in saturation and response time? Photovoltaic or photoconductive mode? Explain the physics behind your answers.

Answer 1

1.Photodiode is a p–n junction. When a photon of sufficient energy strikes the diode, it creates an electron–hole pair. If the absorption occurs in the junction's depletion region, or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. So it converts light to current.

Photodiodes are used in compact disc players, smoke detectors, medical devices and recievers for infrared remote control devices.

2. Photoconductive mode performs better in saturation and response time because in this mode the diode is reverse biased, that is, with the cathode driven positive with respect to the anode. This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's capacitance and increases the region with an electric field that will cause electrons to be quickly collected. Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects.The leakage current of a good PIN diode is so low .