Question

Describe the operating principle of photodiode with sketch

Answer 1

Working principle: A photodiode is a PN- junction diode which uses light energy to produce electric current. It is also called photo-detector, light detector, or photo-sensor. Photodiode works in reverse bias condition, it means that the P-side of the photodiode is connected to the negative terminal of the battery and n-side is connected to the positive terminal of the battery. When a photon of ample energy strikes the diode, it makes a pair of electron-hole. This mechanism is called inner photoelectric effect. If the absorption arises in the depletion region junction, then the carriers are removed from the junction by the inbuilt electric field of the depletion region. Therefore, holes in the region move toward the anode, and electrons move toward the cathode, and a photocurrent will be generated. 

Answer 2

Operating principle of photodiode: A photodiode is a PN junction or PIN structure. When a photon of sufficient energy strikes the diode, it excites an electron, thereby creating a mobile electron and a positively charged electron hole. 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 field of the depletion region. Thus holes move toward the anode, and electrons toward the cathode, and a photocurrent is produced. Photovoltaic mode: When used in zero bias or photovoltaic mode, the flow of photocurrent out of the device is restricted and a voltage builds up. The diode becomes forward biased and "dark current" begins to flow across the junction in the direction opposite to the photocurrent. This mode is responsible for the photovoltaic effect, which is the basis for solar cells—in fact, a solar cell is just an array of large area photodiodes. Photoconductive mode: In this mode the diode is often (but not always) reverse biased. This increases the width of the depletion layer, which decreases the junction's capacitance resulting in faster response times. The reverse bias induces only a small amount of current (known as saturation or back current) along its direction while the photocurrent remains virtually the same. Diagram optional: