A photo diode is made from semiconductors like silicon, germanium, indium gallium, arsenide phosphide and indium gallium arsenide. It converts light or photons into electric current. It made from junctions of semiconductor, photodiodes are P-N junction , between the junction is a tiny instrinic (pure) semiconductor. Photo diodes also known as photodetector or light detector can be operated in reverse bias with the N-type comnected to the negative terminal of a battery and the positive terminal of the battery is connected to the P-Type of the photodiode.
Circuit symbol of a photo diode.
An area of photo diodes semiconductor is intentionally exposed to receive light and photons and also have optical filters, a built lense for concentrating the light and a big surface area to generate much electrons. All semiconductors are subjected to optical charge generation when exposed to light, that's why most diodes that doesn't need light to operate are encased in an opaque plastic casing.
Working principle of photodiodes.
When all semiconductor diodes have their p-n junction exposed to light, the convalent bond is ionised(Electrons starts moving), this produces electrons and protons (holes) by this action photo-current or photo electricity is produced. For photoelectricity to take place the a minimum of 1.1ev light must hit that exposed part.
When photodiodes are exposed to light the mobile electrons are negatively charged while that mobile holes are positively charged. In the depletion region, depletion energy will built in an electric field, due to the electric field electrons moves away from the junction to the cathode and the holes moves from the junction to the anode constituting a photocurrent.
Modes of operation of photodiodes.
•Photo voltaic mode.
When a photo diode produces current when exposed to electricity it is said to be in photovoltaic mode. The current produced by photodiodes are very small that a piece can't power an LED. So photodiodes when use basically for current generation, much are connected together in series to produce much voltage.
•Photoconductive mode.
In this mode a photo diode conducts electricity when the polarities are reversed i.e the cathode is made the anode and the anode is made the cathode.
Circuit applications of photodiodes.
•Substitute as solar cell.
Photodiodes undergoes photoelectric effect just like solar cells. It can be use to convert light into direct current in terms of few volts. It is not as efficient as solar cell. • As beam senser.
Photo diodes conducts when it is exposed to light, but blocks current from flowing when light is obstructed from it. Thus it can be use as motion sensor in rooms. When someone or an object moves in the room and blocks the path of light to the photodiode connected to a depletion mode MOSFET it can be use to sound a buzzer.
• Smartphone camera system.
At daylight smartphones don't flash light when taking pictures at daylight, but flashes it when it's dark. A photodiode makes this possible, when there is light or dark it sends signals to an integrated circuit which then controls the FLash light from an LED.
Tags:
Power Electronics