A solar cell or solar battery is also a PN junction diode. It converts Solar energy into electrical energy. It is also solar energy converter
A solar cell is made up of germanium or silicon material. A glass window is provided at the top of the P‑type layer.
The thickness of the P layer is small so that the incident light can easily reach the junction of the diode. A nickel plated ring is provided around the P‑layer which acts as a positive terminal. A metal contact provided at the bottom of the N‑layer acts as a negative terminal.
When lights fall on the solar cell the light photons collide with valence electrons. Hence the valence electrons leave from the parent atoms.
Thus free electrons are generated in P region and holes are generated in N region. Now the electrons move towards the N region and holes move towards the P region and produce minority current
The amount of minority current depends upon the intensity of incident light and also the surface area of the diode. If an external load is connected across the solar cell it develops a current flow through it.
Uses of Solar Cell
- 1. Used as Power source in satellites
- 2. Used to charge storage batteries
- 3. Used as solar heater
Avalanche Photo Diode
Avalanche Photo Diode is special purpose opto electronic device similar to zenar diode. It converts an incident optical signal into equivalent electrical signal and effectively amplifiers it by means of avalanche effect.
The p type substrate has n and p layers diffused into it before insulation and metallization are applied.
When a sufficient amount of reverse voltage is applied to the avalanche photo diode an extremely high current will flow due to avalanche effect.
Normally more reverse voltage but less than avalanche threshold is applied breakdown may occur and large current will flow when light strikes the junction.
This high current requires less amplification than the small current in the standard photo diode. The avalanche photo diodes are also working very fast. It can handle the data rates of very high giga bit per second
A photo transistor is a type of NPN transistor. It is generally used in common emitter configuration. The bias voltage is applied between the collector emitter circuit with the base left open. The function of transistor is controlled by light energy.
The construction of a photo transistor is just like a conventional NPN transistor with a little hole made on the surface near the collector base junction. A small lens is fixed on this hole for allowing a focused light beam to concentrate the collector base junction.
The modern photo transistors uses highly light effective materials instead of making a hole and fixing a lens on it.
The voltage applied to the transistor makes emitter base junction Je forward biasing and collector base junction Jc reverse biasing.
When the transistor is kept in darkness there will be very few minority charge carriers flow. This makes negligible collector current.
On light being focussed at the collector base junction the transistor starts flowing through the reverse biased junction. The amount of current flow depends upon the intensity of focussed light
Infrared Emitting Diode
Infrared emitting diodes are fabricated by using gallium arsenide indium phosphide materials. When the junction is forward biased electrons from the n region will recombine with excess holes of the p region
During this recombination process energy is generated from the device in the form of photons. The generated photons will either be reabsorbed in the structure or leave the surface of the device as radiant energy.
The wavelength of infrared light is just below red light. This light is not visible by the naked eye
Optical Transmitter Using Infrared LED
The binary pulses to be transmitted are applied to a logic gate. These pulses are used to drive the transistor Q that turns LED, ON and OFF. A positive pulse at the NAND gate input causes the NAND gate output to go zero.
This turns OFF the transistor Q so the LED is forward biased through Rc and turns ON, produce brilliant high intensity light. High intensity is required if data is to be transmitted to long distances
LED transmitters are good for only short distances. Turn OFF and turn ON times are no faster than several nanoseconds and therefore transmission rates are limited.
The receiver part of the optical communication system is relatively simple. It consists of a detector that will sense the light pulses and convert them into an electrical signal. This signal is then amplified and shaped into original serial digital data
The most widely used light sensor is a photo diode. It is more sensitive to light. The diode is normally reverse biased by the supply voltage. Only a small leakage current flows through the diode under in reverse bias condition.
Whenever light strikes the diode this leakage current will increase significantly. It will flow through a resistor and develop a voltage across it. The result is an output voltage pulse.
The voltage pulse is very small so it is amplified properly by using amplifier circuit
This output is then passed through a logic gate. So that correct binary voltage levels are produced.
Laser is the shortened form of Lic Amplification by Stimulated Emission of Radiation. laser emits radiation of essentially one wavelength, or a very narrow band of wave lengths. This means that the light has single colour, is monochromatic Laser light is also referred to as coherent light.
Laser diode is a special type PN junction diode, fabricated by using galium arsenide (GaAs) materials. The length of junction is related to the wavelength of the light to be emitted.
The ends of the junctions are each polished to a mirror surface and may have an additional reflective coating. The purpose of their polishing and coating is to reflect internally generated light back into the junction.
One end is only partially reflective, so that light can pass through when lasing occurs.When the junction is forward biased, the forward current increases. It will increase a number of charge carriers, and enters in the depletion region and excite the atoms that they strike.
The atoms first emit photons of energy randomly as electrons are raised to a high energy level and then fall back to a lower level. Sooner and later several photons strike the reflective ends of the junction perpendicularly.
So that they are reflected back along their original (incident) path. These reflected photons are then reflected back again from the other end of the junction. The reflection back and forward continuous thousands of times, make more amplification of the initial reflected photons of light.
The beam of laser light emerges through the partially reflective end of the junction. The intensity of light is directly proportional to the input current.Because of high energy density, a laser beam can be quite dangerous. Eye protection must be worn when working with these devices.