What is meant by SOLAR CELL | Avalanche photo diode | Infrared Emitting Diodes| Optical Receiver | Laser Diode

Solar cell

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. 1. Used as Pow­er source in satellites 
2. 2. Used to charge stor­age batteries 
3. 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 

Photo Transistor

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. 

Optical Receiver

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 diode

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.

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