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Definition of solar cell

 Solar cell is that cell which converts light energy to electrical energy. Working principal of solar cell is related to emission of light or absorption. It is an electronic device. Actually solar cell is p-n junction device which convert solar energy to electrical energy.

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There are two types of semiconductors

(i)  Intrinsic or pure semiconductor

(ii)  Extrinsic or impure semiconductor

(i)  Intrinsic or pure semiconductor: The conduction band of silicon and germanium is empty and the valence band is fully filled up with electrons at very low temperature. Germanium and silicon have four valence electrons. Each atom of germanium silicon shares one electron with its neighboring atom. Thus covalent bond is made. So, there is no free electron in germanium and silicon. For this reason there is no conduction of electricity in them. Such pure semiconductors are known as intrinsic semiconductors. If pure semiconductors are heated at high temperature due to thermal agitation electrons of pure semiconductors becomes free by breaking the bonds. The electrons can pass forbidden gap if the energy of their electrons is very large and transferred into conduction band. When an electron goes into conduction band from valence band there a vacancy occurs. The vacancy makes a hole and this hole is equal to a positive charge.

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 1.   Insulator: Through these materials electricity cannot pass is called insulators. Plastic, glass, wood etc are the examples of insulators. The valence band of those material remains full of electrons. The conduction band of those material remains empty. The forbidden energy gap between the conduction band and the valence band is widest. The difference is more than 10ev. Crossing the forbidden energy gap from valence band to conduction band large amount of energy is needed.

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What is Light emitting diode: Light emitting diode is a diode through which light emits. Popular name of light emitting diode is LED. It is short form of full name. When current flows through the LED, it becomes illuminated.  By increasing of current flowing the brightness or intensity increases. LED is used extensively in radio, tape-recorder, amplifier etc. It is also used in some calculator and electronic meters as digit shower.

LED Light emitting diode is a p-n junction diode. It emits light when it is forward biased.

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We use sinusoidal a.c. voltage in our house. A.C sinusoidal voltage changes its polarity continuously. Many electronic appliances we use in d.c voltage in our house like audio, amplifier, radio, tape, recorder etc. To operate those D.C. electronic appliance we need D.C. voltage but we are using a.c. voltage. We can solve the problem using p-n junction diode which rectifies A.C. voltage to D.C. voltage.

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 When a p-n junction p-region is connected negative terminal of a battery or a potential difference and n-region is connected with positive terminal of the battery this method of connection is called reverse bias. A circuit diagram is shown in figure.

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Definition of forward biasing

When a battery or a potential difference is used to the p-region of p-n junction for making p-region positive then this supplying method is called forward bias. The positive terminal of the battery is connected with the p-region and the negative terminal of the battery is connected with the n-region of the diode. When the positive charges from the battery go to the p-region, it repels the holes. On the other hand the negative charge from the battery go to the n-region and repels electrons. So, the barrier of p-n junction decreases and the resultant field decreases. Since the voltage of the potential barrier is very small about 0.1V to 0.3V, hence application of a small forward voltage decreases the barrier.

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Zener diode is an important diode. It is used extensively electronic device. Many of electronic circuit require D.C. voltage to operate. But it is not always possible normally. The voltage can be stable if Zener diode is used. It gives regular voltage. A p-n junction diode meant to operate in the break down region is called a Zener diode. There is no difference between Zener diodes and normal diode except, it is properly doped. So as to have a sharp breakdown voltage. Zener diode is always connected in reverse mode so it is always reverse biased. For a semiconductor diode when reverse bias is increased then the critical voltage is gained where the reverse current increases sharply to a high value and the resistance of the diode reduces to zero. It is known as knee voltage or breakdown region. When the reverse current increases in the ordinary diode and cross the Zener voltage, it is destroyed or burnt. But Zener diode cannot burn or destroy for its construction. Zener diode is made of highly doped p and n type semiconductors.

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