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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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When an n type semiconductor is joined to a p type semiconductor such that the crystal structure remains continuous at the boundary, the junction is called the p-n junction and the resulting combination is called a p-n junction diode or semiconductor diode.

At high temperature one half p-type semiconductor is mixed with the half n-type semiconductor such way p-n junction made. We know that electrons are majority carriers in n type semiconductors and holes or positive charges are majority carrier in p type semiconductors. So, p type portion contains positive charges majority carrier and n type portion has electrons as majority carrier at p-n junction.

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A pentavalent impurity like arsenic, phosphorous is added with pure semiconductor like silicon or germanium for making n type semiconductor. The most important requirement is that the size of the impurity atom should be nearly equal to the size of the atoms in the pure semiconductor silicon or germanium in this case. At a high temperature arsenic or antimony pentavalent atoms are added by special technology. The main structure of silicon or germanium should not be changed so the amount of impurity is controlled at the time of mixing rather these atoms are incorporated in the crystal lattice.

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Trivalent element like gallium or aluminum is mixed with pure semiconductor like silicon or germanium to make p type semiconductor. Three valence electrons of aluminum make bond with neighboring electrons of silicon. Here three covalent bonds are made. But the fourth bond is not completed. It is needed one electron to complete the bond. One electron of neibouring bond jumps into the vacancy and make a covalent bond with the vacancy. Now the bond is completed. A positively charge produces with covalent bond. In this way for each impurity atom accepts an election and a positive charge or hole produces in the semiconductor.

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The substances by which electricity or charges flow is called medium. It is classified into three types are

(i) Conductor,

(ii) Semiconductor

(iii) Non conductor

(i) Conductor: conductor is this substance in which electricity or charges can flow easily. Examples of conductors are copper, gold, iron etc metal, human body, acids, bases etc. through these substances charges can flow easily so they are conductors. Electrical conductivity of electricity conductors are also depends upon temperature. Conductivity of conductor in electricity decreases when temperature increases and conductivity increases when temperature decreases.

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Three types substances are mainly we have viz solid, liquid and gases. Solids are classified in three types according to electrical conductivities. This substances through electricity or charges can pass easily facing no prevention is called conductors. Conductors of electricity are silver, gold, iron, copper etc from metal. From these substances electricity cannot pass easily called insulators. The examples of insulators are rubber, mica, quartz, ceramics, glass ebonite etc.

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