Semiconductor physics | Part-02 ~ MONU TUTORIALS ACADEMY

Conductivity of semiconductor

The resultant conductivity of a semiconductor depend upon;

The concentration of mobile charge carriers example electron and holes and
The mobile TV of the charge carriers.

Silicon versus germanium

in the early days of semiconductor device germanium was considered to be the best semiconductor material. But nowadays it is rarely used in new design of semiconductor device. Silicon is considered to be the best for the preparation of semiconductor device. it is because at room temperature former a silicon crystal has almost no free electron compared with the germanium crystal because of this property the semiconductor device made from silicon material give for better performance than the semiconductor device made form germanium.

Thus, silicon has totally overshadow germanium in the fabrication of semiconductor device example diode and transistor thyristor etc.

Extensive semiconductor

all through and intrinsic semiconductor is capable to conduct a little current even at room temperature but as it is not useful for the preparation of various electronic devices. to make it conductivity a small amount of suitable imported is added. It is called then extensive impure semiconductor.

Doping :- the process by which an impurities added to a semiconductor is known as doping.

a semiconductor to which an impurity at controlled rate is added to make a conductive is known as extrinsic semiconductor.

As discuss earlier the purpose of adding impurity in the semiconductor crystal is to increase the number of free electron or holes to make it conductive. If a pentavalent impurities is added to a pure semiconductor a large number of free electron will exit in IT. Whereas if a trivalent impurities as a addeda large number of holes will exit in the semiconductor fullstop depending upon the type of impurity added extrinsic semiconductor may be classified as.

N type semiconductor
P type semiconductor

N type semiconductor

when a small amount of pentavalent impurity is added to a pure semiconductor providing a large number of free electron in the extensive semiconductor thus formed is known as and type semiconductor.

the addition of pentavalent impurities such as arsenic atomic number 33 and antimony atomic number 51 provide a large number of free electron in the semiconductor crystal such impurities which produce and type semiconductor are known as donor impurity because each atom of them donate one free electron to the semiconductor crystal has explain.

The following points are worth noting;

a large number of free electron are made available by the addition of pentavalent impurities.
a minute quantity of free electron are made available by generation of hole electron pair when thermal energy at room temperature is imparted to the semiconductor crystal. This electron leave behind holes in the valence band.
the number of free electron provided by the pentavalent impurities far exceed the number of holes coma it is due to this pen dominance of electron over holes that the material is called and type semiconductor material.

P type semiconductor

when a small amount of trivalent impurity is added to a pure semiconductor providing a large number of holes in it the extrinsic semiconductor thus formed is known as P type semiconductor.

the addition of trivalent impurities such as gallium atomic number 31 and indium atomic number 49 provide a large number of holes in the semiconductor crystal. such impurities which produce P type semiconductor are known as acceptor impurity because each atom of them create one hole which can accept one electron.

Charge on n-type and p-type semiconductors

it has been discussed above that in n type semiconductors comic induction is due to free electron donated by the pentavalent impurity atoms. This is electron are the access electron with regard to the number of electrons needed to fill the covalent bonds in the semiconductor crystal. This excess electrons does not create any charge on the n-type semiconductor, since impurities atom as well as germanium atom all are electrically neutral themselves.

Similarly, in p-type semiconductors, conduction is due to holes created by a trivalent impurity atoms these holes are the positively charged vacant space which can accept the electrons. The host just represent the difficulty of electron with regard to the number of electrons needed to fill the covalent bonds in the semiconductor crystal. this defect of electron do not create any charge on the p-type semiconductor, since impurity atoms as well as germanium atoms all are electrically neutral themselves.

Majority and minority carriers

when a small amount of pentavalent impurity is added to a pure semiconductor it provides a large number of free electron in the crystal forming and type semiconductor. However it may be recalled that even at room temperature commerce some of the covalent bonds break releasing a small number of all electron pairs.

Thus, an n type semiconductor container large number of free electrons but only a few number of holes see in figure. therefore, in n type semiconductor the most of the current conduction is due to free electron available in the semiconductor.

Thus,in n type semiconductor the electron are the majority carrier, whereas the holes are the minarti carriers see figure 8.32

giving the similar explanation it can be concluded that in p type semiconductor comedy the holes are the majority carriers whereas the electrons are the minority carriers.

PN junction

when a p type semiconductor is suitably joined to an n type semiconductor coma the contact surface so formed is called PN junction.

Formation of PN junction

in actual practice the PN junction is not formed by Justin bringing at P type semiconductor block near to an n type semiconductor block as its looks. ActuallyPN junction is fabricated by special techniques namely growing, lying and diffusion method.

the most common method of making PN junction is called alloying. And alloy junction is made from an n type silicaof semiconductor by melting a palette of trivalent Indium placed on the Slice full stop this is done by hitting the system at about 500 degree Celsius. The indiumis absorbed into the germanium of silicon to produce a p reason and hence a PN junction is formed as shown in figure

Properties of PN junction

to understand the properties of a PN junction, consider two types of extrinsic semiconductor one P type and the other and type as shown in figure the p-type semiconductor is having negative acceptor ions and positive charge the holes where as the n-type semiconductor is having positive donor ions and negative charge electron.

the two pieces are joined together and suitably treated they form a PN junction. The moment they form a p-n junction some of the conduction electrons from antic material diffuse over to the p-type material and undergo electron-hole cream combination with the holes available in the valence band. simultaneously holes from P type material diffuse over the n-type material and undergo Holi electron combination with the electron available in the conduction band. This process is called diffusion. in this process some of the free electron moving across the junction from and type of p-type leaving behind positive donor ions as they are robbed of the free electron. This stabilizes simultaneously the free electron which cross over the the junction be combined with the holes of p-type and uncover some of the negative receptor iron as shown in figure.

desi stabilizers and negative charge on the P side of the junction. This process of diffusion continue still a sufficient number of donor and acceptor impurities ions are uncovered and steplizer request potential difference full stop after this form of other diffusion is prevented because now positive charge on and side repel holes to cross from P type 2 and type and negative charge on side ripples free electron to enter from and type 2 p type. A potential difference created across the junction act as barrier which for the movement of charge carriers this is called a potential barrier or junction barrier Vo.

Potential barrier :-

a potential difference built up across the PN junction which restricts for the moment of charge carrier across the junction is known as potential barrier.

it may be noted that on the both side of the junction a layer is formed this layer is depleted of free electron and holes. This reason is called depletion layer.

Depletion layer :-

arisen around the junction from which the charge carriers free electron and hole sa depleted is called depletion layer.

Behaviour of a PN junction under biasing

when a PN junction is connected across an electric supply the junction is said to be under biasing.

the potential difference across the PN junction can be applied in two ways namely forward biasing and reverse biasing.

Forward biasing :- when the positive terminal of DC source or battery is connected to P type and negative terminal is connected to n type semiconductor of NPN junction as shown in figure the junction is said to be forward biasing.

Reverse biasing :- when the positive terminal of DC source or battery is connected to and type and negative terminal is connected to P type semiconductor of a PN junction as shown in figure the junction is said to be in the reverse biasing.

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