Quick Revision :: Electronic Devices Class 12 CBSE

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Classification of metals, insulators and semi-conductors on the basis of energy bands

* In metals, the conduction band and valence band partly overlap each other and there is no forbidden energy gap.

* In insulators, the conduction band is empty and valence band is completely filled and forbidden gap is quite large = 6 eV. No electron from valence band can cross over to conduction band at room temperature, even if electric field is applied. Hence there is no conductivity of the insulators.

* In semiconductors, the conduction band is empty and valence band is totally filled. But the forbidden gap between conduction band and valence band is quite small, which is about 1 eV. No electron from valence band can cross over to conduction band. Therefore, the semiconductor behaves as insulator. At room temperature, some electrons in the valence band acquire thermal energy, greater than energy gap of 1 eV and jump over to the conduction band where they are free to move under the influence of even a small electric field. Due to which, the semiconductor acquires small conductivity at room temperature.

* Hole is a seat of positive charge which is produced when an electron breaks away from a covalent bond in a semiconductor. Hole has a positive charge equal to that of electron. Mobility of hole is smaller than that of electron.

* Extrinsic semiconductor - A doped semiconductor or a semiconductor with suitable impurity atoms added to it is called extrinsic semiconductor. Extrinsic semiconductor are of two types : N-type and P-type.

* n-type semiconductor - When a pure semiconductor of Ge or Si is doped with a controlled amount of pentavalent atoms, say arsenic or phosphorous or antimony or bismuth. We get n-type semiconductor or donor type semiconductor.

It is called n-type semiconductor because the conduction of electricity in such semiconductor is due to motion of electrons i.e., negative charges, or n-type carriers. It is called donor type, because the doped impurity atom donates one free electron to semiconductor for conduction.

In n-type semiconductor electrons are majority carriers and holes are minority carriers.

* p-type semiconductor - When a pure semiconductor of Ge or Si is doped with a controlled amount of trivalent atoms, say indium or boron or aluminium, we get p-type semiconductor or acceptor type semiconductor. It is called p-type because the conduction of electricity in such semiconductors is due to motion of holes i.e., positive charges. It is called acceptor type semiconductor because the doped impurity atom creates a hole in semiconductor which accepts the electron, resulting conduction in p-type semiconductor.

In p-type semiconductor, holes are majority carriers and electrons are minority carriers.

* Electrical conductivity of semiconductor (a) is the reciprocal of its resistivity (r) and is given by

Where ne, nh are the number density of electrons and holes and me mh are the mobility of electrons and holes in the given semiconductor.

* P-N junction - When a p-type crystal is brought into close contact with n-type crystal, the resulting arrangement is called p - n junction or junction diode.

* Depletion region is a layer created around the junction which is devoid of free charge carriers and has immobile ions.

* Forward biasing of p-n junction. When positive terminal of external battery is connected to p-side and negative to n-side of p-n junction, the p-n junction is said to be forward biased. In forward biasing, the conduction across p-n junction takes place due to migration of majority carriers ( i.e. electrons from n-side to p-side and holes from p-sides to n-side). The size of the depletion region decreases. The resistance of p-n junction becomes low.

* Reverse biasing of p-n junction. A p-n junction is said to be reverse biased if the positive terminal of the external battery is connected to n-side and the negative terminal to p-side of p-n junction. In reverse biasing, the conduction across the p-n junction does not takes place due to majority carriers but takes place due to minority carriers if the voltage of external battery is large. The size of the depletion region increase. The resistance of the p-n junction becomes high in reverse biasing.

* Junction-Transistor is a semiconductor device which is obtained by growing a thin layer of one type semiconductor in between two thick layers of other smaller type semiconductor i.e., the semiconductor device having two junctions and three terminals.

If central thin layer is of p-type and outer thick layers are of n-type semiconductor, we get n-p-n transistor. If central thin layer is of n-type and outer thick layers are of p-type semiconductor, we get p-n-p transistor.

The thin layer of junction transistor is said to form the base (B). One of the thick layers serves as emitter (E) and the other thick layer serves as collector (C).

The function of emitter is to emit the majority carriers. Function of collector is to collect the majority carriers and base provides the proper interaction between the emitter and the collector.