CH 9 SEMI-CONDUCTOR DEVICES #) Energy band of a solid. The large number of energy levels confined in a small region of energy range of a given solid, constitute what is known as energy bands.

In some solids, there is an energy gap in between the energy bands. This energy gap is called forbidden gap. The energy band above the forbidden gap is called conduction band and the energy band below the forbidden gap is called valence band.

The conductivity of a solid depends upon the number of electrons present in the conduction band and number of holds present in valence band.

#) Classification of metals, insulators and semi-conductor on the basis of energy bands

(A) In metals. The conduction band and valence band partly overlap each other and there is no forbidden energy gap.

(b) 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.

(C) In semiconductor. The conduction band is empty and valence band is totally filled at 0K but the forbidden gap between conduction band and valence band is quite small, which us about 1eV. No electron from valence band can cross over to conduction band at 0K. Therefore ,the semiconductor behaves as insulator at 0K. 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. It 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.

#)Doping. It is a process of deliberate addition of a desirable impurity to a pure semiconductor to modify its properties in a controlled manner. The impurity atoms added are called do pants. The impurity added may be = 1 in 1010 atoms.

#) 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

(I) n-type semiconductor. When a pure semiconductor of Ge or Si is doped with a controlled amount of pentavalent atoms, say arsenic or phosphorus or antimony or bismuth. We get n-type semiconductor or donor type semi-conductor.

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 carries and holes are minority carries.

(ii) 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 aluminum, we get p-type semiconductor or accepter type semiconductor. It is called p-type because the conduction of electricity in such semiconductor is due to motion of holes I.e. positive charges. It is called acceptor type semi-conductor because the doped impurity atom creates a hole in semiconductor which accepts the electron, resulting conduction in p-type semi-conductor.

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

#)Effect of temperature om conductivity of semi conductor. With the increase in temperature, more number of covalent bonds is broken, resulting large

increase in current carrier concentration. Due to which the conductivity of semiconductor increase.

Mobility of electrons as well as holes in a semiconductor decreases with increase in temperature.

#)p-n junction. When a p-type crustal is brought into close contact with n-type crystal, the resulting arrangement is called p-n junction or junction diode.

#)Depletion region. It 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. The size of the depletion region decreases. The resistance of the 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 take place due to majority carriers but takes place to minority carriers if the voltage of external battery is large. The size of the depletion region increases. The resistance of the p-n junction becomes high in reverse biasing..

#)Characteristics of p-n junction diode. The graphical relations between forward bias voltage and forward current are called forward characteristics. The graphical relations between reverse bias voltage and reverse current is called reverse characteristics of p-n junction.

#) Knee voltage. It is the forward voltage beyond which the current through the junction starts to increase rapidly with linear variation. But below the knee voltage, the characteristics curve is non linear.

The knee voltage for Si is 0.7V and for Ge is 0.3 V.

#)Junction-Transistor. It is a semiconductor devise which is obtained by growing thin layer of one type semiconductor in between two thick layers of

other similar type semiconductor I.e. the semiconductor device is 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 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 interact between the emitter and the collector.

#) Analogue signal. A continuous time varying current or voltage signal is called analogue signal.

#)Analogue circuit. An electronic circuit which gives out any type of analogue signals is called analogue circuit.

#)Digital signal. A signal which has two levels of voltage is called digital signal.

#)Logic gate. A digital circuit which either allows a signal to pass through or stops it, is called gate. Such gate allows the signal to pass through only when some logical conditions are satisfied. Hence they are called logic gates.

#)Truth table. It is a table that shows all possible input combination and the corresponding output combination for a logic gate. It is also called a table of combinations.