Transistor for std to 10 C B S E Course

3.5  TRANSISTORS

The term ‘transistor’ is a shortened form for transfer resistor. Transistor is a specially constructed three terminal semiconductor device. It is a device which consists of either one p-type region sandwiched between two n-type regions (npn transistor) or one n-type region sandwiched between two p-type regions (pnp transistor). The middle region is called the base and the two end regions are called the emitter and the collector. Figure shows npn transistor with its circuit symbol.

Transistor

Transistor was first fabricated in 1947 at the Bell Telephone Laboratories, by three American physicists John Bardeen, Walter H. Brattain and William B. Shockley. They were awarded Nobel prize for physics in 1956.

npn Transistor

A pnp transistor with its circuit symbol is shown in Fig. The   base is very thin compared to the emitter and the collector. In a transistor, the emitter is more doped then the collector region and the base is lightly doped. The function of the emitter is to emit electrons (in the case of npn transistor) or holes (in the case of pnp transistor) into the base. The base controls the flow of charge carriers into the collector which collects them from the base. Emitter of a transistor is always forward biased and the collector is always reverse biased.

pnp Transistor

Action : in the circuit shown in fig. using a n-p-n transistor the voltage from a source is applied to the base of the transistor. The emitter n layer has a small negative voltage with respect to the base p layer to permit the forward flow of current across the n-o junction. The collector n layer has a large positive voltage with respect to base p layer to prevent the reverse flow of current. Electrons from the emitter are attracted to the collector and a current flows through the outer circuit.

Circuits of npn transistor

To understand the practical application of the circuit, in Fig  3.6, circuit shown in fig 3.7 is considered. A variable weak voltage is applied across a resistor in the emitter base circuit(This is called input voltage) . The output voltage is taken across a resistor (called load) in the collector emitter circuit. Small changes in the input voltage result in large changes in output voltage; in other words, there is amplification.

Amplication of npn transistor

Applications of transistors :

            Transistor is used in

                     (i)        Amplifier

                    (ii)        Oscillator

                   (iii)        Switching circuits.

            Oscillator is a device for producing electric oscillations of a desired frequency.

Activity

Draw an appropriate circuit diagram, using p-n-p transistor.

For more see on LABLE list on right side.

Types of Semiconductors Std 8 to 10

3.3  TYPES OF S SEMICONDUCTORS

In a pure semiconductor, holes and electrons are always present in equal numbers and the resulting conductivity is called intrinsic conductivity. Pure semiconductors are called intrinsic semiconductors.

            The conductivity of semiconductors can be enhanced by the addition of minute traces of impurities (other elements) called dopants. The process of adding dopants is called doping and the resulting semiconductor is called doped semiconductor or extrinsic semiconductor. The conductivity in this case, is called extrinsic conductivity.

            There are two types of extrinsic semiconductors depending upon the types of impurities. They are classified as

(A) n-type semiconductors and

(B) p-type semiconductors.

These are used in the manufacture of diodes and transistors.

(A)   n-type semiconductors : if a very small amount of pentavalent impurity for example antimony (or arsenic or phosphorous), is introduced into the crystal of germanium, four out of five electrons of each impurity atom enters into bonds with the nearest germanium atoms, to form covalent bonds. The fifth electron is set free. Such free electrons act as current carries.

The added pentavalent impurity donates electrons to the semiconductor and the semiconductor becomes n-type semiconductor. The name n-type indicates that the majority charge carries are electrons. There will be free-electrons and holes in equal numbers due to breaking of some covalent bonds.

n-type Semiconductors

(B)   p-type semiconductors : When a trivalent impurity like boron, indium, gallium or aluminum is introduced into a germanium crystal, each impurity atom takes away one electron from the neighboring germanium atoms, to form covalent bonds. This results in the creation of  holes. Electrons from neighboring atoms can fill up these holes and there will be apparent motion of holes. Thus holes act as current carries. The added trivalent impurity accepts electrons from the semiconductor and the semiconductor becomes a p-type indicates that the majority charge carries are holes equivalent to positive charge. There will be free electrons and holes in equal numbers due to breaking of some covalent bonds.

p-type semiconductors