Transistor

TRANSISTOR The name is transistor derived from ‘transfer resistors’ indicating a solid state Semiconductor device. In addition to conductor and insulators, there is a third class of material that exhibits proportion of both. Under some conditions, it acts as an insulator, and under other conditions it’s a conductor. This phenomenon is called Semi-conducting and allows a variable control over electron flow. So, the transistor is semi conductor device used in electronics for amplitude. Transistor has three terminals, one is the collector, one is the base and other is the emitter, (each lead must be connected in the circuit correctly and only then the transistor will function). Electrons are emitted via one terminal and collected on another terminal, while the third terminal acts as a control element. Each transistor has a number marked on its body. Every number has its own specifications. There are mainly two types of transistor (i) NPN & (ii) PNP NPN Transistors: When a positive voltage is applied to the base, the transistor begins to conduct by allowing current to flow through the collector to emitter circuit. The relatively small current flowing through the base circuit causes a much greater current to pass through the emitter / collector circuit. The phenomenon is called current gain and it is measure in beta. PNP Transistor: It also does exactly same thing as above except that it has a negative voltage on its collector and a positive voltage on its emitter.

Transistor is a combination of semi-conductor elements allowing a controlled current flow. Germanium and Silicon is the two semi-conductor elements used for making it. There are two types of transistors such as POINT CONTACT and JUNCTION TRANSISTORS. Point contact construction is defective so is now out of use. Junction triode transistors are in many respects analogous to triode electron tube. A junction transistor can function as an amplifier or oscillator as can a triode tube, but has the additional advantage of long life, small size, ruggedness and absence of cathode heating power. Junction transistors are of two types which can be obtained while manufacturing.

The two types are: 1) PNP TYPE: This is formed by joining a layer of P type of germanium to an N-P Junction

P

N

P

2) NPN TYPE: This is formed by joining a layer of N type germanium to a PN Junction.

N

P

N

Both types are shown in figure, with their symbols for representation. The centre section is called the base, one of the outside sections-the emitter and the other outside sectionthe collector. The direction of the arrowhead gives the direction of the conventional current with the forward bias on the emitter. The conventional flow is opposite in direction to the electron flow.

OPERATION OF PNP TRANSISTOR:A PNP transistor is made by sand witching two PN germanium or silicon diodes, placed back to back. The centre of N-type portion is extremely thin in comparison to P region. The P region of the left is connected to the positive terminal and N-region to the negative terminal i.e. PN is biased in the forward direction while P region of right is biased negatively i.e. in the reverse direction as shown in Fig. The P region in the forward biased circuit is called the emitter and P region on the right, biased negatively is called collector. The centre is called base.

The majority carriers (holes) of P region (known as emitter) move to N region as they are repelled by the positive terminal of battery while the electrons of N region are attracted by the positive terminal. The holes overcome the barrier and cross the emitter junction into N region. As the width of base region is extremely thin, two to five percent of holes recombine with the free electrons of Nregion which result in a small base current while the remaining holes (95% to 98%) reach the collector junction. The collector is biased negatively and the negative collector voltage aids in sweeping the hole into collector region. As the P region at the right is biased negatively, a very small current should flow but the following facts are observed:1)

A substantial current flows through it when the emitter junction is biased in a forward direction.

2)

The current flowing across the collector is slightly less than that of the emitter, and

3)

The collector current is a function of emitter current i.e. with the decrease or increase in the emitter current a corresponding change in the collector current is observed.

The facts can be explained as follows:1.

As already discussed that 2 to 5% of the holes are lost in recombination with the electron n base region, which result in a small base current and hence the collector current is slightly less than the emitter current.

2.

The collector current increases as the holes reaching the collector junction are attracted by negative potential applied to the collector.

3.

When the emitter current increases, most holes are injected into the base region, which is attracted by the negative potential of the collector and hence results in increasing the collector current. In this way emitter is analogous to the control of plate current by small grid voltage in a vacuum triode.

Hence we can say that when the emitter is forward biased and collector is negatively biased, a substantial current flows in both the circuits. Since a small emitter voltage of about 0.1 to 0.5 volts permits the flow of an appreciable emitter current the input power is very small. The collector voltage can be as high as 45 volts.