TRANSISTOR

What is transistor ?

Transistor – A transistor consists of an any type of semiconductor (let p-type) in which a layer of other type semiconductor (let n-type) 25 sandwiched between them. Thus transistor is a three terminal semiconductor device, where the terminals are called –

i) emitter (E)

ii) base (B)

iii) Collector (C)

Why is so named ; transistor ?

The transistor is the name which is abbreviated from “transfer resistor”, as the current is transferred from low

resistance region to high resistance region.

Categories of transistors –

There are two types of transistors –

1) N-P-N transistor ->

2) P-N-P transistor ->

 

Biasing of a transistor –

Any transistor consists of two junctions –

i) emitter junction, JE (between emitter and base)

ii) collector junction, Jc (between collector and base).

For forward biasing –

Emitter junction is always forward biased.

Collector junction is always reversed biased.

 

Explain the doping profile of a transistor –

For a transistor as the emitter junction is at forward biased thus the majority carriers enter to the base region from emitter region. Some of these carriers will recombine with the majority carriers of the base region and the rests are able to reach the collector region.

To reduce recombination, the doping at the base region should be small. On the other hand to avoid pinch through the doping at the base region should be large. Thus it should be make e compromise.

In transistor, as the current flows from low resistance to high resistance, thus emitter region will be of low resistance and the collector region will be of high resistance.

To make it, the doping at the emitter region  should be, large whereas at the collector region should be small.

 

The base region of a transistor is made very thin compared with emitter and collector region – why?

The width of the base is made very thin-

i) To increase the electron concentration gradient in the base region of an N-P-N transistor, thereby enhancing the diffusion current and

ii) to diminish the number of electrons lost by recombination in the base region.

 

Show with a net diagram of different current components in a P-N-P transistor. Find the relation between emitter, base and collector current.

Transistor current components for a forward biased emitter junction and a reverse biased collector junction is shown in figure.

A) Emitter current –

The emitter current IE consist of

i) hole current IPE ;  due to hole form E to B..

ii) electron current INE ; due to electron from B to E.

Thus

Practically base is lightly doped, hence

 

B) Base Current

The base current IB consists of,

i) The diffusion of electron from base to emitter, we get current component INE , called current component due to diffusion of electron from base to emitter.

ii) Some of the holes entering into the base from emitter recombine with the electrons in the base region and are able to reach the collector which gives collector current. The recombination base current

iii) Since collector junction is reverse biased thus we get a current due to drift of minority carriers, this current is called ‘leakage current’.

This leakage current Ico has two components –

a) Ipco ; due to drift of holes from B to C.

b) Inco ; due to drift of electrons from C to B.

 

C) Collector current –

i) The rest holes enter into the collector from base which gives collector hole current lpc.

ii) Leakage current due to minority carriers, lco

Thus

From equation 1, 2 and 3

Therefore, in the transistor, a current flows from a low resistance input circuit is transferred to a high resistance output circuit with almost unchanged magnitude and we get a power gain. Hence the name ‘transfer – resistor’ is abbreviated as transistor.

The value of α- increases with increasing reverse bias voltage of the collector junction – why like that?

With increasing reverse bias of the collector junction the effective base width decreases. This reduces the possibility of recombination of the carriers injected from the emitter into the base, leading to the increase of α.

 

What is ‘thermal run away’?

If temperature increases then the leakage current within the transistor with also increases. As a result of which more heat develops and again temperature increases, hence leakage current again increases. This process will continue in nature and the finally, the transistor will or may burn out. This phenomena is called ‘thermal runaway’.

 

Two p-n junction diodes, joined externally in opposition not form a transistor – explain.

Two p-n junction diodes having metal leads and connected back to back will not make a transistor because then the base region will be thicker. Again the contact potential at the metal conduction junction will not give the desired energy.

In a transistor, the emitter region is heavily doped the collector region must be highly doped. Moreover,for commercial transistor, collector -base junction’s area is made considerably larger than that of the emitter base junction.

When two diodes are joined back to back the above conditions are not obtained, hence it is impossible to make a transistor by joining two diodes back to back.

 

Thermal noise in CE circuit is much higher than that in a CB circuit – explain.

We have,

Therefore, the unwanted reverse saturation current which has got its origin in thermal generation, is multiplied  times in CE mode. Hence thermal noise in CE circuit is much higher than that in a CB circuit.

 

What is “early effect” ?

If the reverse bias voltage I VCB l of the collector junction is increased the width of the depletion region of the collector base junction increases. The effective width of the base region is reduced. This change of effective base width by collector voltage is known as Early effect. It is also known as base width modulation.

 

What do you mean by “punch through” ?

When the reverse bias of the collector junction is increased, the effective base width decreases due to Early effect. At a certain reverse bias of the collector junction the depletion region, covers the base i.e. effective base width becomes zero. Then the potential barrier at the emitter junction is lowered. For this, an excessively large emitter current flows. This phenomena is called punch through.

Why is biasing of transistor is necessary ?

Biasing of a transistor means d.c. sources connected to the emitter base junction such that the function is forward biased and d.c. sources are connected to the collector – base junction such that the junction in reverse biased. Also the establishment of the d.c. operating point at suitable location on the active region of the characteristics must necessary for stability of biasing.

Thus d.c. sources are necessary to make the transistor able to amplify signals. So to analyse the performance of a transistor amplifier it becomes important.

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