More topics on Transistors:
Transistors are the fundamental building block of modern electronic devices basically used for controlling, amplifying and generating electrical signals. We have covered basics of transistors in our previous tutorial. In this article, we will focus our attention on the Transistor working and construction.
A Transistor is a three layer semiconductor device in which one type of semiconductor is sandwiched between two similar types of semiconductor.
It is of two types:
(i) P-N-P Transistor: In this type of transistor, n-type semiconductor piece is sandwiched between two pieces of p-type semiconductor layers.
(ii) N-P-N Transistor: In this type of transistor, p-type semiconductor piece is sandwiched between two pieces of n-type semiconductor layers.
In each type of the transistor, the following points may be noted:
- Most important factor in the function of a transistor is the middle section which is of a very thin layer.
- Transistor is a combination of two p-n junction, that’s why it is considered as a combination of two diodes connected back to back.
For the sake of convenience, the transistors are represented below by a schematic diagram. The symbols of NPN and PNP transistors are shown distinctly:
Note: The emitter is shown by an arrow which indicates the direction of flow of conventional current with the forward bias. In NPN transistor, the conventional current flows out of the emitter as indicated by the outgoing arrow while in PNP transistor the current flows into the emitter as indicated by the inward arrow.
Transistors comprise of three sections of doped semiconductors. The portion on one side is the emitter and the portion on the opposite side are the collector. The Middle portion is known as the base which forms two junctions between the emitter and the collector as shown in the figure.
The 3 terminals of transistor is explained briefly:
(i) Emitter: It is a heavily doped terminal through which electrons enters. It is always forward biased w.r.t. base so that it can supply a large number of majority carriers. As shown in figure below, the emitter of PNP transistor is forward biased and supplies hole charges to its junction with the base while the emitter of NPN transistor (above figure) supplies electron charges to its junction.
(ii) Base: It is lightly doped terminal through which electrons passes from the emitter to collector. The base-emitter junction is forward biased and allows low resistance to the emitter circuit while the base-collector junction is reverse biased and provides high resistance in the collector circuit.
(iii) Collector: It is a moderately doped terminal which basically collects electron. The collector is always reverse biased and its function is to remove charges from its junction with the base. As shown in the figure above, the collector of the PNP transistor is reverse biased and receives hole charges that flow in the output circuit while the collector of NPN transistor receives electron charges.
Transistor Working and its operation
For an appropriate working of transistor, the emitter-base junction is forward biased and collector base junction is reverse biased. For instance, if we ignore emitter-base junction then practically due to reverse bias no current will flow in the collector circuit. However, if the emitter-base junction is also present then due to its forward bias emitter current starts to flow to the collector circuit. It is seen that this emitter current almost entirely flows in the collector circuit.
For e.g. if the emitter current is zero then the collector current is also zero while if the emitter current is 2mA then the collector current is also 2mA. This is the basic operation of a transistor.
Now we are going to explain working of NPN and PNP transistors. Have a look over it.
(i) Npn Transistor Working:
In this transistor, the forward bias causes the electrons in the n-type emitter to flow towards the base which constitutes the emitter current IE . When these electrons flow towards the base, it got combined with the majority carriers i.e. holes of the base. As we know, the base is very thin & lightly doped, therefore, to constitute base current IB only a few electrons i.e. less than 5% got combine with the holes. The remaining electrons i.e. more than 95% passes to the collector region to constitute collector current IC. In this way, the entire emitter current flows in the collector circuit. Therefore, it is clear that the emitter current is equal to the sum of collector current and base current i.e.
IE = IB + IC
(ii) Pnp transistor working:
In this transistor, the forward bias causes the holes in the p-type emitter to flow towards the base which constitutes the emitter current IE . When these holes flow towards the base, it got combined with the majority carriers i.e. electrons of the base. As we know, the base is very thin & lightly doped, therefore to constitute base current IB only a few holes i.e. less than 5% got combine with the electrons. The remaining holes i.e. more than 95% passes to the collector region to constitute collector current IC. In this way, the entire emitter current flows in the collector circuit.
Note: Current conduction in NPN type transistor is by electrons while in PNP transistor is by holes.
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