What Is a Transistor
When a third doped element is added to a crystal diode in such a way that two pn junctions are formed, the resulting device is known as a transistor.
A transistor consists of two pn junctions is formed by sandwiching either a p-type or n-type semiconductor between a pair of n-types or p-types semiconductors respectively.
There are two types of transistors, namely:
(i) n-p-n transistor (ii) p-n-p transistor
In an n-p-n transistor, two n-type semiconductors are separated by a thin layer of p-type semiconductor as shown in Fig.1(i). And in a p-n-p transistor, two p-type semiconductors are separated by a thin layer of n-type as shown in Fig.1(ii).
The following points can be noted in each type of transistors:
(i) These are two pn junctions. So, a transistor may be considered as a combination of two diodes connected back to back.
(ii) There are three terminals, one taken from each type of semiconductor.
(iii) The middle layer is very thin. This is the most important factor in the function of a transistor.
A transistor(npn or pnp) has three sections of doped semiconductors. The section on one side is called emitter and the section on the opposite side is called collector. The middle section is called the base and it forms two junctions between the emitter and collector.
(i) Emitter: The section on one side that supplies charge carriers( electrons or holes) is called the emitter. The emitter is always forward biased w.r.t. base so that it can supply a large number of majority carriers. In fig.2(i), the emitter (p-type) of pnp transistor is forward biased and supplies holes to its junction with the base. Similarly in fig.2(ii), the emitter (n-type) of npn transistor has a forward bias and supplies free electrons to its junction with the base.
(ii) Collector: The section on the other side that collects the charges is called the collector. The collector is always reverse biased. Its function is to remove charges from its junction with the base. In fig.2(i), the collector (p-type) of pnp transistor has a reverse bias and receives hole charges that flow in the output circuit. Similarly in fig.2(ii) ,the collector (n-type) of npn transistor has reverse bias and receives electrons.
(iii) Base: The middle section which forms two pn junctions between the emitter and collector is called the base. The base-emitter junction is forward biased and allows low resistance for the emitter circuit. The base-collector junction is reverse biased and provides high resistance in the collector circuit.
Before discussing transistor action, it is important to keep in mind the following facts about the transistor:
(i) The transistor has three regions, emitter,base and collector. The base is much thinner than the emitter while the collector is wider than both. However for the sake of convenience the emitter and collector are usually shown to be of equal size.
(ii) The emitter is heavily doped so that it can inject a large number of charge carriers into the base.
(iii) The base is lightly doped and very thin. So that less than 5% of charge carriers coming from the emitter can recombine in the base. And it can pass most of the emitter injected charge carriers to the collector.
(iv) The collector is moderately doped.
(v) The transistor has two pn junctions that means it is like two diodes. The junction between emitter and base may be called emitter-base diode or simply the emitter diode.The junction between base and collector may be called collector-base diode or simply collector diode.
(vi) The emitter diode is always forward biased and the collector diode is always reverse biased.
(vii) The resistance of emitter diode is very small as compared to collector diode. Therefore, forward bias applied to the emitter diode is generally very small whereas reverse bias on the collector diode is much higher.
Working of npn transistor
Fig.3 shows the npn transistor with forward bias to the emitter-base junction and reverse bias to the collector-base junction.
The forward bias cause the electrons in the n-type emitter to flow towards the base. This constitutes the emitter current IE.
As these electrons flow through the p-type base, they tend to combine with holes. As the base is lightly doped and very thin, therefore,only a few electrons i.e less than 5% combine with holes to constitute base current IB.
The remainder i.e more than 95% cross over into the collector region to constitute collector current IC.
In this way almost entire emitter current flows in the collector circuit.
The emitter current is the sum of collector and base current.
IE = IB + IC
Working of pnp transistor:
Fig.4 shows the basic connection of pnp transistor.
The forward bias causes the holes in the p-typ emitter to flow towards the base.This constitute the emitter current IE.
As these holes cross into n-type base,they tend to combine with the electrons. As the base is lightly doped and very thin, therefore only a few holes i.e. less than 5% combine with the electrons. This constitute the base current IB.
The remainder holes i.e. more than 95% cross into the collector region to constitute collector current IC.
Importance of transistor action
A transistor transfers the input signal current from a low-resistance circuit to a high-resistance circuit. This is the key factor responsible for the amplifying capability of the transistor.
The symbols used for npn and pnp transistors are shown below in fig.5(i) and fig.5(ii).
It can be noted that the emitter is shown by an arrow which indicates the direction of conventional current flow with forward bias.
For npn connection, the conventional current flows out of the emitter as indicated by the outgoing arrow in fig.5(i).
Similarly, for pnp connection, the conventional current flows into the emitter as indicated by the inward arrow in fig.5(ii).
Transistor circuit as an Amplifier
A transistor raises the strength of a weak signal and thus act as an amplifier.
Fig.6 shows the basic circuit of a transistor amplifier.
The weak signal is applied between the emitter and base. The output is taken across the load RC, connected between the collector and base.
To achieve faithful amplification, the input circuit should always be forward biased. To do so, a d.c. voltage VEE is applied in the input circuit in addition to the signal to be amplified.
This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the input circuit forward biased regardless of the polarity of the signal.
As the input circuit is forward biased ,it has low resistance,therefore, a small change in signal voltage causes an appreciable change in emitter current.This causes almost the same change in collector current due to transistor action.
The collector current flowing through a high resistance RC produces a large voltage across it.
Thus, a weak signal applied in the input circuit appears in the amplified form in the collector circuit.
It is in this way that a transistor acts as as amplifier.
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