Transistors and Its Types | Semiconductors | Electrical Engineering

A ‘transistor’ is an amplifying device in which an input signal is transmitted at an increased magnitude material. It was invented in 1948 by J. Bardeen and W.H. Brattain of Bell Telephone Laboratories.

When a thin layer of p-type or n-type semiconductor is between a pair of opposite type semiconductors it constitutes a transistor.

A transistor consists of two p-n junction diodes placed back to back.

The following are the two common types of junction transistors:

1. Grown-junction type

2. Alloy-junction type.

Fig. 7.26 (a) shows a grown p-n-p junction transistor.

Fig. 7.26 (b) shows the form of n-p-n junction transistor.

In the manufacture of grown junction transistors, the single crystal-growing process is employed.

The left hand section or region is called the emitter whereas right hand section is known as collector. The middle section called base region or base is extremely thin as compared to either the emitter or collector and is lightly doped. Function of emitter is to inject majority charge carriers into the base and that of the collector is to collect or attract these carriers through the base.

p-n-p Transistor:

Following facts may be kept in mind while understanding the basic mechanism of transistor operation:

(i) Since emitter is to provide carriers, it is always forward biased.

(ii) First letter of transistor type indicates the polarity of the emitter voltage with respect to base e.g. in pnp transistor, emitter is always at positive potential.

(iii) Collector’s job is to collect or attract those carriers through the base, hence it is always reverse biased e.g. in pnp transistor, collector is at negative potential.

(iv) Second letter of transistor type indicates the polarity of collector voltage with respect to the base.

Working:

Fig. 7.27 shows p-n-p transistor connected in the common base (or grounded base) configuration (it is so called because both the emitter and collector are returned to the base terminals). The emitter junction is forward-biased whereas the collector junction is reverse-biased. The holes in the emitter are repelled by the positive battery terminal towards the p-n or emitter junction. The potential barrier at the junction is reduced due to the forward-bias, hence holes cross the junction and enters the n-type base.

Because the base is thin and lightly doped, majority of the holes (about 95%) are able to drift across the base without meeting electrons to combine with. The balances of 5% of holes are lost in the base region due to recombination with electrons. The holes which after crossing the n-p collector junction enter the collector region are swept up by the negative collector voltage V_e.

The following points need particular attention:

(i) In a p-n-p transistor majority charge carriers are holes.

(ii) The collector current is always less than the emitter current because some recombination of holes and electrons take place (I_c = l_e – I_b).

α = I_c (collector current)/I_e (emitter current) < 1

(iii) Emitter arrow shows the direction of flow of conventional current. Evidently, electron flow will be in the opposite direction.

n-p-n Transistor:

Fig. 7.28 shows a n-p-n junction transistor. The emitter is forward-biased and the collector reverse-biased. The electrons in the emitter region are repelled by the negative battery terminal towards the emitter or n-p junction. The electrons cross over into the p-type base region because potential barrier is reduced due to forward bias.

Since the base is thin and lightly doped, most of the electrons (about 95%) cross over to the collector junction and enter the collector region where they are readily swept up the positive collector voltage, V_c. Only about 5% of the emitter electrons combine with the holes in the base and are lost as charge carriers.

The following points need particular attention:

(i) In a n-p-n transistor, majority charge carriers are electrons.

(ii) I_c (collector current) is less than I_e (emitter current) so that α < 1.

(iii) Emitter arrow shows the direction of flow of conventional current.

The junction transistors have been made in power ranges from a few milli-watts to tens of waits. The tiny junction transistors are unparalleled in that it can be made to work at power level as 1 micro-watt.