Junction Transistor

Junction Transistors

A junction transistor is formed by sandwiching a thin wafer of one type of semiconductor between two layers of another type. The n-p-n transistor has a p-type wafer between two n-type layers. Similarly the p-n-p transistors has a n-type wafer between two p-type layers.

Emitter (E):
It provides majority charge carries by which current flows in the transistor. Therefore the emitter semiconductor is heavily doped.

Base (B): the based region is lightly doped and thin.

Collector (C): The size of collector region is larger than the two other regions and is moderately doped

(i) p-n-p Transistor
Figure shows a p-n-p transistor, in which a thin layer of n-type semiconductor is sandwiched between two p-type semiconductors. The middle layer (called the base) is very thin  as compared to the widths of the two layers at the sides layer (Called collector) is moderately doped. Figure shows the symbol of p-n-p transistor.

(ii)n-p-n Transistor
In n-p-n transistor p-type semiconductor is sandwiched between two n-type semiconductors. Symbol of p-n-p transistor is shown in figure .

More points about a transistor
A transistor is basically a three-terminal device. Terminals come out from the emitter, base and the collector for external connections. In normal operation of a transistor, the emitter-base junction is always forward biased and collector-base junction is reverse biased.

The arrow on the emitter-base line shows the direction of current between emitter and base. In an n-p-n transistor for example, there are a large number of conduction electrons in the emitter and a large number of holes in the base. If the junction is forward biased the electrons will diffuse from emitter to the base and holes will diffuse from the base to the emitter. The direction of electric current at this junction is therefore from the base to the emitter. A transistor can be operated in three different modes.
(i) common emitter (or grounded-emitter)
(ii) common collector (or grounded-collector) and
(iii) common base (or ground-base)

In common emitter mode, emitter is kept at zero potential. Similarly in common collector mode collector is at zero potential and so on.

Working of Transistor
(1) There are four possible ways of biasing the two P-N junctions (emitter junction and collector junction) of transistor.
(i) Active mode : also known as linear mode operation.
(ii) Saturation mode : Maximum collector current flows and transistor acts as a closed switch from collector to emitter terminals.
(iii) Cut-off mode : Denote operation like an open switch where only leakage current flows.
(iv) Inverse mode : The emitter and collector are inter hanged.
(2) A transistor is mostly used in the active region of operation i.e., emitter base junction is forward biased and collector based junction is reverse biased.

Working of a p-n-p Transistor

Let us consider the working of a p-n-p transistor in common base mode. In emitter (p type) holes are in majority. Since, emitter-base is forward biased, holes move toward base. Few of them combine with electrons in the base and rest go to the collector. Since, base-collector is reverse biased, holes coming from base move toward the terminal of collector. They combine with equal number of electrons entering from collector terminal.

Ie = Ib + Ic

Note that Ib is only about 2% of Ie , or roughly around 2% of holes coming from emitter to base combine with the electrons. Rest 98% move to collector.

Working of n-p-n Transistor
A common base circuit of an n-p-n transistor is shown in figure. Majority charge carriers in the emitter (n-type) are electrons. Since, emitter-base circuit is forward biased. The electrons rush from emitter to base. Few of them leave the base terminal (comprising ib) and rest move to collector. These electrons finally leave the collector terminal (give rise to ic). Electrons coming from base and form collector meet at junction O and they jointly move to emitter, which gives rise to ie.
Thus, here also we can see that

Ie = Ib + Ic

Note that although the working principle of p-n-p and n-p-n transistors are similar but the current carriers in p-n-p transistor are mainly holes whereas in n-p-n transistors the current carriers are mainly electrons. Mobility of electrons are however more than the mobility of holes, therefore n-p-n transistors are used in high frequency and computer circuits where the carriers are required to respond very quickly to signals.

α and β-parameters:  α and  β-parameters of a transistor are defined as,

α = Ic / Ie and  β = Ic / Ib

As Ib is about 2 to 5% of Ie  ,  α is about 0.95 to 0.99 and β is about 20 to 100. Simply we can prove that,

$\large \beta = \frac{\alpha}{1-\alpha}$

Also Read :

→ Energy Levels & Energy Bands
→ Intrinsic semiconductor & Extrinsic Semiconductor
→ P-N Junction & P-N Junction Diode
→ Junction Diode as Rectifier
→ Zener diode
→ Transistor as Amplifier

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