NPN Transistor – Definition, Working, Symbol, Types, Uses & Characteristics

Define NPN Transistor?

What is NPN Transistor?

At the point when a single p-type semiconductor layer is sandwiched between two n-type semiconductor layers, a npn semiconductor is framed.
NPN Transistor Symbol
The circuit image/symbol and diode analogy of npn transistor is displayed in the below figure.
In the above figure, it is shown that the electric current generally moves from p-region to n-region.


NPN transistor construction
The npn transistor is comprised of three semiconductor layers: one p-type semiconductor layer and two n-type semiconductor layers.

The p-type semiconductor layer is sandwiched between two n-type semiconductor layers.

The npn semiconductor has three terminals: emitter, base and collector. The emitter terminal is connected with the left side n-type layer. The collector terminal is connected with the right side n-type layer. The base terminal is connected with the p-type layer.

The npn semiconductor has two p-n junctions. One junction is framed between the emitter and the base. This junction is called emitter base junction or emitter junction. The other junction is shaped between the base and the collector. This junction is called collector base junction or collector junction.

Working Principal of a NPN transistor
Unbiased NPN Transistor
Whenever no voltage is applied to a transistor, it is supposed to be an unbiased transistor. At the left side n-region (emitter) and right side n-region (emitter), free electrons are the larger part carriers and holes are the minority carriers though in p-region (base), holes are the larger part carriers and free electrons are the minority carriers.

We realize that the charge carriers (free electrons and holes) consistently attempt to move from higher concentration region to the lower concentration region.

With the expectation of free electrons, n-region is the higher concentration region and p-region is the lower concentration region. Also, for holes, p-region is the higher concentration region and n-region is the lower concentration region.

Consequently, the free electrons at the left side n-region (emitter) and right side n-region (collector) experience a repulsive force from one another. Accordingly, the free electrons at the left side and right side n-regions (emitter and collector) will move into the p-region (base).

During this interaction, the free electrons meet the holes in the p-region (base) close to the junction and fill them. Subsequently, depletion region (positive and negative ions) is shaped at the emitter to base junction and base to collector junction.

At emitter to base junction, the depletion region is infiltrated more towards the base side, comparatively; at base to collector junction, the depletion region is entered more towards the base side.

This is on the grounds that at emitter to base junction, the emitter is intensely doped and base is delicately doped so the depletion region is entered more towards the base side and less towards the emitter side. Likewise, at base to collector junction, the collector is intensely doped and base is daintily doped so the depletion region is infiltrated more towards the base side and less towards the collector side.

The collector region is delicately doped than the emitter area, so the depletion layer width at the collector side is more than the depletion layer width at emitter side.

Why depletion region penetrates more towards lightly doped side than the heavily doped side?

We realize that doping is the method involved with adding impurities to the intrinsic semiconductor to work on its electrical conductivity. The electrical conductivity of the semiconductor is relies upon the doping level added to it.

Assuming that the semiconductor material is intensely doped, its electrical conductivity is exceptionally high. That implies the heavily doped semiconductor material has an enormous number of charge carriers which transmit electric current.

In the event that the semiconductor material is lightly doped, its electrical conductivity is exceptionally low. That implies the softly doped semiconductor material has few charge carriers which transmit electric current.

We realize that in n-type semiconductor, free electrons are the larger part charge carriers and holes are the minority charge carriers.

In npn transistor, the left side n-region (emitter) is intensely doped. So the emitter has countless free electrons.

We realize that in p-type semiconductor, holes are the larger part charge carriers and free electrons are the minority charge carriers.

The p-region (base) is lightly doped. So the base has few holes.

The right side n-region (collector) is reasonably doped. Its doping level lies between that of emitter and base.

Whenever the atom loses or donates an electron, it turns into a positive ion. On the other hand, when the atom gains or accepts an electron, it turns into a negative ion.

The atoms which give electrons are known as donors and the atoms which accept electrons are known as acceptors.

Emitter base junction:

Allow us to expect to be that, at left side n-region (emitter), every atom has three free electrons, and at p-region, each atom has one hole.

During the diffusion process, the free electrons move from emitter (n-region) to base (p-region). Likewise, the holes move from base (p-region) to emitter (n-region).

At emitter-base junction, when the n-region (emitter) atoms meet the p-region (base) atoms, every n-region atom gives three free electrons to three p-region atoms. Subsequently, the n-region (emitter) atom which gives three free electrons will turn into a positive ion and the three p-region (base) atoms which accepts (each accept one free electron) three free electrons will become negative ions. In this way, every n-region (emitter) positive ion produces three p-region (base) negative ions.

Subsequently, the depletion region at the emitter-base junction contains more negative ions than the positive ions. The negative ions live at the p-region (base) close to the junction and the positive ions live at the n-region (emitter) close to the junction.

Subsequently, the depletion region is entered more towards the p-region (base) than the n-region (emitter).

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