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Thursday, 17 October 2019

PNP and NPN Junction Transistor

A semi conductor arranged in a particular fashion offers low resistance to the flow of current in one portion while as high resistance in the other position. This kind of semi conductor is known as transistor. A transistor is a device which transforms current flow from low resistance path to high resistance path. The transfer of current through resistance has given the name to the device as device transfer resistors as transistors. Mainly there are two types of transistors point contact and junction contact. Point contact construction is defective and also obsolete. Junction transistors are in many respect analogous to that of a triode electron tube. Junction transistors are also of two types PNP junction transistor and NPN junction transistor. Each has three elements base, emitter and collector. The three elements are made of P and N materials.


PNP Junction Transistor:

A PNP junction is made up of two PN Silicon or Germanium junction diodes placed back to back. The centre or N-type portion of the sandwich is extremely thin in comparison to the P regions.
The P regions are negative with respect to the centre N region, or the N-region is positive with respect to the P-regions. The mobile electrons in the N-region, therefore, move away from both junctions in the direction of the positive connecting terminal. The holes from the P-region also move away from the junctions to the negative terminals. The current flows through them after the displacements of these holes and electrons.

When the battery terminals are reversed as shown in Fig, the holes in the left P-region known as emitter are repelled by the positive battery terminal towards the emitter junction (the junction that is forward biased in a transistor is termed emitter junction).
Under the influence of the electric field, the holes overcome the barrier and cross the emitter junction into the N-type or base region. The region is very thin and lightly doped with impurity atoms, so that the majority of the holes are able to drift across the base without meeting electrons to combine with. A small number of holes (about 5%) however, are lost in the area because of recombination with electrons. The remaining penetrate through the base region and flow across the right junction into the P-region or collector. The negative collector voltage Vce rapidly sweeps up the holes that pass into the collector region. For each hole reaching the collector electrode, an electron is emitted from the negative battery terminal (Vee) and neutralises the holes.

For each hole lost by combination with an electron in the collector and base areas, a covalent bond near the emitter electrode breaks down. The liberated electron leaves the emitter electrode and enters the positive battery terminal (VCE). The new hole that is formed then moves immediately towards the emitter junction, and the process is repeated. Current conduction with the PNP transistor thus takes place by holes from emitter to collector. Further the collector current is less than the emitter current by an amount proportional to the number of electron hole combinations occurring in the base area.

NPN Junction Transistor :

An NPN junction transistor is shown in Fig. The positions of N and P-type silicon have been interchanged and the battery polarities have been reversed with respect to PNP transistor. The emitter junction is still forward biased since the electrons are repelled from the negative emitter battery terminal (Vce ) towards the junction. Likewise, the collector junction has reverse bias because electrons are flowing away from the collector junction toward the positive collector battery terminal (Vcc ). The main difference between the PNP and NPN transistor is at current conduction in the latter is carried by electrons while the charge carriers in the former are holes.
With the emitter voltage Vce applied as shown, electrons are repelled from the negative terminal and injected into the emitter junction. Since the P-region is very thin and is lightly doped, most of the electrons diffuse through the case and reach the collector junction. A few electrons of about 4 to 5% combine with the holes present in the P-region and are lost as charge carriers. The remaining move into the collector region, where they are immediately swept up by the positive collector voltage (Vce). For each electron flowing out of the collector and entering the positive battery terminal, an electron enters the emitter from the negative emitter battery terminal. Electron conduction thus takes place continuously in the direction shown in the Figure.


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