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  Theory of Bipolar Junction Transistor

Introduction:

The Bipolar Junction Transistor (or BJT) has three terminals joined to three doped semiconductor regions. In the NPN transistor, thin and lightly doped P-type material is sandwiched among two thicker N-type materials; whereas in a PNP transistor, the thin and lightly doped N-type material is sandwiched among two thicker P-type materials.

The BJT comprises of 3 alternating layers of n- and p-type semiconductor termed emitter (E), base (B) and the collector (C).

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The majority of current enters the collector, crosses base region and exits via the emitter. A small current as well enters the base terminal, crosses base emitter junction and leaves via the emitter.

Carrier transport in an active base region directly under the heavily doped (n+) emitter dominates the i-v characteristics of BJT.

The bipolar junction transistor (or BJT) was the first solid-state amplifier element and begin the solid-state electronics revolution. Brattain, Bardeen and Shockley, at Bell Laboratories, invented it in the year 1948 as part of a post-war effort to substitute vacuum tubes with solid-state devices. Solid-state rectifiers were previously in use at the time and were preferred over the vacuum diodes since of their smaller size, lower weight and higher reliability. The solid-state substitution for a vacuum triode was expected to outcome similar benefits. The work at Bell Laboratories was highly culminated and successful and Bardeen, Brattain and Shockley receiving the Nobel Prize in the year 1956.

Their work led them primary to the point-contact transistor and then to bipolar junction transistor. They employed germanium as the semiconductor since it was possible to get high purity material. The extraordinarily big diffusion length of minority carriers in germanium offered functional structures in spite of the large dimensions of early devices.

BJTs come in two kinds, or polarities, termed as PNP and NPN based on the doping kinds of three main terminal regions. An NPN transistor includes two semiconductor junctions which share a thin p-doped anode region, and a PNP transistor includes two semiconductor junctions which share a thin n-doped cathode region.

In usual operation, the base–emitter junction is forward biased, that means that the p-doped side of the junction is at a much positive potential than n-doped side, and base–collector junction is reverse biased. In NPN transistor, whenever positive bias is applied to the base–emitter junction, the equilibrium is disturbed among the thermally produced carriers and the repelling electric field of n-doped emitter depletion region. This permits thermally excited electrons to inject from emitter to the base region. Such electrons diffuse via the base from the region of high concentration close to the emitter towards the region of low concentration close to the collector. The electrons in base are termed as minority carriers since the base is doped p-type that makes the holes as majority carrier in base.

To reduce the percentage of carriers which rejoin before reaching the collector base junction, the transistor's base region must be thin adequate that carriers can diffuse across it in much less time than semiconductor's minority carrier lifetime. In specific, the thickness of base should be much less than the diffusion length of electrons. Collector–base junction is reverse-biased, and therefore little electron injection takes place from the collector to base, however electrons which diffuse via the base towards collector are swept into collector by the electric field in depletion region of collector–base junction. A thin shared base and asymmetric collector–emitter doping is what distinguishes the bipolar transistor from two separate and oppositely biased diodes joined in series.

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