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  • Writer's pictureSunil Rawat

Bipolar Junction Transistors (BJT)

Updated: Jan 5, 2022

A transistor transfers a signal from a low resistance to high resistance. The prefix ‘trans’ means the signal transfer property of the device while ‘istor’ classifies it as a solid element in the same general family with resistors.


The bipolar junction transistor is used in two broad areas of electronics as a linear amplifier to boost an electrical signal and as an electronic switch


The bipolar junction transistor consists of two back-to-back P-N junctions manufactured in a single piece of a semiconductor crystal. These two junctions give rise to three regions called emitter, base and collector. The emitter, base and collector are provided with terminals which are labelled as E, B and C.

Transistors using diodes
Transistor Diode Analogy

A junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type.


A PNP transistor consists of a layer of N-type material sandwiched between two layers of P-type material.


PNP Transistor Construction
PNP Transistor

A NPN transistor consists of a layer of P-type material sandwiched between two layers of N-type material.

PNP Transistor Construction
PNP Transistor

The two junctions are : emitter-base (E/B) junction and collector-base (C/B) junction.

The symbols employed for PNP and NPN transistors.

NPN and PNP Transistor Symbols
Transistor Symbols

The arrowhead is always at the emitter (not at the collector) and in each case, it's direction indicates the conventional direction of current flow.


For a PNP transistor, arrowhead points from emitter to base meaning that emitter is positive with respect to base (and also with respect to collector)


For NPN transistor, it points from base to emitter meaning that base (and collector as well) is positive with respect to the emitter.

Emitter

Base

Collector

Medium Size

Thin Size

Large Size

Heavily Doped

Very Lightly Doped

Medium Doping

Supply Charge

Transfer Charge

Collector Charge

For proper working of a transistor, it is essential to apply voltages of correct polarity across its two junctions.

  1. Emitter-base junction is always forward- biased and

  2. Collector-base junction is always reverse-biased.

This type of biasing is known as FR biasing.


For a PNP transistor, both collector and base are negative with respect to the emitter (the letter N of Negative being the same as the middle letter of P N P). Of course, collector is more negative than base.


Similarly, for NPN transistor, both collector and base are positive with respect to the emitter (the letter P of Positive being the same as the middle letter of N PN). Again, collector is more positive than the base.

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