Introduction

The bipolar junction  transistor (BJT) is a three-terminal electronic device constructed from the doped semiconductor  material  and  can  be  used  in   amplifying  or  switching  applications.  The Bipolar transistors are so named because their operation involves electrons and  holes both. Charge flow in the BJT is because of bidirectional diffusion of charge carriers across the junction between 2 regions of different charge concentrations.  This  mode  of  operation  is  contrasted  with the unipolar transistors, like  field-effect transistors, in which only one carrier type is involved in the charge flow because of  drift. By design, majority of the BJT collector current is because of the flow of charges injected from the high-concentration emitter into base where they are  minority carriers which diffuse toward the collector, and thus BJTs are classified as the minority-carrier devices.

Schematic symbols for PNP- and NPN-type BJTs.

NPN BJT with forward-biased E-B junction and reverse-biased B-C junction

The NPN transistor can be considered as 2  diodes with a shared  anode. In characteristic operation, the base-emitter junction is  forward biased and base-collector junction is  reverse biased. In the NPN transistor, for instance, when a positive voltage is applied to base-emitter junction, the equilibrium in between thermally generated carriers and repelling electric field of the depletion region becomes unbalanced, allowing thermally excited electrons to inject into base region. These electrons wander through the base from region of the high concentration near the emitter towards region of the low concentration near the collector. The electrons in the base are called as minority carriers because base is  doped P type which would make holes the majority carrier in base.

To  minimize  percentage  of  carriers  which   recombine  before  reaching  collector-base junction, the transistor's base region should be thin enough that carriers can diffuse across it in less time than the semiconductor's minority carrier lifetime. Specifically, the thickness of base should be less than the diffusion length of electrons. The collector-base junction is reverse-biased, and so little electron injection occurs from collector to the base, but the electrons which diffuse through the base towards collector are swept into collector by electric field in depletion region of collector base junction. The thin shared base and asymmetric collector-emitter doping is what differentiates the bipolar transistor from 2 separate and oppositely biased diodes connected in the series.

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