The Bipolar junction transistor or BJT is constructed by three doped semiconductor regions. While studying BJT it is important to understand the basics of semiconductors and doping phenomenon first.
Semiconductor materials are bad conductors of current in their intrinsic state because they have limited number of electrons and holes in their valence shell. The doping process is used to increase the number of electrons and holes. In this process, different impurities are added so as to change the properties of materials.
N-type and P-type semiconductors
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