Tech Trill

A bipolar (junction) transistor (BJT) is a three-terminal electronic device constructed of
doped semiconductor material and may be used in amplifying or switching applications.
Bipolar transistors are so named because their operation involves both electrons and
holes. Charge flow in a BJT is due to bidirectional diffusion of charge carriers across
a junction between two regions of different charge concentrations.

This mode of operation
is contrasted with unipolar transistors, such as field-effect transistors, in which only one
carrier type is involved in charge flow due to drift. By design, most of the BJT collector
current is due to the flow of charges injected from a high-concentration emitter into the
base where they are minority carriers that diffuse toward the collector, and so BJTs are
classified as minority-carrier devices.
An NPN transistor can be considered as twodiodes with a shared anode. In typical operation
, the base-emitter junction is forward biased and the base–collector junction is reverse
biased. In an NPN transistor, for example, when a positive voltage is applied to the
base–emitter junction, the equilibrium between thermally generatedcarriers and the repelling
electric field of thedepletion region becomes unbalanced, allowing thermally excited electrons
to inject into the base region. These electrons wander (or "diffuse") through the base from
the region of high concentration near the emitter towards the region of low concentration near
the collector. The electrons in the base are called minority carriersbecause the base is
doped p-type which would make holes the majority carrier in the base.
To minimize the percentage of carriers that recombine before reaching the collector–base
junction, the transistor's base region must be thin enough that carriers can diffuse across
it in much less time than the semiconductor's minority carrier lifetime. In particular, the
thickness of the base must be much less than the diffusion length of the electrons. The
collector–base junction is reverse-biased, and so little electron injection occurs from the
collector to the base, but electrons that diffuse through the base towards the collector are
swept into the collector by the electric field in the depletion region of the collector–base
junction. The thin shared base and asymmetric collector–emitter doping is what differentiates
a bipolar transistor from two separate and oppositely biased diodes connected in series.