Rajeev Ranjan E.C.E

Main article: Transistor
An n–p–n bipolar junction transistor
structure
Bipolar junction transistors are formed from
two p–n junctions, in either n–p–n or p–n–p
configuration. The middle, or base , region
between the junctions is typically very narrow.
The other regions, and their associated
terminals, are known as the emitter and the
collector. A small current injected through the
junction between the base and the emitter
changes the properties of the base-collector
junction so that it can conduct current even
though it is reverse biased. This creates a
much larger current between the collector
and emitter, controlled by the base-emitter
current.
Another type of transistor, the field-effect
transistor , operates on the principle that
semiconductor conductivity can be increased
or decreased by the presence of an electric
field . An electric field can increase the
number of free electrons and holes in a
semiconductor, thereby changing its
conductivity. The field may be applied by a
reverse-biased p–n junction, forming a
junction field-effect transistor ( JFET ) or by an
electrode isolated from the bulk material by
an oxide layer, forming a metal–oxide–
semiconductor field-effect transistor ( MOSFET ).
The MOSFET, a solid-state device, is the most
used semiconductor device today. The gate
electrode is charged to produce an electric
field that controls the conductivity of a
"channel" between two terminals, called the
source and drain . Depending on the type of
carrier in the channel, the device may be an
n-channel (for electrons) or a p-channel (for
holes) MOSFET. Although the MOSFET is
named in part for its "metal" gate, in modern
devices polysilicon is typically used instead.