Pronounced MAWS-feht. Acronym for
metal-oxide
semiconductor field-effect
transistor, a common type of transistor
in which charge carriers, such as electrons, flow along
channels. The width of the channel, which determines how
well the device conducts, is controlled by an electrode
called the gate, separated from channel by a thin layer
of oxide insulation. The insulation keeps current from
flowing between the gate and channel.
MOSFETs
are useful for high-speed switching
applications
and also on integrated circuits in computers.
If a bias voltage is applied to the
Gate metal, relative to the silicon substrate, in excess
of the Threshold Voltage, Vt, then charge
carriers are gethered in sufficient concentration under
the Gate oxide. The type of charge carriers conducting
the channel current is opposite to the substrate. For
example, if the substrate is p-type silicon, then
electrons are the channel charge; for n-type silicon
substrate, it is the holes. This is due to the need to
separate electrically the MOSFET device from the silicon
bulk.
In an n-channel MOS structure,
fabricated on a p-type silicon substrate, a Gate bias
Vg greater than Vt will create an inversion
n-channel under the gate oxide. The inversion layer
charge, QN, is given by
QN = - Cox
(Vg - Vt) for Vg >= Vt