Mosfet

Jovany Tavera August 7, 2011 ET 475 Electronic Design William Shockley proposed the Field Effect Transistor (FET) in 1952. FETs are majority carrier devices and its operation depends on its applied voltage to control the majority carriers. The applied voltage controls the current in the device by means of an electric field. FETs are voltage sensitive with high input impedance. FETs are more temperature stable then Bipolar Junction Transistor (BJT). They are easier to fabricate than BJTs.

FETs, with their high input impedance, can store charge long enough to be used as storage elements. A later version of FETs is the metal-oxide semiconductor FET (MOSFET). There are four terminals in total: source, gate, drain and the substrate. The MOSFET is constructed with the gate insulated with a silicon dioxide dielectric. Depletion and enhancement is the two modes on the MOSFET. MOSFETs are also known as IGFETs (Insulated Gate Field Effect Transistor). Just as BJTs has either npn or pnp, MOSFETs can be either NMOS or PMOS.

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The depletion mode was design with a physical channel connected between the drain and source. In operation, a negative gate-to-source voltage pushes out electrons from the channel region, therefore depleting the channel. When the gate-to-source voltage reaches the threshold voltage the channel is pinched off. A positive charge of the gate-to-source voltages increases the channel size, as a result, an increase of drain current. Due to the insulated gate, the gate current is extremely small.

Enhancement MOSFET is different from depletion MOSFET due to not having the thin n-layer. In order to establish a channel it requires a positive voltage between the gate and the source. Positive gate-to-source voltage accumulates electrons at the surface beneath the oxide layer. When the voltage reaches the threshold voltage a number of electrons are attracted to the substrate region therefore acting as an conducting n-channel. No current exist in the drain until gate-to-source voltage exceeds the threshold voltage.

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