In an n-channel enhancement-mode device, a positive gate-to-source voltage is necessary to create a conductive channel.The positive voltage attracts free-floating electrons within the body towards the gate, forming a conductive channel. If the depletion region expands to completely close the channel, the resistance of the channel from source to drain becomes large, and the FET is effectively turned off like a switch. The FET operation is as follows: apply a voltage to the gate this voltage sets up an electric field in the “body” of the device electric field inhibits/supports the flow of charge from source to drainĥ Depletion mode device in an n-channel depletion-mode device, a negative gate-to-source voltage causes a depletion region to expand in width and encroach on the channel from the sides, narrowing the channel. The FET controls the flow of electrons (or electron holes) from the source to drain. FETs are “Unipolar” (conduct either electrons or holes, not both) A FET has three terminals: gate: as in the “gate” keeper of the current source: the source of the current drain: the destination of the current FETs can be made in NPN or PNP variety. it behaves like a voltage-controlled resistor. Other MOSFET typesĢ History The principle of field-effect transistors was first patented by Julius Edgar Lilienfeld in 1925 and by Oskar Heil in 1934, but practical semi-conducting devices were only developed much later after the transistor effect was observed and explained by the team of William Shockley at Bell Labs in 1947.ģ Definition of a FET The field-effect transistor (FET) is a generic term for a device that controls current through a circuit via an applied voltage, i.e. MOSFET composition 6.1 metal-oxide-semicoductor structure 6.2 MOSFET structure and channel formation 6.3 modes of operation 7.
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