The low-voltage and minimal heat generation of field effect transistors (FET) make them attractive components for miniaturisation and for tightly packed integrated circuits. There are two basic forms of the FET transistor, the JFET and the MOSFET. Each type of device is useful in a wide range of circuits, often displacing older bipolar transistor designs.
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JFET & MOSFET Design
Both the junction field effect transistor (JFET) and the metal oxide semiconductor field effect transistor (MOSFET) share a common unipolar base design. Voltage is carried across a conductive channel from the source and drain junctions. The resistance to this current varies according to the voltage input at the gate lead. The gate of the MOSFET is isolated from the channel material by a layer of silicon-dioxide that enables it to function in either depletion or enhancement mode.
The design of the JFET limits its channel to being utilised in a depletion mode only. A depletion channel increases resistance in response to voltage sensed at the gate lead. A MOSFET channel may be fabricated in either depletion or enhancement forms. The enhancement-type MOSFET channel becomes less resistive as voltage is applied to the gate. Both transistors are available in n-channel and p-channel configurations.
The base configuration of the two types of transistors displays a fundamental difference with regard to current flow across the channel. A JFET is normally on, that is, it allows current to flow freely between the source and drain leads when there is no voltage input at the gate. Depletion increases the resistance on this channel until the gate reaches the "pinch" voltage, which shuts off the channel. A depletion-type MOSFET device is also a normally-on channel. The enhancement --type MOSFET is a normally off channel. Voltage at the gate of this type of device reduces the resistance over this channel enabling current to flow.
The JFET input impedance is high, which means that the device will draw little or no current from the circuit to which it is attached. No unwanted current enters the circuit from the device either, making the device useful in a wide range of circuit designs. The MOSFET design has even higher impedance than the JFET. The price for this characteristic is that the MOSFET device is more susceptible to damage by a build-up of static electricity.
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