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Junction Field-Effect Transistors (JFETs) Three-terminal semiconductor devices are far more useful than two-terminal devices. Used in applications ranging from signal amplifiers to the realization of logic and memory functions. Basic principle involved is the use of the voltage between two terminals to control the current flowing in the third terminal. Three-terminal devices can therefore be used to realize a controlled source which is the basis of amplifier design In the extreme the control signal can be used to cause the current in the third terminal to change from zero to a large value thereby acting as a switch. The name Field-Effect transistor arises from the fact that current flow between two of the device terminals is controlled by a electric field which in turn is established by a voltage applied to the third terminal. JFETs or FETs are called unipolar transistors because current is conducted by charge carriers (electrons or holes) flowing through one type of semiconductor (n-type in n-channel FETs or p-type in p-channel FETs) This is in contrast to bipolar transistors where current passes through both n- type and p-type semiconductor material in series. JFETs are useful in the design of special amplifier circuits, especially those with very high input impedances. Example op-amps with very high input impedances usually have a first stage made up of JFETs. JFETS can also be combined with bipolar transistors to provide high- performance linear circuits (BiFET circuits) The JFET structure employing a metal-semiconductor (Schottky) junction is used with the semiconductor GaAs to form the MESFET a JFET like device suitable for application in amplifiers and logic circuits operating in the GHz range Physical Operation Two types of JFETs: n-channel and p-channel Examine the n-channel The p-channel JFET works in a similar manner except for a reversal of polarities of all currents and voltages
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Consists of a slab of n-type Si with p-type Si diffused on both sides The n-region is called the channel The p-regions are electrically connected to form the gate Metal contacts are made to both ends of the channel with the terminals called the Source (S) and the Drain (D). Similarly a metal contact is made to the p-type region to provide the gate
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