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__NOTOC__ The junction gate field-effect transistor ('''JFET''' or '''JUGFET''') is the simplest type of Field Effect Transistor . Like other transistors, it can be used as an Electronically -controlled Switch . They are also used as voltage-controlled resistances. An Electric Current flows from one connection, called the ''source'', to a second connection, called the ''drain''. A third connection, the ''gate'', determines how this current flows. By applying an increasing negative (for an n-channel JFET) bias Voltage to the gate, the current flow from source to drain can be impeded by pinching off the channel, in effect switching off the transistor. STRUCTURE The JFET consists of a long channel of Semiconductor material. This material is Doped so that it contains an abundance of positive Charge carriers (''p-type''), or of negative charge carriers (''n-type''). There is a contact at each end; these are the source and drain. The third control Terminal , the ''gate'', surrounds the channel, and is doped opposite to the doping-type of the channel. FUNCTION With no gate voltage, current flows easily when a Voltage is applied between the source and drain. The current flow is modulated by applying a voltage between the gate and source terminals. The Polarity of the gate voltage is such that it puts the p-n junction between the gate and channel in reverse Bias , increasing the width of the depletion region in the junction. As the current-carrying channel shrinks with increasing gate voltage, the current from source to drain also shrinks. In this way, the gate controls the Conductance of the channel, just like in the MOSFET . Unlike most MOSFETs, JFETs are always depletion-mode devices — they're "on" unless a gate voltage is applied. The operation of a JFET can easily be understood by considering a garden Hose . The flow of Water through a garden hose can be controlled by squeezing it and reducing its Cross Section ; the flow of Electric Charge through a JFET is controlled by constricting the cross section of the current-carrying channel. COMPARISON WITH OTHER TRANSISTORS The JFET gate presents a small current load which is the reverse leakage of the gate-to-channel Junction . The MOSFET has the advantage of extremely low gate current (measured in Picoamps ) because of the insulating oxide between the gate and channel. However, compared to the base current of a Bipolar Junction Transistor the JFET gate current is much lower, and the JFET has higher Transconductance than the MOSFET. Therefore JFETs are used to advantage in some low- Noise , high input-impedance Op-amps and sometimes used in switching applications. The JFET had been predicted as early as 1925 by Julius Lilienfeld , and the theory of operation of the device was sufficiently well known by the mid 1930's for a patent to be issued for it. However, technology at the time was not sufficiently advanced to produce doped crystals with enough precision for the effect to be seen until many years later. In 1947 , researchers John Bardeen , Walter Houser Brattain , and William Shockley were attempting to construct a JFET when they discovered the Bipolar Junction Transistor . The first practical JFETs were thus constructed many years after the first bipolar junction transistors, in spite of having been invented much earlier. MATHEMATICS Current in N-JFET due to a small voltage VDS is given by: where
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