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The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field effect transistor. Like other transistors, it is an electronically-controlled switch. 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 a voltage to the gate, the current flow from source to drain is impeded (switched off).
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 by 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 to other transistors
JFETs have several advantages over the bipolar junction transistor (BJT). The former do not require any input current to function, which makes them useful for circuits requiring a high input impedance. However, their gain is usually relatively low in comparison. They are used in low-noise, low-signal level analog applications, and sometimes used in switching applications.
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