JFET

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Electric current flow from source to drain in a p-channel JFET is restricted when a voltage is applied to the gate.

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.

1 Structure
2 Function
3 About the drawing symbols
4 Comparison with other transistors
5 Mathematics
6 External links

[edit] Structure

Circuit symbol for an n-Channel JFET

Circuit symbol for a p-Channel JFET

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.Then, a pn junction is formed at the interface of the two types of the material and one has to make sure that the terminal made with the semiconductor are usually made Ohmic.

[edit] 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.

[edit] About the drawing symbols

Sometimes the JFET gate is drawn in the middle of the channel instead of at the drain/source electrode as in these examples. The US style of the symbol is to draw the whole component inside a circle, whilst the european style is to draw it without a circle.

The symmetric variation is hinting at that the channel is indeed symmetric in sense that drain and source are interchangeable physical terminals.

In every case the arrow head is telling the direction, where the P-N-junction of the gate is in relationship to the channel. In order to pinch off the channel, one must produce around 2 volts in reverse direction (V_GS) of that junction.

In N-type channel that is hinting to us that negative voltage at the gate in comparison of the source is called for. In P-type channel the hint is for positive V_GS.

[edit] 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 picoamperes) 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.