JFET

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. It is also used as a voltage-controlled resistance. An electric charge flows from one connection, called the "source", to a second connection, called the "drain". A third connection, the "gate", determines how much charge flows. By applying an increasing negative (for an n-channel JFET) bias voltage to the gate, the electric current 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, forming a p-n junction at the interface of the two types of the material. Terminals to connect with the outside are usually made Ohmic.

Function

The operation of a JFET is analogous to 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. It is a device in which the current through the conducting region is controlled by an electric field.

Schematic symbols

Sometimes the JFET gate is drawn in the middle of the channel instead of at the drain or source electrode as in these examples. This symmetric variation is hinting that the channel is indeed symmetric in the sense that "drain" and "source" are interchangeable physical terminals. So this symbol variation should be used only for JFETs where drain and source indeed are interchangeable, which is not true for all JFETs.

Traditionally, the US style of the symbol was drawn with the whole component inside a circle, although this has been simplified in favor of the European style to draw it without a circle.

In every case the arrow head indicates the polarity of the P-N-junction of the gate in relationship to the channel. As with a diode, the arrow points from P to N, indicating the direction of conventional current when forward-biased. A mnemonic for remembering the N-channel device is that the arrow "points in".

In order to pinch off the channel, one must produce a certain voltage in reverse direction (V_GS) of that junction. The precise value of this "pinch-off voltage" varies considerably with individual JFETs, even with JFETs of the same type, for instance V_GS(off) for the Temic J201 device varies from -0.8V to -4V. [ [http://docs-europe.origin.electrocomponents.com/webdocs/0027/0900766b80027bd1.pdf J201 data sheet] ] Typical values anywhere between 0.3V and 10V may be encountered.

The appropriate voltage bias can be remembered easily, since the n-channel device requires a negative gate-source voltage (V_GS) to switch off the JFET, while the p-channel device requires a positive gate-source voltage (V_GS) to switch off the JFET.

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 1930s 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 point-contact transistor. The first practical JFETs were thus constructed many years after the first bipolar junction transistors, in spite of it having been conceived much earlier.

Mathematical model

The current in N-JFET due to a small voltage V_DS is given by:::$I\_\{DSS\}\; =\; (2a)\; frac\{W\}\{L\}\; q\; N\_d\; mu\_n\; V\_\{DS\}$where
* "2a" = channel thickness
* "W" = width
* "L" = length
* "q" = electronic charge = 1.6 x 10^-19 C
* "μ_n" = electron mobility
* "N_d" = n type doping concentrationIn the saturation region:::$I\_\{DS\}\; =\; I\_\{DSS\}left\; [1\; +\; frac\{V\_\{GS\{V\_P\}\; ight]\; ^2$In the linear region::$I\_D\; =\; (2a)\; frac\{W\}\{L\}\; q\; N\_d$mu}_n} left [1 - sqrt{frac{V_{GS{V_P ight] V_{DS}or (in terms of $I\_\{DSS\}$):::$I\_D\; =\; frac\{2I\_\{DSS\{V\_P^2\}\; (V\_\{GS\}\; -\; V\_P\; -\; frac\{V\_\{DS\{2\})V\_\{DS\}$

External links

* [http://ist-socrates.berkeley.edu/~phylabs/bsc/PDFFiles/bsc5.pdf Physics 111 Laboratory -- JFET Circuits I pdf]
* [http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/jfet.html Interactive Explanation of n-channel JFET]