JFET

The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals. By applying a bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

Contents

Structure

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers (p-type), or of negative carriers (n-type). Contacts at each end from the source(S) and drain(D). The gate(G) (control) terminal has doping opposite to that of the channel, which surrounds it, so that there is a P-N junction at the interface. Terminals to connect with the outside are usually made ohmic.

Function

JFET operation is like that of a garden hose. The flow of water through a hose can be controlled by squeezing it to reduce the cross section; the flow of electric charge through a JFET is controlled by constricting the current-carrying channel. The current also depends on the electric field between source and drain (analogous to the difference in pressure on either end of the hose).

Schematic symbols

The JFET gate is sometimes drawn in the middle of the channel (instead of at the drain or source electrode as in these examples). This symmetry suggests that "drain" and "source" are interchangeable, so the symbol should be used only for those JFETs where they are indeed interchangeable (which is not true of all JFETs).

Officially, the style of the symbol should show the component inside a circle (representing the envelope of a discrete device). This is true in both the US and Europe. The symbol is usually drawn without the circle when drawing schematics of integrated circuits. More recently, the symbol is often drawn without its circle even for discrete devices.

In every case the arrow head shows the polarity of the P-N junction formed between the channel and gate. As with an ordinary diode, the arrow points from P to N, the direction of conventional current when forward-biased. An English mnemonic is that the arrow of an N-channel device "points in".

To pinch off the channel, it needs a certain reverse bias (VGS) of the junction. This "pinch-off voltage"(Vp) varies considerably, even among devices of the same type. For example, VGS(off) for the Temic J201 device varies from -0.8V to -4V.[1] Typical values vary from -0.3V to -10V.

To switch off an n-channel device requires a negative gate-source voltage (VGS). Conversely, to switch off a p-channel device requires VGS positive.

In normal operation, the electric field developed by the gate must block conduction between the source and the drain.

Comparison with other transistors

JFET gate current (the reverse leakage of the gate-to-channel junction) is comparable to that of a MOSFET (which has insulating oxide between gate and channel), but much less than the base current of a bipolar junction transistor. The JFET has higher transconductance than the MOSFET and is therefore used in some low-noise, high input-impedance op-amps.

History of the JFET

The JFET was predicted by Julius Lilienfeld in 1925 and by the mid-1930s its theory of operation was sufficiently well known to justify a patent. However, it was not possible for many years to make doped crystals with enough precision to show the effect. In 1947, researchers John Bardeen, Walter Houser Brattain, and William Shockley were trying to make a JFET when they discovered the point-contact transistor. The first practical JFETs were made many years later, in spite of their having been conceived long before the junction transistor. To some extent it can be treated as a hybrid of a MOSFET and a BJT though an IGBT resembles more of the hybrid features.

Mathematical model

The current in N-JFET due to a small voltage VDS is given by:

I_{DSS} = (2a) \frac{W}{L} q N_d \mu_n V_{DS}

where

In the saturation region:

I_{DS} = I_{DSS}\left[1 - \frac{V_{GS}}{V_P}\right]^2

In the linear region

I_D = (2a) \frac{W}{L} q N_d {{\mu}_n} \left[1 - \sqrt{\frac{V_{GS}}{V_P}}\right]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}

See also

References

  1. ^ J201 data sheet

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