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 reverse bias voltage to a "gate" terminal, the channel is "pinched", so that the electric current is impeded or switched off completely.

Structure

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

The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (p-type), or of negative carriers or electrons (n-type). Ohmic contacts at each end form the source (S) and drain (D). A pn-junction is formed on one or both sides of the channel, or surrounding it, using a region with doping opposite to that of the channel, and biased using an ohmic gate contact (G).

Function

I–V characteristics and output plot of a JFET n-channel transistor.

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).

Construction of the conducting channel is accomplished using the field effect: a voltage between the gate and source is applied to reverse bias the gate-source pn-junction, thereby widening the depletion layer of this junction (see top figure), encroaching upon the conducting channel and restricting its cross-sectional area. The depletion layer is so-called because it is depleted of mobile carriers and so is electrically non-conducting for practical purposes.

When the depletion layer spans the width of the conduction channel, "pinch-off" is achieved and drain to source conduction stops. Pinch-off occurs at a particular reverse bias (V_GS) of the gate-source junction. The pinch-off voltage (V_p) varies considerably, even among devices of the same type. For example, V_GS(off) for the Temic J201 device varies from -0.8V to -4V. Typical values vary from -0.3V to -10V.

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

In normal operation, the electric field developed by the gate blocks source-drain conduction to some extent.

Some JFET devices are symmetrical with respect to the source and drain.

Schematic symbols

Circuit symbol for an n-Channel JFET

Circuit symbol for a p-Channel JFET

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.

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".

Comparison with other transistors

At room temperature, 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 conception long before the junction transistor. To some extent it can be treated as a hybrid of a MOSFET (metal–oxide–semiconductor field-effect transistor) and a BJT though an IGBT resembles more of the hybrid features.

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

• I_DSS = drain-source saturation current
• 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 concentration

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 $I_{DSS}$):

$I_D = \frac{2I_{DSS}}{V_P^2} (V_{GS} - V_P - \frac{V_{DS}}{2})V_{DS}$

where ${V_P}$ is the pinch off voltage, the minimum voltage across G-S to completely turn off conduction. When $V_{DS}$ is small compared with $V_{GS}$ - $V_P$, the device acts like a voltage controlled resistor.

See also

• Constant-current diode

External links

• Physics 111 Laboratory -- JFET Circuits I pdf
• Interactive Explanation of n-channel JFET