JFET
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JFET
The junction gate field-effect transistor (JFET or JUGFET) is the simplest type of field-effect transistor. They are
three-terminal semiconductor devices that can be used as electronically-controlled switches, amplifiers, or
voltage-controlled resistors.
Unlike bipolar transistors, JFETs are exclusively voltage-controlled in that they do not need a biasing current.
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. A JFET is usually on when there is no potential difference between its gate and source terminals. If a
potential difference of the proper polarity is applied between its gate and source terminals, the JFET will be more
resistive to current flow, which means less current would flow in the channel between the source and drain terminals.
Thus, JFETs are sometimes referred to as depletion-mode devices.
JFETs can have an n-type or p-type channel. In the n-type, if the voltage applied to the gate is less than that applied
to the source, the current will be reduced (similarly in the p-type, if the voltage applied to the gate is greater than
that applied to the source). A JFET has a large input impedance (sometimes on the order of 10
10
ohms), which means
that it has a negligible effect on external components or circuits connected to its gate.
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
IV characteristics and output plot of an n-channel JFET
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).
JFET
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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.
[1]
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 J202
device varies from -0.8V to -4V.
[2]
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, as well as lower flicker noise, 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 (metaloxidesemiconductor field-effect transistor) and a BJT though an IGBT resembles
more of the hybrid features.
JFET
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Mathematical model
The current in N-JFET due to a small voltage V
DS
(that is, in the linear ohmic region) is given by treating the
channel as a rectangular bar of material of electrical conductivity :
where
I
D
= drainsource current
b = channel thickness for a given gate voltage
W = channel width
L = channel length
q = electron charge = 1.6 x 10
19
C
n
= electron mobility
N
d
= n-type doping (donor) concentration
The drain current in the saturation region is often approximated in terms of gate bias as:
where
I
DSS
is the saturation current at zero gatesource voltage.Wikipedia:Please clarify
In the saturation region, the JFET drain current is most significantly affected by the gatesource voltage and barely
affected by the drainsource voltage.
If the channel doping is uniform, such that the depletion region thickness will grow in proportion to the square root
of (the absolute value of) the gatesource voltage, then the channel thickness b can be expressed in terms of the
zero-bias channel thickness a as:Wikipedia:Citation needed
where
V
P
is the pinchoff voltage, the gatesource voltage at which the channel thickness goes to zero
a is the channel thickness at zero gatesource voltage.
Then the drain current in the linear ohmic region can be expressed as:
or (in terms of ):Wikipedia:Citation needed
JFET
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References
[1] [1] For a discussion of JFET structure and operation, see for example
[2] J201 data sheet (http:/ / docs-europe.origin.electrocomponents. com/ webdocs/ 0027/ 0900766b80027bd1. pdf)
External links
Physics 111 Laboratory -- JFET Circuits I pdf (http:/ / ist-socrates. berkeley. edu/ ~phylabs/ bsc/ PDFFiles/ bsc5.
pdf)
Interactive Explanation of n-channel JFET (http:/ / www-g. eng. cam. ac. uk/ mmg/ teaching/ linearcircuits/ jfet.
html)
Article Sources and Contributors
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Article Sources and Contributors
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Image Sources, Licenses and Contributors
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