April 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
Adolph Mangieri, who authored
articles in other electronic magazines in the 1970s and 1980s, provides a good introduction
to junction field effect transistors (JFETs) in this 1973 piece in Popular Electronics
As mentioned, JFETs were a relative newcomer at the time to the commercial electronics
world because of high fabrication costs. Obtaining consistent pinch−off voltages
and gains was largely responsible for the relatively high production costs due
to substrate purity and doping issues. Semiconductor processing and some
circuit application examples are included. One of the first big commercial applications
of the JFET was probably transistorized multimeters, which enabled a very high input
impedance. Doing so helped minimize the loading effect on the meter on the
circuit under test.
Acting like a vacuum tube, the versatile JFET has many applications
By Adolph A. Mangieri
Junction field effect transistors (JFETs) are
now available in quantities and prices comparable to those of the bipolar transistor.
Although the JFET was developed at about the same time as the bipolar, its appearance
on the market was delayed because of the high cost of production, which has now
been greatly reduced by advances in manufacturing technology - including the planar
The JFET has a transverse conductive channel whose cross section is varied by
application of an electric field perpendicular to the current path. The field is
applied by gate junctions. The controlled load current consists either of electrons
or holes (but not both) and it passes through only one type of semiconductor material
- hence, the term unipolar transistor.
Fig. 1 - A bar junction FET is shown at (A), a planar junction
FET at (B). At (C), channel height is reduced by increase of depletion zone height.
Channel is cut off (D) by merged depletion zones. At (E) is shown onset of saturation
region with the drain voltage equal to pinch-off voltage. With high drain voltage,
the saturation current remains constant in (F).
Fig. 2 - Static characteristics of the JFET showing the
linear, or triode, region and the saturation, or pentode, region of the vacuum tube.
Types of construction of the JFET include the n-channel bar (Fig. 1A) and
the economically fabricated planar double-diffused unit formed on one side of the
silicon substrate as shown in Fig. 1B. Gate regions are heavily doped p regions
and channels are lightly doped n regions. This provides considerable "leverage"
of control of the depletion zone by relatively small gate voltages.
Channel ends are terminated by source and drain connections by metalized ohmic
contacts (linear, non-rectifying). Source and drain leads are interchangeable in
symmetrical JFET's; and, although the gate leads are usually tied together, they
may be separated to form a dual-gate JFET.
Figure 1A shows the normal voltage polarities and depletion zones (shaded) for
an n-channel JFET. Consider first the effect of varying gate voltage alone at low
drain-source voltage. At zero gate voltage, channel height is maximum, and channel
resistance is minimum. At an intermediate gate voltage (Fig. 1C) channel height
is reduced by penetration of the depletion zone. Channel resistance is higher because
the depletion zone is much like a non-conductive insulator. Finally, at a particular
gate voltage, usually between one and eight volts, the depletion zones merge, cutting
off the current (Fig. 1D). This occurs at gate pinch-off voltage (VP).
Now, consider the effect of varying the drain voltage with zero gate voltage.
As shown in Fig. 2, drain current increases with increasing drain voltage until
VDS equals VP. Channel saturation commences at this point
and the depletion zones merge initially at point A in Fig. 1E. With further
increase in drain voltage, the drain current remains constant in the saturation
region. Depletion zone merging progresses toward the source, as shown at point B
in Fig. 1F. At sufficiently high drain voltage, the gate junction avalanches
in the breakdown region.
Although zone merging is shown in Figs. 1E and 1F, current continues to flow
by virtue of carrier injection at points A and B, effected by high current concentration
and electric fields at these points. In the saturation region, the JFET is a constant-current
source with gate voltage control.
At a lower gate voltage (V1 in Fig. 2) saturation occurs at a
lower drain voltage which is equal to VP-V1 at a lower drain
current. Finally, at VP, the JFET is cut off for all values of drain
voltage. The linear region may be termed the triode region and the saturation region
the pentode region by analogy to vacuum tube characteristics.
Applications. Frequently used as a low-level preamplifier in
the common-source connection (Fig. 3A), the JFET permits direct inputs from
a high impedance device such as a crystal microphone. Source resistor RS
provides gate bias and also negative feedback which linearizes the input-output
characteristics at the expense of voltage gain. For higher ac gain, a capacitor
can be connected across the resistor.
Fig. 3 - Common-source voltage amplifier (A) has high input
and output impedances, with voltage and power gain. Source follower (B) has high
input impedance and low output impedance, with power gain but less than unity on
voltage gain. FET can be used as a constant current source (C) and source follower
and a constant-current supply combined provide near unity gain (D). At (E), FET-bipolar
combination boosts voltage and the power rating of FET.
The source follower, Fig. 3B, is another common application. Output voltage
is across the source resistor. Operationally, the output voltage follows the input
but at less than unity gain. In addition, the output impedance is less than the
source resistance. This circuit is used to step down impedance levels while preserving
bandwidth and linearity. For example, a high-impedance device can be coupled to
a low-impedance coaxial cable without sacrificing frequency response. The circuit
can be used to step up the input impedance of the bipolar transistor. In effect,
the source follower is an impedance transformer with power gain.
Linear sawtooth generators and long-delay timers may use the JFET in a circuit
which is equivalent to a constant-current diode (Fig. 3C). Current can be adjusted
from loss with the source resistor set to zero and to fractions of a microampere
with large values of RS. The circuit operates in the saturation region
where drain voltage changes have little effect on drain current.
To obtain a voltage gain near unity, the source resistor must be large in value.
This requires higher supply voltages. By replacing the source resistor with a constant-current
source (Fig. 3D), a high equivalent source resistor is achieved with a lower
dc drop across the current source.
Present limitations on JFET voltage and power are circumvented by the FET-bipolar
cascode circuit shown in Fig. 3E. The drain voltage of Q1, which drives Q2,
is about equal to the battery voltage. By using a high-voltage transistor for Q2,
,VC can be much larger than VD, permitting large RL
values and much higher output voltage and power. Cascode circuits inherently have
low reverse feedback or coupling. As such, the circuit is particularly suited to
high-frequency tuned amplifiers since it eliminates the need for neutralization
to prevent oscillations.
Transistorized voltmeters sometimes use a JFET to provide high input impedances
(11 megohms or more) with sensitivities exceeding those of a VTVM. Figure 4A shows
the simplest FET voltmeter circuit, a dc bridge with the FET in one leg. The source
resistor provides negative feedback to give high linearity in the response. The
circuit requires a regulated supply voltage - easily obtained by using a zener diode
- and can be used with full-scale dc ranges as low as 200 millivolts.
Higher dc sensitivities are obtained in the differential amplifier dc voltmeter
circuit shown in Fig. 4B. The circuit also has high common-mode rejection.
Emitter-follower, constant-current source Q3 fixes the drain currents to the zero
drift points (or near them) and also reduces effects of supply voltage changes.
The source resistors improve stability and linearity. For optimum results, Q1 and
Q2 must be closely matched pairs.
Fig. 4 - Basic dc FET voltmeter (A) uses a source follower
as part of bridge to give high stability, linearity, and sensitivity. A dc voltmeter
using a FET differential and constant-current source is shown at (B). Chopper (C)
has low noise and offset.
The JFET chopper circuit shown in Fig. 4C, when operated with sources having
high impedance and amplifiers with high input impedance, is better than a bipolar
transistor chopper. A chopper converts low-level dc to low-level ac, which is more
readily amplified. The JFET chopper has an offset voltage near zero. The gates of
Q1 and Q2 are driven by square waves 180 degrees out of phase so that one transistor
is on while the other is off. Transistor Q2 reduces noise by shorting the amplifier
input when Q1 is off or open. Chopper transistors are designed for low on resistance
in the linear region. (The on resistance may vary from a low of 10 ohms for low-speed
choppers to 150 ohms for higher speeds.)
Other JFET applications include their use in mixers or converters, in which advantage
is taken of their nonlinear characteristics. Dual-gate or tetrode JFET's are used
in agc and other dual-input circuits. Digital circuit logic elements have high fan-out
and low power requirements as a result of high input impedances. The relatively
high gate voltage swings which change the state from on to off, provide high noise
immunity in FET logic elements. The switching speed is inversely related to the
operating drain current; but, within the same current range, the switching speed
of JFET logic is somewhat comparable to many junction transistors.
Posted March 29, 2023
(updated from original post