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,
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 electronic magazines in the 1970s and 1980s, provided 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 pinchoff voltages and gains
was largely responsible for the costs. Semiconductor processing and some circuit
application examples are included. One of the first big commercial applications of the
JFET was probably transistorized multimeters in order to obtain a high input impedance.
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 process.
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
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
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
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
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
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)