February 1967 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
|
Yesterday was the 71st
anniversary of the announcement of the transistor's invention by Drs. Shockley,
Bardeen, and Brattain at Bell Labs, but it was a Sunday so not as many RF Cafe visitors
saw the commemorative title graphic I used (see it
here).
Their transistor was a current-controlled signal amplifying device as opposed to
the field-effect transistor (FET) which is a voltage-controlled signal amplifying
device - as is the vacuum tube. I never thought about it before, but maybe that
had something to do with the electronics world's hesitancy to adopt the transistor
as a replacement for the tube. Early in the transistor's history, practical applications
were limited due to low reliability, low power handling, low frequency, lack of
ruggedness in harsh operating conditions, and other shortcomings compared to established
and much refined vacuum tubes was reason enough to shun the newfangled technology,
but that current-controlled thing could have been a barrier inhibiting adaptation
as significant as any of the aforementioned obstacles. By the time FETs became widely
available for commercial use, the transistor vs. tube battle was already tipping
in favor of the transistor. FETs initially enjoyed a huge cadre of cheerleaders
in the digital circuit realm due to their extremely low power consumption. One of
the most notable uses of FETs in the analog world was as high impedance inputs to
opamps and voltmeters.
Meet Mr. FET ... the Transistor That Thinks it's a Tube
By Louis E. Garner, Jr., Semiconductor Editor
This Little Fellow and His Family are Taking over Solid-State
It's hard to imagine, in the light of present scientific and technological achievements,
that just a few short years ago there were no transistors and no integrated circuits.
In fact, there are still many old-timers who remember the "prehistoric" age when
there were no vacuum tubes, either. In those days, radio transmitters were weird
spark-sputtering electromechanical monsters which bore a nostalgic resemblance to
the fire-eating dragons of a yet earlier era.
Radio receivers were simple, too. A huge antenna hooked up to a couple of oversized
coils, a tiny bit of mineral-galena - with a cat's whisker (fine wire), a pair of
headphones ... and that was the receiver. The galena, a crystal detector, was cheap,
but it was insensitive and temperamental, too. It was on a quest for a better detector
that Prof. J. A. Fleming developed the diode vacuum tube which, rightly enough,
came to be known as the "Fleming valve."

Fig. 1 - Cutaway view illustrates the internal construction
of a triode vacuum tube. The schematic symbol representing this tube is shown below
the cutaway view.
|

Fig. 2 - Basic junction transistor cross section shows sandwich
arrangement of semiconductor material for pnp unit. Note direction of arrow in the
schematic symbol.
|

Fig. 3 - Cross section of n-channel junction field-effect
transistor shows p-type regions diffused into n-type substrate. Symbol has not been
fully standardized yet.
|
A short time later Dr. Lee De Forest, inventor and scientist, added the control
grid which, for the first time, enabled the vacuum tube to amplify, oscillate and
detect electrical signals.
With the development of the vacuum tube came a giant industry with a record of
spectacular achievements in radio broadcasting, electronic surveillance, computer
technology, and industrial control. During the course of this industrial revolution,
the vacuum tube was enlarged, miniaturized, modified and refined in many ways, including
the addition of more electrodes. But there was a proverbial fly in the ointment.
Most tubes generated so much heat that they had a relatively short useful life,
and this resulted in a high failure rate for tube-type electronic equipment.
Then, early in 1948, Drs. Shockley, Bardeen, and Brattain - all scientists at
the Bell Telephone Laboratories - announced the invention of a completely new device:
a triode "crystal" which they claimed could amplify as well as detect electrical
signals. Dubbed a Transistor (from TRANsfer and re-SISTOR), the device was nothing
more than a tiny cube of crystalline semiconductor material with two fine wire cat's
whiskers. A minute voltage applied to the base crystal (thereafter called the base)
controlled a much larger current flowing between the two whiskers, one of which
was called the emitter, and the other a collector. The early transistors were expensive,
noisy, and not too reliable. But these disadvantages were offset by their extremely
small size, high efficiency and, potentially at least, manufacturing simplicity.
By 1951, long before this early point-contact transistor posed even a mild threat
to the supremacy of the vacuum tube, a radically new type of transistor, the now
common and widely used junction transistor was introduced.
Of Tubes and Transistors. Although a godsend in many ways, transistors brought
a host of new problems to circuit designers. Essentially a current amplifier, the
device could not be used as a direct replacement for the vacuum tube, which is a
voltage amplifier. It had a low-to-moderate input impedance in contrast to the very
high input impedance of vacuum tubes. In addition, because the transistor has a
direct resistive connection between its input (base) and output (collector) terminals,
a multiplicity of circuit feedback problems had to be solved.
Improved design methods were developed later, and transistorized receivers, amplifiers,
transmitters, hearing aids, toys and industrial controls were produced in vast quantities.
But there were still many circuit requirements where only high-impedance vacuum
tubes could fill the bill, and many designers yearned for a miracle-like device
- a transistor with tube-like characteristics.
As time went by, transistors got better and better. Output voltage and current
ratings were being extended, as were the upper operating frequency limits. But no
matter how the newer transistors were improved, they still had the basic characteristics
of earlier types.
Meanwhile, back at the laboratory, scientists were experimenting with a new solid-state
device, based on a molecular principle described by Lilienfeld as far back as 1928.
Shockley, one of the co-inventors of the original transistor, had proposed a practical
transistor-like device based on Lilienfeld's principle as early as 1948, but it
was not until the mid 1950's that a workable device was developed in the laboratories,
and practical, reliable units were not manufactured until the early 1960's.
The new device combined the most desirable features of the versatile vacuum tube
and the efficient transistor. It had high input impedance and offered good isolation
between input and output electrodes. Capable of high gain, it was, at the same time,
as small as conventional transistors and extremely efficient. And, oddly enough,
it exhibited at least one of the important operating characteristics of the vacuum
tube - the control of a current by means of a varying electric field - in a solid-state
medium rather than in a vacuum.

Fig. 4 - Diffusion of p-type regions into n-type substrate
provides a means of controlling the current flow between source and drain electrodes.
|

Fig. 5 - When gate is reverse-biased, an electric field
is set up to repel the current carriers, creating a depletion area and restricting
region in which current flows.
|

Fig. 6 - As the reverse gate bias is increased, depletion
areas spread into the channel until they meet, creating an almost infinite resistance
between source and drain.
|
Identified by a variety of names - Fieldistor, unipolar transistor, and so on
- during its gestation period, the device is now known as the field-effect transistor
(FET). It is, indeed, a transistor which "thinks" and "acts" like a tube.
Meet Mr. FET. Pictorial and schematic representations of a triode vacuum tube,
junction transistor, and field-effect transistor are illustrated in Figs. 1 through
3. Of the three schematic symbols, the FET symbol is the least standardized at present.
In a vacuum tube (Fig. 1), the plate current is simply a flow of free electrons
which are literally "boiled" off of the cathode by the heated filament (in some
high-power tubes, the filament is used directly) and are attracted by the positively-biased
plate. The electrons leaving the cathode must travel through the intervening grid.
A negative bias on the grid establishes an electric field which tends to repel
the electrons flowing from cathode to plate, limiting the plate current. The plate
current can also be controlled, within limits, by the plate voltage. However, since
the grid e is much closer to the cathode than the plate, a smaller variation in
grid voltage has essentially the same or greater effect on the plate current as
a larger variation in plate voltage. It is this characteristic that e-ables a vacuum
tube to amplify a signal.
Plate current saturation occurs when the plate is attracting all available free
electrons. When this point is reached, a further increase in plate voltage does
not cause a corresponding increase in plate current.
The basic junction transistor (Fig. 2) consists of three sandwich layers
of two different semiconductor materials. Here, the emitter-collector current consists
of a movement of two types of particles: electrons, which are negatively charged,
and "holes" (essentially, the absence of an electron in an otherwise stable crystalline
structure) which carry a positive charge. If the electrons predominate, they are
called majority carriers and the holes minority carriers, with the material identified
as an n-type semiconductor. By the same token, a material in which the positive
holes predominate is called a p-type semiconductor.
The transistor's emitter-collector current is controlled by the injection of
minority carriers into the base region. Since the base is quite thin, a relatively
small current change there can control a much larger emitter-collector current.
The junction transistor, then, is a current amplifying or control device, in contrast
to the vacuum tube, which is essentially a voltage amplifier. In addition, since
a base current flow, however minute, is essential to operation, the device must
have a low input impedance.
The basic field-effect transistor consists of a slab of either n- or p-type semiconductor
material with an electrode at either end, and two electrodes along the sides as
shown in Fig. 3. Observe that the side electrodes are tied together and thus
function as a single element. By convention, the terminal into which current is
injected is called the source, and the output terminal is called the drain. The
remaining electrode, which serves as a control element, is called the gate. Notice
how FET terminology thus differs from that of both vacuum tubes and junction transistors.

Fig. 7 - A JFET can be manufactured by diffusing p-type
gates on either side of an n-type substrate, and then attaching suitable electrodes.
|

Fig. 8 - This junction FET features single-ended construction.
Here, an n-type channel is formed on one side only of a p-type substrate by photo-masking,
etching, and impurity diffusion processes. The surface is covered with an insulating
oxide layer through which holes are cut for electrode connections.
|

Fig. 9 - Cross-section view of insulated-gate field-effect
transistor (IGFET) shows gate metal contacts insulated by a thin layer of oxide
which, together with the semiconductor channel, forms a capacitor. The metal contacts
serve as one plate while the substrate material serves as other plate of capacitor.
|
How the FET Works. The basic junction FET (JFET) is essentially a bar of doped
silicon that behaves like any ordinary resistor. Refer to Fig. 4 and assume
that the FET is made up of an n-type substrate (material). Then, current through
the device will consist principally of electrons as majority carriers. Consider
what happens when a d.c. voltage is applied to the source and drain electrodes,
while the gate is at zero bias. Under these conditions the device behaves more or
less like an ordinary resistor. Within limits, source-drain current flow is directly
proportional to applied voltage.
Now suppose a reverse bias is applied to the gate. (This would be a voltage of
the same polarity as the majority carriers; that is, negative for n-type material,
positive for p-type material.) The gate voltage would then set up an electric field
to repel the current carriers, and restrict the region through which they flow.
This action is shown in Fig. 5. In essence, the current-carrying channel is
depleted of current carriers within areas immediately adjacent to the gate electrode.
Logically enough, the regions where current movement is restricted are termed the
depletion areas (sometimes referred to as zones or regions rather than areas).
A further increase in the reverse gate bias further expands the depletion areas,
as shown in Fig. 6, further reducing drain-to-source current. Thus, with a
given fixed gate bias, the drain current will vary with the signal applied to the
gate. Note, also, that since the gate is reverse-biased, the FET has a very high
input impedance when there is little or no drain current flow. The FET behaves much
like a vacuum tube in that drain current is controlled by an electric field set
up by the gate voltage.
Consider what happens when the gate bias is zero and the source-drain voltage
is gradually increased. Up to a point, drain current will increase linearly as in
a resistor. However, the drain current flowing along the channel sets up an internal
reverse bias along the surface of the gate. This, in turn, establishes an electric
field which causes a gradual increase in the depletion areas similar to the effect
produced by the application of an external gate bias. Eventually, the increase in
the depletion areas, which tends to limit drain current, reaches the point where
it counterbalances the drain current increase. From then on, there can be no further
increase in drain current regardless of any further increase in drain-source voltage.

Fig. 10 - Schematic symbols currently used for field-effect
transistors include (a) n-channel JFET, (b) p-channel JFET, and (c) one form of
p-channel IGFET.

Fig. 11 - This FET voltmeter, featuring a matched pair of
Siliconix U112 FET's in a differential amplifier arrangement, has a sensitivity
of 0.5 - 1.0 volt full scale.
In effect, the drain current has reached saturation (that should be a familiar
term!). The point at which this current limiting takes place is called the drain-source
pinch-off voltage. And there is, as you might suspect, a pinch-off voltage for any
given gate bias. With higher gate bias voltages, pinch-off occurs at much lower
drain currents, of course.
If drain current is plotted against drain-source voltage for a given gate bias,
a FET characteristic curve is developed. A family of such curves may be prepared
by plotting drain-source current vs. drain-source voltage for a number of different
gate bias voltages. When compared to corresponding families of vacuum tube characteristic
curves, the typical FET is found to have characteristics which are virtually identical
to those of a pentode vacuum tube.
The FET Family. Field-effect transistors are manufactured using techniques that
are almost identical to those used in the manufacture of the familiar junction transistor.
For example, a FET can be assembled by diffusing or alloying p-type gates on either
side of an n-type substrate and then attaching suitable metallic electrodes, giving
the appearance of Fig. 7.
From a production standpoint, it is often easier to carry out all diffusion and
processing operations from one side of the substrate. This type of single-ended
construction is illustrated in Fig. 8. Manufacture starts with a wafer of p-type
material. Photo-masking, etching, and impurity diffusion processes form an n-type
channel on one side of the material. A p-type gate is then diffused into the n-type
channel, and the entire surface is covered with an insulating protective oxide layer,
with holes etched through the oxide for the final metallic electrode connections.
If you have been wearing your "thinking cap," you may be wondering, at this point,
just why the gate electrode is joined electrically to the channel material. After
all, the gate is reverse-biased in use, causing the p-n junction to behave as if
it were a dielectric. Furthermore, the operation of the device is based on the presence
of a varying electric field on the gate and not upon the movement of current carriers
from the gate to the channel region.
So, why not insulate the gate? Good question, but someone else thought of it
before. As a matter of fact, insulated-gate FET's (IGFET's) are actually being produced
by several major manufacturers. One type of construction is illustrated in Fig. 9.
Here, the gate is insulated by a thin layer of oxide. The gate metal area is overlayed
on the oxide and in conjunction with the insulating oxide layer and the semiconductor
channel forms a capacitor. The metal area serves as the top plate of the capacitor,
while the substrate material is the bottom plate.
In some cases, the IGFET's are assembled as tetrode devices, with the substrate
body (often identified as gate 2) connected to a separate electrode. Since the drain
and source are isolated from the substrate, any drain-to-source current in the absence
of gate voltage is extremely low because, electrically, the structure is equivalent
to two diodes connected back to back.

Table 1 - JFETs for the Experimenter
Insulated-gate FET's have extremely high input impedances - higher, in fact,
than many vacuum tubes - but are very sensitive to stray electrical charges and
can't be destroyed by body static. Input impedances higher than 10 million megohms
are not uncommon. Manufacturers generally wrap IGFET leads in metallic foil, or
supply them with the leads held together by a metal eyelet as a protective measure.
Extra care must be taken during installation, wiring, and testing of the IGFET to
prevent its destruction.
The junction field-effect transistor (JFET) shown in Figs. 7 and 8 can be made
as an n-channel or a p-channel device. As with conventional junction transistors,
JFET's are identified by the slightly modified schematic symbols shown in Figs.
10(a) and 10(b). With the source considered common, an n-channel FET requires a
positive drain voltage and a negative gate bias; the p-channel FET is operated with
a negative drain voltage and a positive gate bias.
As shown in Fig.10(c), the IGFET is identified by an entirely different symbol.
This general type of FET is offered in two basic forms and in many individual types
with different electrical specifications and operating characteristics. Unlike the
JFET, however, a given IGFET may require either a positive or negative gate bias,
with respect to its source, depending on mode of operation.
In addition to regular FET's, light-sensitive FET's are being produced by a number
of manufacturers. Called photoFET's, they are similar to conventional FET's but
are equipped with transparent lenses that focus external light on their sensitive
surface areas. The photoFET can be up to ten times as sensitive as a junction phototransistor,
and has a better gain bandwidth factor, in addition to offering exceptional isolation
between input and output circuitry.
Terminology. As with any new technology, a number of terms are used to describe
FET devices, and their characteristics. Some terms are used primarily by manufacturers,
others chiefly by circuit designers. Unfortunately, the terms and symbols have not
yet been fully standardized, with the result that different manufacturers may use
different terms and symbols to represent the same thing.
During its early developmental stages, the FET was identified by different names.
At various times it has been called a Fieldistor, UNIFET, and Unipolar field-effect
transistor. The UNIFET and Unipolar terms were derived from the single-junction
construction of the FET as contrasted to the two-junction (or bipolar) construction
of the junction transistor.

Fig. 12. This high-frequency crystal-controlled oscillator
employing a Siliconix 2N2608 p-channel FET has a useful operating range of 1 megahertz.

Fig. 13. Modified Baxandall hi-fi tone control employs a
single p-channel FET (Siliconix 2N2843). Separate bass and treble controls are provided.

Fig. 14. Definitely not recommended for the experimenter,
this single-stage preamplifier features an insulated-gate field-effect transistor
(IGFET).
The name Fieldistor is practically ob-solete today. And so are the other names,
although one firm still refers to its products as UNIFETS. Generally, junction-type
units are simply referred to as FET's, although some firms use the more specific
designation JFET.
Insulated-gate field-effect transistors are also called MOSFET's in recognition
of the importance of the metal-oxide-semiconductor (MaS) insulating film used in
their construction. But some de-signers refer to the same device simply as MOST.
The latter could lead to an expression such as "Gosh, Mr. FET, you're the MOST."
At times, the full expressions used to identify a specific transistor may assume
an awe-inspiring length. For example, a data sheet from one firm identifies a specific
unit as a - hold your breath - low-noise, n-channel epitaxial planar silicon tetrode
field-effect transistor!
In addition, not all manufacturers describe their products using the same specifications.
A parameter which is considered important by one company may be completely ignored
by another. As a general rule, however, the majority of manufacturers do give maximum
voltage ratings, input and output capacitances, maximum power dissipation, and typical
gate cutoff current. Many even specify the common source forward transconductance
(in μmhos, as in tube specifications) for typical operating conditions.
Naturally, references are still made to n-channel or p-channel types, as well
as to enhancement or depletion modes of operation. The fact that both n- and p-channel
types are available permits FET's to be used in a variety of complementary circuits,
a characteristic that FET's do not share with vacuum tubes.
Some firms, in striving to simplify matters, have adapted type designations to
indicate the intended mode of operation of the device. Thus, Type A FET's are characterized
for depletion-mode operation; Type B are intended for either depletion or enhancement
modes; and, finally, the Type C designation is reserved strictly for enhancement-mode
types. But please don't confuse these designations with Class A, B or C amplifiers!
Typical FET Applications. With high input and output impedances and other tube-like
operating characteristics, FET's may be considered as almost the solid-state equivalents
of vacuum tubes, and can be used in virtually identical circuits, provided power
ratings are observed. The common source configuration is the most popular, and corresponds
to the common-cathode tube circuit arrangement. Typical FET circuits are illustrated
in Figs. 11 through 14.
Figure 11 is a FET voltmeter with a matched pair of p-channel FET's (Q1 and Q2)
used in a differential amplifier arrangement. In general, FET voltmeters compare
favorably with good-quality VTVM's.
A high-frequency crystal-controlled oscillator employing a p-channel FET is shown
in Fig. 12. .Gate bias is provided, as in a vacuum tube circuit, by source
resistor R2, bypassed by C2. The feedback needed to start and sustain oscillation
is furnished by the FET's inter-electrode capacity as well as by stray wiring capacities.
Figure 13 features a single p-channel FET, Q1, in a modified Baxandall hi-fi
tone control circuit which can be used as part of a stereo control center. Potentiometer
R2 serves as the bass control, and R5 as the treble control.
Finally, a simple preamp circuit using an IGFET (MOSFET, or MOST, take your choice)
is given in Fig. 14. Here, gate bias is provided by a 22-megohm resistor, R1,
returned to the drain electrode.
These circuits illustrate a few of the many practical applications of the FET.
They are not intended for use in construction projects as shown, since some component
values might have to be changed to compensate for the use of different FET's. In
any case, only an experienced technician should attempt to use an IGFET in the application
shown in Fig. 14. Practical FET projects will be covered in future issues.
One thing is certain: Mr. FET is a real "comer," and should have a brilliant
future!
Posted December 14, 2018
|