concept of a field effect transistor (FET) has been around in theory
for a long time*, but manufacturable devices arrived in designers'
labs not until the early 1960s. This article from the October 1966
edition of QST magazine gives a good introduction to the physics
of a basic FET as well as the junction FET (JFET) and the insulated
gate FET (IGFET), all of which are still in widespread use today.
What you learn about them here is applicable today. In fact, I swear
some of the drawings are the same ones that appeared in my college
semiconductor physics text books (admittedly from the late 1980s,
so not too much of a surprise).
October 1966 QST
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present. All copyrights hereby acknowledged.
Wikipedia, "The field-effect transistor was first patented by
Julius Edgar Lilienfeld in 1926 and by Oskar Heil in 1934, but practical
semi-conducting devices (the JFET) were only developed much later
after the transistor effect was observed and explained by the team
of William Shockley at Bell Labs in 1947. The MOSFET, which largely
superseded the JFET and had a more profound effect on electronic
development, was first proposed by Dawon Kahng in 1960."
What They Are-How They Work
By Jim George,
The field-effect transistor, for the last few years a tantalizing
promise of better things in semiconductor devices, is now rapidly
coming into the market at prices attractive to amateurs. We've
already shown you one application in Walt Lange's audio oscillator
(July QST). Here's the background story of the FET - how it's
made and how it functions, and why it will be superior to older
transistor types in many applications.
A recently introduced semiconductor device called the
field-effect transistor, or FET for short, combines some of the
best features of both the vacuum tube and the transistor. The FET
is appearing in new electronic circuits of all types and will soon
be used in amateur radio equipment. A great deal of excitement centers
about this solid-state device which acts much like a vacuum tube,
and it is the purpose of this article to describe its construction
Before getting into device characteristics,
let us quickly review some basic concepts in semiconductors. Useful
semiconductor material is either p-type, where holes (a hole is
actually a place where there is no electron) are concentrated relatively
heavily, or n-type which features an excess of free electrons. Both
the holes and free electrons are the current carriers in a piece
of semiconductor, just as free electrons only are the current carriers
in a piece of copper. As a memory aid, remember that the "p" in
p-type material stands for the positive charge of the current carriers
(holes) and the "n" stands for the negative charge of the current
carriers (free electrons) in n-type semiconductor material. When
p-type material and n-type material are brought together, a p-n
junction is formed as shown in Fig. 1A.
The Junction Diode
Fig. 1 - Action of p-n junction when bias is applied.
Fig. 2 - Reverse bias causes electrons and holes to move away
from the p-n junction, leaving a depletion region (shaded area)
and no current flows.
Fig. 3 - The junction field-effect transistor (JFET).
Fig. 4 - Operation of the JFET under applied bias. A depletion
region (light shading) is formed, compressing the channel and
increasing its resistance to current flow.
Fig. 5 - The insulated-gate field-effect transistor (IGFET).
action of a p-n junction is well known. When a forward bias is applied
as shown in Fig. 1B, a large current will flow. The total current
is made up of the electron current plus the hole current. When reverse
bias (Fig. 1C) i applied, only a small reverse current flows - the
p-n junction acts as a diode. It is important to note the reverse-bias
case. We know that when a voltage is applied to a resistive network,
this voltage is dissipated by IR drops until the applied voltage
is "used up" by the sum total of all the voltage drops. The same
principle applies to the case of the reverse-biased p-n junction
diode. We have applied V volts of reverse bias and this voltage
must be used up somehow in our circuit, the p-n junction. The resulting
action is shown in Fig. 2, where charges are separated in a region
close to the junction. Enough charges are moved until the electric
field across the junction produces a voltage drop which is approximately
equal to our applied voltage. The important point here is that the
region around the p-n junction is now depleted of all its holes
and free electrons, thus there are no current carriers available.
This region is commonly called the "depletion region" and it sounds
reasonable that its thickness depends on the magnitude of reverse
voltage which is applied. It will be an important point in the operation
of field-effect transistors that no current can flow in the depletion
region since there are no current carriers in that region.
The Junction FET (JFET)
transistors are divided into two main groups: junction FETs, and
insulated-gate FETs. We will discuss the Junction FET, or JFET,
first. The basic JFET device is shown in Fig. 3.
all, note the location of the terminals where voltages can be applied.
The reason for the terminal names will become clear later. A d.c.
operating condition is set up by starting a current flow between
source and drain. This current flow is made up of free electrons
since the semiconductor is n-type in the channel, so a positive
voltage is applied at the drain. This positive voltage attracts
the negatively-charged free electrons and the current flows (Fig.
4A). The next step is to apply a gate voltage of the polarity shown
in Fig. 4B. Note that this reverse-biases the gates with respect
to the source, channel, and drain. This reverse-bias gate voltage
causes a depletion layer to be formed which takes up part of the
channel, and since the electrons now have less volume in which to
move the resistance is greater and the current between source and
drain is reduced. If we apply a large gate voltage, we cause the
depletion regions to meet, and in this case the source-drain current
is reduced nearly to zero. Since we changed the large source-drain
current with a relatively small gate-voltage, we have a device which
acts as an amplifier.
Further, note that in the operation
of the JFET, the gate terminal is never forward biased, because
if it were the source-drain current would all be diverted through
the forward-biased gate junction diode.
The resistance between
the gate terminal and the rest of the device is very high, since
the gate terminal is always reverse biased, so the JFET has a very
high input resistance. The source terminal is the source of current
carriers, and they are drained out of the circuit at the drain.
The gate opens and closes the amount of channel current which flows.
It is seen how the operation of a FET closely resembles the operation
of the vacuum tube with its high grid input impedance. Comparing
the JFET to a vacuum tube, the source corresponds to the cathode,
the gate to the grid, and the drain to the plate.
Insulated-Gate FET (IGFET)
The other large
family which makes up field-effect transistors is the insulated-gate
field-effect transistor, or IGFET, which is pictured schematically
in Fig. 5. In order to set up a d.c. operating condition, a positive
polarity is applied to the drain terminal. The substrate is connected
to the source, and both are at ground potential, so the channel
electrons are attracted to the positive drain and we now have a
d.c. source-drain current. In order to regulate this current, we
apply voltage to the gate contact. Note that the gate is insulated
from the rest of the device by a piece of insulating glass so this
is not a p-n junction between the gate and the device - thus the
name insulated gate. When a negative gate polarity is applied, positively-charged
holes from the p-type substrate are attracted towards the gate and
the conducting channel is made more narrow; thus the source-drain
current is reduced. When we connect a positive gate voltage, the
holes in the substrate are repelled away, the conducting channel
is made larger, and the source-drain current is increased. As can
be seen, the IGFET is more flexible since we can apply either a
positive or negative voltage to the gate. The resistance between
the gate and the rest of the device is extremely high because they
are separated by a layer of glass - not as clear as your window
glass, but it conducts just as poorly. Thus the IGFET has an extremely
high input impedance. In fact, since the leakage through the insulating
glass is generally much smaller than through the reverse-biased
p-n gate junction in the JFET, the IGFET has a much higher input
impedance. Typical values of Rin for the IGFET are over
a million megohms, while Rin for the JFET ranges from
megohms to over a thousand megohms.
- Typical JFET characteristic curves.
Fig. 7 - Typical IGFET
Fig. 8 - Typical vacuum-tube characteristic
The characteristic curves for the FETs described above are shown
in Figs. 6 and 7, where drain-source current is plotted against
drain-source voltage for given values of the gate voltage. Note
the similarity to the family of a vacuum-tube pentode as shown in
Fig. 8, where plate current is plotted against plate voltage for
varying amounts of grid voltage.
Fig. 9 - Symbols for most-commonly available field-effect transistors.
Fig. 10 - Typical JFET crystal-oscillator circuit.
In discussing the JFET
so far we have left both gates separate so the device can be used
as a tetrode in mixer applications. However, the gates can be internally
connected for triode applications. When using the IGFET the substrate
is always a.c.-shorted to the source, and only the insulated gate
is used to control the current flow. This is done so that both positive
and negative polarities can be applied to the device, as opposed
to JFET operation where only one polarity can be used, because if
the gate itself becomes forward biased the unit is no longer useful.
are classed in to two main grouping for application in circuits,
enhancement mode and depletion mode. The enhancement-mode devices
are those specifically constructed so that they have no channel.
They become useful only when we apply a gate voltage which causes
a channel to be formed. IGFETs can be used as enhancement-mode devices
since both polarities can be applied to the gate without the gate
becoming forward biased and conducting current.
unit corresponds to Figs. 3 and 5 shown earlier, where a channel
exists with no gate voltage applied. For the JFET we can apply a
gate voltage and deplete the channel, causing the current to decrease.
With the IGFET we can apply a gate voltage of either polarity so
the device can be depleted (current decreased) or enhanced (current
To Slim up, a depletion-mode FET is one which
has a channel constructed; thus it has a current flow for zero gate
voltage. Enhancement-mode FETs are those which have no channel,
so no current flows with zero gate voltage . The latter type devices
are especially useful in logic application.
symbols approved for FETs are shown in Fig. 9. Both depletion-mode
and enhancement-mode devices are illustrated.
Some applications for FETs are shown in Figs. 10 and 11.
In Fig. 10 a JFET oscillator is pictured, and a versatile FET d.c.
voltmeter (FETVM) is shown in Fig. 11.
The voltmeter features
two Motorola 2N4221 JFETs and offers the high input impedance (22
megohms on all ranges in Fig. 11) of a v.t.v.m, but with more stability.
The circuit is essentially a differential amplifier which works
on the principle that the current through a resistance is directly
proportional to the difference between the voltages at its ends.
When R1 and R2 are adjusted so that the voltage
at the emitter of Q2 is equal to the voltage at the emitter
of Q3, with no input signal, the voltage difference is
zero and no current will flow through the meter.
Fig. 11 - D.C. voltmeter circuit
using field-effect transistor. Resistances are
in ohms (K = 1000);
fixed resistors are 1/2 watt. The second
position of S3
is used for checking battery voltage.
1 section, 1 pole,7 positions.
R2 R3-Linear controls (R2 and R3
are panel adjustments; R1 can be internal).
inc.-Trimming potentiometers, for internal
mounting (Mallory MTC or equivalent).
the voltage at the gate, G, of Q1 is raised to 0.3 volt
the voltage across the meter circuit also is approximately 0.3 volt
since the source followers and emitter followers have a voltage
gain of approximately one. R3 is then adjusted to give
full-scale meter deflection. The calibration resistors, R4-R9
inclusive, are individually adjusted for exactly full-scale deflection
on each range when the maximum voltage for that range is applied
to the input terminals through the range switch, S2.An
accurate voltmeter should be used to check the applied voltage when
setting the calibration resistors.
These two circuits were
furnished by Don Wollesen and Walter Birks of the Applications Engineering
Group at Motorola Semiconductor Products Division.
article has provided a basic look into the FET and some of its applications.
In general, the FET offers much improved noise performance, stability,
and cross-modulation resistance over either vacuum tubes or standard
transistors (p-n-p, n-p-n). Its many features, such as high input
impedance and desirable high-frequency performance, insure its design
into a vast number of electronic circuits in the near future. It
is probably the most recent radical improvement in device design,
and should prove most interesting to amateur radio operators everywhere,
allowing them greatly improved freedom in circuit design.
* c/o Motorola Semiconductor
Products, Inc., 5005 East McDowell Road, Phoenix, Ariz. 85008