Esaki invented the
tunnel diode in 1957 while working at Sony (Tokyo Tsushin Kogyo
at the time). Tunnel diodes have a very narrow, heavily doped p–n
junction only around 10 nm (100 Å) wide that exhibits a broken
bandgap, where conduction band electrons and therefore on the n-side
are approximately aligned with valence band holes on the p-side
facilitate the quantum mechanical tunneling process after which
the diode is named. A negative differential resistance in part of
their operating range makes them useful for high frequency oscillators.
This article in a 1960 edition of Popular Electronics introduces
the device's characteristics and potential uses.
September 1960 Popular Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published October 1954 - April 1985. All copyrights are hereby acknowledged.
The Tunnel Diode
By Donald L. Stoner, W6TNS
of semiconductors, the tunnel diode is unique in its field. Learn
why it's unique, then build a simple transmitter and put it to work.
By now, just about everyone has heard of the tunnel diode, latest
"miracle" from the semiconductor industry. Though related to the
tube and transistor, the tunnel diode ordinarily has only two terminals.
Yet it differs from other two-terminal devices (resistors, capacitors,
and so on) in a very special way. Apply voltage to a resistor, for
example, and you can determine current flow by Ohm's law. Increase
the voltage across the resistor, and the current flow through the
resistor will increase in proportion. But this is not so with the
The effect which brought about the practical
construction of this unique semiconductor was discovered by Dr.
Leo Esaki, a brilliant Japanese scientist. Dr. Esaki determined
that unusual doping of the germanium-diode junction would cause
the current flow to decrease, even though the applied voltage was
increased. This effect, known as negative resistance, enables the
tunnel diode to perform its unusual feats.
To understand the term negative resistance
and what causes it, let's study a more familiar object - a tetrode
Figure 1(A) shows a tetrode vacuum tube with a fixed screen voltage
of 200 volts and a plate voltage that can be varied between 0 and
300 volts. The tube's control grid is grounded, since we need no
input signal to the tetrode for the purposes of this example.
Fig. 1. Tetrode vacuum tube circuit (A) displays curves (B)
somewhat like those of a tunnel diode. See text.
Fig. 2. Tunnel-diode forward characteristic curve. In negative-resistance
slope range, current through diode decreases even though voltage
across diode increases.
Fig. 3. Typical crystal oscillator circuit using tunnel diode.
denotes internal resistance of battery.
Fig. 4. Load line for typical tunnel-diode oscillator. Load
must be as low as possible to restrict diode to negative-resistance
portion of curve.
Fig. 5. Low internal-resistance power supply for tunnel diode
circuits. Drain through resistor RI is heavy but unavoidable
due to required design.
Let's vary the plate voltage between 0 and 300 volts and record
the changes in the tetrode's plate current as shown on the milliammeter
- see Fig. 1(B). Note that the plate current increases in the normal
fashion as the plate voltage is increased until the plate voltage
reaches a value of about 100 volts.
At this point a peculiar
phenomenon occurs due to the secondary emission from the plate -
the plate current decreases as the plate voltage increases. This
decrease in plate current with increase in plate voltage is called
negative resistance, which is a well-known characteristic of tetrodes.
When the plate voltage reaches the value of the screen voltage,
200 volts in this example, the plate current increases as before.
Negative resistance is seemingly contrary to Ohm's law.
If we were to apply a steadily increasing voltage across a resistor,
for example, the current through the resistor would increase proportionately.
If we carried this far enough, the resistor would eventually go
up in smoke. But in this case, steadily increasing voltage on the
tetrode's plate brings steadily decreasing current. The tetrode
in this example actually exhibits a negative resistance at plate
voltages between about 100 and 200 volts.
Now that we know
what negative resistance is, let's return to the tunnel diode. The
slope of the tunnel diode's forward-characteristic curve is very
much like the tetrode's plate-characteristic curve. See Fig. 2.
Note that as the diode voltage is increased positively from zero
to Vp, the tunnel-diode curve is similar to that for any conventional
semiconductor or vacuum-tube diode. However, at Vp we reach the
peak voltage of the negative- resistance portion of the tunnel-diode
slope. Now the tunnel-diode current decreases as the voltage across
it increases until the potential Vv, the valley voltage, is reached.
At this point, the diode reverts back to type and the current increases
as the voltage is increased above Vv. By operating the tunnel diode
on the negative-resistance portion of its curve, we can make it
function as a negative-resistance oscillator, as will the tetrode
shows a typical crystal oscillator circuit made possible by the
development of the tunnel diode. Actually, any negative-resistance
device (a tetrode tube, operated at a plate voltage well below its
screen voltage as discussed previously, for example) could be used;
the arrangement is known as a negative- resistance oscillator.
One of the greatest advantages of this circuit, known as a Dynatron
oscillator in its tube version, is its inherent simplicity - it
requires only a power source, a negative-resistance device, and
a tuned circuit. Although the circuit is relatively unstable in
contrast to other oscillators, its oscillatory properties depend
solely on the use of a negative-resistance device between battery
B1 and tuned-circuit L1-C2.
Depending on the impedance of
tuned-circuit L1-C2, the circuit in Fig. 3 will function as an amplifier
or an oscillator. To oscillate, the diode's operating point must
be in its negative-resistance region, and the impedance of L1-C2
must be greater than the negative resistance of the diode.
One factor to consider with the tunnel diode is the internal
resistance of the battery, Rint. This resistance is equivalent
to the plate-load resistor in a vacuum-tube circuit. Figure 4 shows
typical load lines that are possible for a tunnel-diode oscillator.
Note that all load lines are drawn from point Vb which is the power
supply voltage. The actual value of Rint is important
to us. We know that the internal resistance will always be present
so that a resistance of zero is impossible in practice. If Rint
is too high, the tunnel diode will be operating on the positive
portion of its slope, which we want to avoid. Hence, it is desirable
to have a resistance as close to zero as possible.
tunnel diodes have negative slope resistance between 20 and 40 ohms,
and Rint should be on the order of 10 ohms or less for
the oscillator circuit to operate. The action of C1 in Fig. 3 helps
to reduce the internal resistance of B1. However, a low-value bleeder
resistor connected in parallel with C1 would greatly improve the
operation of the circuit.
Figure 5 shows a low internal-resistance
power supply that can be used to power tunnel-diode circuits. If
you own a low-voltage power supply (one for powering transistors
is ideal), it can be used in place of the circuit shown in Fig.
5. Dry cells cannot be used with much success because their voltage
and internal resistance are too high. If a bleeder is placed across
the dry cell, the large currents passing through the resistor will
result in a steadily increasing internal resistance in the dry cell.
For experimental purposes, dry cells can be used if they are of
the D size or larger and are new. However, they are usable only
for a short time.
For a better understanding of just
what a tunnel diode can do, let's try an experimental hookup using
it in a midget, or "Micro-QRP," 80- or 40-meter transmitter. Even
though the tunnel diode is a low-power device, such a transmitter
is capable of delivering a usable signal. The "Micro-QRP" tunnel-diode
transmitter runs on about 0.6 volt at 1.8 ma., or approximately
one milliwatt input. It is crystal-controlled on either the 80-
or 40-meter bands, but can be used on any frequency between 3.5
and 10 mc. with the values shown.
There are only nine working
components in the tunnel-diode transmitter - a key jack, a 1.5-volt
battery, a 1000- ohm potentiometer, a 100-ohm resistor, a .01-uf.
disc capacitor, a 200-uf., 3-volt electrolytic capacitor, the tunnel
diode, and a coil and crystal. See Fig. 6.
Mount the components
in a 1 5/8" x 2 3/4" x 2 1/8" chassis box. The meter jack is mounted
on the front panel along with the bias potentiometer; the coil,
crystal, and tunnel diode are mounted on the top of the chassis;
the battery is located under the chassis and supported by leads
soldered to the terminals.
Fig. 6. Circuit of tunnel-diode
transmitter for operation between 3.5 and 10 mc. Battery power
supply (shaded portion) can be replaced with supply shown in Figure
5 if desired.
tunnel-diode pin connections.
A large solder lug should be installed under the coil and
used as the common ground terminal for the entire transmitter. It
is important that both disc capacitors be returned to this point,
with very short leads.
The meter jack is connected in an
unusual manner to eliminate the need for an on-off switch. Use a
two-circuit jack, with the frame grounded to the chassis. The outer
contact connects to the minus end of the battery and the center
contact is wired to the coil. When a meter plug is inserted, it
shorts the outer pin to the chassis, thereby completing the battery
circuit. The tunnel-diode circuit (through the coil) is completed
by the meter.
The General Electric 1N2939 and 1N2940 tunnel
diodes plug into a standard transistor socket and are therefore
easy to work with. The RCA TD-100, on the other hand, will have
to be modified by trimming away some of the gold foil lead to make
a "pin" of each terminal; be sure to remove sufficient material
so that the "pin" will fit snugly in the socket gripper. Once the
tunnel diode has been mounted, wire the "Micro-QRP" transmitter
as shown in Fig. 6, and you're ready to check it out.
Testing the Transmitter.
Tunnel diode transmitter is adjusted with external multimeter
to determine the diode's negative-resistance region.
Connect a milliammeter
as shown in the schematic diagram; any meter between 5 and 15 ma.
full scale will do. Turn the bias potentiometer to the minimum resistance
end of rotation and plug in the meter. The reading should be a little
over .01 ma. As the potentiometer is rotated, the current will increase.
When the meter indicates 1-3 ma. (depending on what type tunnel
diode you use), the reading will suddenly jump to a lower current.
The point at which the drop occurs is called the peak current; the
value to which the meter drops is called the valley current. In
between these two points is the unstable or negative resistance
region where the diode oscillates.
By tuning a communications
receiver to the crystal frequency, you should be able to hear the
signal generated by the "Micro-QRP" transmitter. Place a hank of
wire from the receiver antenna terminal near the transmitter, and
you should be able to "peg" the "S" meter.
By winding a 5-turn
link of hookup wire around the coil, the transmitter can be loaded
to an antenna. No claims for transmitting distance are made for
the little unit, since this is almost entirely up to the skill of
Some tunnel diodes will not "take off"
as easily as other types. Depending on your diode, you may find
it necessary to touch the cathode terminal at some point between
the diode and coil through a small capacitor, while adjusting the
bias potentiometer. The static electricity on your body will shock-excite
the transmitter circuit and start it oscillating. Once you have
the circuit oscillating properly, you can adjust the coil for maximum
You can also
use the tunnel diode to demonstrate computer switching techniques.
You will find that at one particular setting of the bias potentiometer
the diode will switch back and forth between the peak and valley
whenever you shock-excite the anode (between the diode and potentiometer
arm). Your body's static electricity acts much the same as the information
fed to the diode in a computer.
Although the meter moves
quite slowly, the diode switches from one state to the other as
fast as a bolt of lightning. In fact, the switching characteristic
of this unique diode occurs almost at the speed of light - 186,000
miles per second! In computers, the tunnel diode is capable of making
a "decision" in less time than it takes the light to travel from
this page to your eyes!
While a tunnel diode may cost you
between $5.00 and $15.00 right now, it will last a lifetime (unless
you step on it) and can be used each time a new circuit is brought