The Tunnel Diode
Esaki invented the
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
September 1960 Popular Electronics
of Contents]People old and young enjoy waxing nostalgic about and learning some of the history of early electronics.
Popular Electronics was published from October 1954 through April 1985. All copyrights (if any) 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 tunnel diode.
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 vacuum
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. Rint
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
Fig. 5. Low internal-resistance power supply for tunnel diode circuits.
Drain through resistor RI is heavy but unavoidable due to required
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
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
3 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.
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.
Building a Transmitter.
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.
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
portion) can be replaced with supply shown in Figure 5 if desired.
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
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 the experimenter.
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 signal.
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 out.