The Tunnel Diode
September 1960 Popular Electronics
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.
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. As time permits, I will be glad to scan articles
for you. All copyrights (if any) are hereby acknowledged.
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.
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.
Tunnel Diode Theory.
the term negative resistance and what causes it, let's study a more familiar object - a tetrode vacuum tube.
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 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 above.
Figure 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.
Present-day 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.
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.
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.
supply (shaded portion) can be replaced with supply shown in Figure 5 if desired.
Note 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.
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
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
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.