July 1969 Electronics World
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
Electronics World, published May 1959
- December 1971. All copyrights hereby acknowledged.
A decade after
were first invented by Nobel Laureate
Leo Esaki, grand
plans for the unique device never played out. Predictions included its use for computer
solid state memories to replace
core arrays. Tunnel diodes benefitted from the aura surrounding their exploitation
of the quantum
mechanical tunnelling phenomenon, which had a futuristic ring to it. Conventional
diodes, having a relatively wide depletion region, require the current carriers (electrons
and holes) to overcome a potential hill in traveling from the valence band to the conduction
band of energies. Since high doping levels are used in the tunnel diode, a narrow depletion
region is formed at the junction. This allows electrons to "tunnel" directly through
from the valence to the conduction bands. It is this mechanism which gives rise to the
tunnel-diode characteristics, including the negative-resistance region.
This edition of Electronics World ran a series of diode articles:
Hot Carrier Diodes,
Microwave Power Diodes,
A Survey of Silicon
Junction Diodes, and
Following Esaki's announcement of the tunnel diode in 1958, expectations ran high
for its potential use in a wide variety of applications. Many people thought that the
tunnel diode would replace all magnetic cores and transistors in the digital computer.
Because of the rapid progress made in conventional discrete diodes and transistors and
the phenomenal development of integrated circuits, most of the early hopes never materialized.
Taking advantage of its good frequency response and low noise characteristics, the greatest
application of the tunnel diode today is for oscillators and amplifiers operating at
video and microwave frequencies.
The volume of sales for tunnel diodes was $4.5 million in 1968; by 1972 it is expected
to reach close to $7 million. Leading manufacturers of tunnel diodes include Centralab,
G-E, Motorola, Philco-Ford, RCA, Raytheon, Sylvania, and Texas Instruments. In quantities
from 1 to 99, prices range from $1 to $15, depending on device characteristics.
A typical tunnel diode characteristic curve and schematic symbol are shown in Fig.
1. Between peak and valley voltages VP and VV, respectively (corresponding
to peak and valley currents IP and IV), diode current decreases
with increasing diode voltage. This behavior results in the region of negative resistance
-RN, which permits the tunnel diode to be used as a switch, oscillator, or
Conventional diodes, having a relatively wide depletion region, require the current
carriers (electrons and holes) to overcome a potential hill in traveling from the valence
band to the conduction band of energies. Since high doping levels are used in the tunnel
diode, a narrow depletion region is formed at the junction. This allows electrons to
"tunnel" directly through from the valence to the conduction bands. It is this mechanism
which gives rise to the tunnel-diode characteristics, including the negative-resistance
region. For voltages exceeding the valley voltage, the tunnel diode assumes the characteristics
of the conventional p-n junction diode,
A small-signal model of the device is shown in Fig, 2. Capacitance CJ represents
junction capacitance; CP is the package capacitance. Series resistance RS
is ohmic and LS is the lead inductance. The negative resistance is denoted
by -RN. Typical values of these parameters for a tunnel diode with a peak
current rating of 10 mA are: LS ≈ 5 nH, RS ≈ 1 ohm,
-RN ≈ -25 ohms, and CJ ≈ 10-20 pF. CP depends
on the mounting and packaging.
Four basic materials used for fabricating tunnel diodes are: germanium (Ge), silicon
(Si), gallium arsenide (GaAs), and gallium antimonide (GaSb), Typical values of significant
tunnel-diode parameters are listed in the table. Besides the peak-to-valley current ratio
IP/IV and the peak and valley voltages VP and VV,
two other parameters of interest are included: the peak current-to-capacitance ratio
IP/C (where C is the sum of the junction and package capacitance) and the
resistive cut-off frequency fR0 (the frequency at which the net negative
resistance reduces to zero). These two parameters serve as figures of merit for tunnel
Tunnel diodes exhibit good resistance to radiation.
The valley current is most sensitive, with significant changes in its value occurring
in the vicinity of an integrated' flux density of 1016 neutrons/cm2.
The peak and valley voltages have negative temperature coefficients of 0.1 and 0.8 mV/°C,
respectively. Typical variation of IP and VP over a temperature
range of -55°C to 150°C is 10 percent. The valley current is temperature sensitive;
its value at 150°C may be 3 times as great as at -55°C.
Because of their low noise, gallium antimonide and germanium diodes are generally
employed for amplifiers. Although germanium exhibits somewhat greater noise than gallium
antimonide, germanium tunnel diodes are more stable with respect to temperature. Gallium
arsenide diodes exhibit the greatest power-output capabilities and are therefore invariably
used for oscillators. For high-speed switching, gallium arsenide and germanium tunnel
diodes are best. Silicon, however, would be required at elevated temperatures (up to
Referring to Fig. 1A, if the forward characteristic is altered to appear as shown
in Fig. 3A, the characteristics of the back diode are obtained. Interchanging the first
and third quadrants results in Fig. 3B. An examination of the "forward" characteristics
in quadrant I of Fig. 3B reveals that the cut-in or threshold voltage is approximately
zero volts. The new reverse characteristic is not too different from that of ordinary
diodes. At room temperature, the cut-in voltage is 0.6 and 0.2 V for silicon and germanium
p-n junction diodes, respectively. Therefore, back diodes are well suited for rectifying
very low-amplitude signals; they are also used as video detectors in microwave applications.
Some companies making back diodes are G-E, Sylvania, and Transitron. In small quantities
prices are about $3 and up.
Posted February 19, 2018