March 1946 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Crystal diodes have been used as
detectors in radio circuits since the 1910s. Originally, the rectification process
was effected via a point contact "whisker" (aka "cats whisker") pressed against
the crystal's face. It was not mechanically rugged (vibration could cause erratic
operation), was sensitive to heat and humidity (if not contained in a hermetic case),
and operated at fairly low frequencies. Vacuum tube diodes provided some improvement,
but were still limited to operation in the lower hundreds of MHz. Once germanium
and silicon crystals became available, operational frequencies climbed into the
upper MHz to lower GHz realm, even though, as shown in this 1946 Radio-Craft
article, the diodes were of the point contact type. PN junctions at those frequencies
were still a few years off. Their smaller size and construction largely mitigated
the environmental issues of the early types. The 1950s- vintage
S-band radar (and
maybe X-band, but I don't recall for certain) I worked on in the USAF used the kind
of point contact crystal diode shown here.
H.F. Crystal Diodes
Fig. 1 - Modern crystal diode, cross section.
The limitations of conventional and even of special types of vacuum-tube diodes
make them unusable at the high frequencies used in the most recent radar equipment.
It was therefore necessary to develop a new kind of detector for use in the signal
frequency sections of u.h.f. and s.h.f. receivers. The answer was found in the crystal
Just a few years ago the crystal detector belonged to the archaeology of radio
communications. With the discovery of the remarkable properties of hollow wave guides
it was resurrected for use in measuring instruments. As development progressed from
the u.h.f. into the s.h.f. ranges, the crystal was pressed into service as a circuit
Considerable research and development work was necessary, however, before these
units became entirely satisfactory. Photo A shows a typical crystal detector of
the first World War period (1919), together with a modern Sylvania 1N21B crystal
Crystal Diode Theory
The present concept of the rectification process is based on quantum mechanics
and the modern theory of solids. Without going into detail we may describe the process
briefly as follows: When two metals are brought together an electrical equilibrium
exists at the point of contact; electrons flow in one direction as readily as in
the other. However, when a metal is brought into contact with a certain class of
materials known as semiconductors equilibrium is not established, electrons flow
more easily in one direction than the other, and a potential difference exists between
the two materials. When an external electric potential is applied to this junction,
the energy either adds to or subtracts from the potential energy of the semiconductor
at the junction. Assuming an initial energy difference between the metal and the
semiconductor, we see that the effect of the external potential will be to either
increase or decrease the total energy difference between the two materials, according
to the polarity of the external potential. The effective resistance to the transfer
of electrons thus varies with the polarity of the applied potential and rectification
Photo A - Crystals of World War I and II.
Photo B - The 1N34, one of the latest of the crystal diodes,
rectifies currents of 50 ma.
Photos courtesy Sylvania Electric Products, Inc.
Manufacture of crystals has been developed to a state of precise process control.
Batches of the extremely pure crystalline material, such as silicon or germanium,
are melted in electric furnaces together with a doping ingredient. In an ordinary
chemically pure semiconductor the resistance to electron flow is high in either
direction, but by the addition of controlled quantities of other elements the conductivity
can be increased. The atoms of these doping elements alter the electronic structure
of the semiconductor and reduce the initial potential difference between the metal
and the semiconductors.
The melt is cast in ingots. These are large extent by proper attention to the
sliced into wafers and the surfaces polished to optical flatness. After fragmentation,
the minute crystals are mounted by a special soldering technique which avoids rectification
at the mount. In the final assembly, an accurately pointed tungsten wire spring
is brought into contact with the crystal and adjusted for pressure and sensitivity.
Fig. 1 shows the cross section of a crystal diode of the 1N21B type.
The principal wartime use of crystal diodes was as first detectors in u.h.f.
and s.h.f. receivers. At these frequencies conventional vacuum tubes are limited
by electron transit-time effects.
Velocity-modulated tubes, such as Klystrons, might be used were it not for the
extremely high thermal noise which they produce. In a crystal diode the transit-time
effect is not a factor. The noise figures are very low and the limiting-feature
is the interelectrode capacitance, which maybe controlled to a large extent by proper
geometry of the holder or mount employed.
Crystal diodes are ruggedly constructed. Their resistance to vibration and shock
is comparable to that of an ordinary receiving tube. They can tolerate temperature
extremes from -40° to +70°C. Preliminary tests have shown that the units
will perform satisfactorily for a period of many years without appreciable deterioration.
The principal precaution is to see that the unit is not subjected to high electrostatic
or magnetic fields. Another precaution is that before inserting a crystal in its
holder, the operator should ground himself by touching the apparatus. In this way
one may eliminate the danger of a high surge current due to a potential difference
between the operator and the set.
High Currents and Voltages
The silicon crystal diodes developed for radar use were designed to operate at
1 or 2 ma. More recent developments have produced a germanium crystal diode, the
type 1N34, which is electrically much more rugged than its predecessors, and which
is useful in many applications up to 500 Mc. The peak inverse anode voltage is 50
volts. It is rated at an average anode current of 22.5 ma and will withstand peak
currents of 60 ma or surges even up to 200 ma.
Fig. 2. Forward and reverse currents, 1N34.
Table I - Properties of Crystal Diodes.
Small interelectrode capacitances and the ability to work into a low resistive
load with high efficiency are two requirements imposed on detectors for use in frequency
modulation and television equipment. The type 1N34 (Photo B) admirably fulfills
these requirements. The cathode-to-anode capacitance of a unit mounted in place
is of the order of 3 μμf. This is several times lower than the value of a conventional
vacuum-tube diode when measured under similar conditions.
The type 1N34 is manufactured with pigtail leads and can therefore be readily
soldered directly into the circuit. This economy of weight and size and the absence
of any heater supply suggests their use in most portable and airborne equipment.
The static characteristics of the type 1N34 are shown in Fig. 2. These characteristics
may be utilized for many applications. In addition to use as detectors, some other
uses which suggest themselves are modulators of all descriptions, voltage regulators,
low frequency oscillators, d.c. restorers and polarizing devices. In meter and instrument
work they may be used as rectifiers and in probes of vacuum tube voltmeters. In
general, the type 1N34 is particularly useful with low impedance loads such as are
exemplified by wide band television and FM circuits.
Eighteen Types Listed
The table shows the principal characteristics of the most commonly used crystals.
Although some eighteen types are listed by RMA and JAN specifications, many are
already obsolete or have been made only in limited quantities for special applications.
Those listed in the table are the types in most common demand today.
The crystal, for many years the play-thing of boys building their first radio,
has again come into its own. The increasing use of u.h.f. will cause it to grow
further in importance.
Posted May 17, 2021