A crystal detector is an obsolete electronic component in some early 20th century radio receivers that used a piece of crystalline mineral as a detector (demodulator) to rectify the alternating current radio signal to extract the audio modulation which produced the sound in the earphones. It was the first type of semiconductor diode, and one of the first semiconductor electronic devices. The most common type was the so-called cat whisker detector, which consisted of a piece of crystalline mineral, usually galena (lead sulfide), with a fine wire touching its surface. The "asymmetric conduction" of electric current across electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystals were first used as a radio wave detector in 1894 by Jagadish Chandra Bose in his microwave experiments. and crystal detectors were first patented by Braun (1899) and Bose (1901). The crystal detector was developed into a practical radio component mainly by G. W. Pickard, who began research on detector materials in 1902 and found hundreds of substances that could be used in forming rectifying junctions. The physical principles by which they worked were not understood at the time they were used, but subsequent research into these primitive point contact semiconductor junctions in the 1930s and 1940s led to the development of modern semiconductor electronics.
The unamplified radio receivers that used crystal detectors were called crystal radios. The crystal radio was the first type of radio receiver that was used by the general public, and became the most widely used type of radio until the 1920s. It became obsolete with the development of vacuum tube receivers around 1920, but continued to be used until World War 2.
The crystal detector consisted of an electrical contact between the surface of a semiconducting crystalline mineral and either a metal or another crystal. The construction of the detector depended on the type of crystal used, particularly how much pressure on the crystal surface was needed to make the most sensitive rectifying contact. Crystals that required a light pressure like galena were used with the wire cat whisker contact; silicon was used with a heavier point contact, while carborundum required the heaviest pressure. Another category used two crystals of different minerals with their surfaces touching. Since the detector would only function when the contact was made at certain spots on the crystal surface, the contact point was almost always made adjustable. Below are the major categories of crystal detectors used during the early 20th century:
Cat whisker detector
Patented by Pickard in 1906 this was the most common type of crystal detector, mainly used with galena but also other crystals. It consisted of a pea-size piece of crystalline mineral in a metal holder, with its surface touched by a fine metal wire or needle (the "cat whisker").  The contact between the tip of the wire and the surface of the crystal formed a crude unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. The wire whisker is the anode, and the crystal is the cathode; current can flow from the wire into the crystal but not in the other direction.
Only certain sites on the crystal surface functioned as rectifying junctions. The device was very sensitive to the exact geometry and pressure of contact between wire and crystal, and the contact could be disrupted by the slightest vibration. Therefore a usable point of contact had to be found by trial and error before each use. The wire was suspended from a moveable arm and was dragged across the crystal face by the user until the device began functioning. In a crystal radio, the user would tune the radio to a strong local station if possible and then adjust the cat whisker until the station or radio noise was heard in the radio's earphones. This required some skill and a lot of patience. An alternative method of adjustment was to use a battery-operated buzzer connected to the radio's ground wire or inductively coupled to the tuning coil, to generate a test signal. The spark produced by the buzzer's contacts functioned as a weak radio transmitter whose radio waves could be received by the detector, so when a rectifying spot had been found on the crystal the buzz could be heard in the earphones, at which time the buzzer was turned off.
The parts of the cat whisker detector were:
- A crystalline mineral formed the semiconductor side of the junction. The most common crystal used was galena (lead sulfide, PbS, varieties were sold under the names "Lenzite" and "Hertzite"), a widely occurring ore of lead, although other crystalline minerals were also used, the more common ones were iron pyrite (iron sulfide, FeS2, fool's gold, also sold under the trade names "Pyron" and "Ferron"), molybdenite (molybdenum disulfide, MoS2), and cerussite (lead carbonate, PbCO3) Not all specimens of a crystal would function in a detector, often several crystal pieces had to be tried to find an active one. Galena with good detecting properties was rare and had no reliable visual characteristics distinguishing it from galena samples with poor detecting properties. A rough pebble of detecting mineral about the size of a pea was mounted in a metal cup, which formed one side of the circuit. The electrical contact between the cup and the crystal had to be good, because this contact must not act as a second rectifying junction, creating two back-to-back diodes which would prevent the device from conducting at all. To make good contact with the crystal, it was either clamped with setscrews or embedded in solder. Because the relatively high melting temperature of tin-lead solder can damage many crystals, a fusible alloy with a low melting point, well under 200 °F (93 °C), such as Wood's metal was used. One surface was left exposed to allow contact with the cat-whisker wire.
- Cat whisker
- The "cat whisker", a springy piece of thin metal wire, formed the metal side of the junction. Phosphor bronze wire of about 30 gauge was commonly used because it had the right amount of springiness. It was mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal could be probed from many directions to find the most sensitive spot. Cat whiskers in simple detectors were straight or curved, but most professional cat whiskers had a coiled section in the middle that served as a spring. The crystal required just the right gentle pressure by the wire; too much pressure caused the device to conduct in both directions. Precision detectors made for radiotelegraphy stations often used a metal needle instead of a cat's whisker, mounted on a thumbscrew-operated leaf spring to adjust the pressure applied. Gold or silver needles were used with some crystals.
Invented in 1906 by Henry H. C. Dunwoody, this consisted of a piece of silicon carbide (SiC, then trade-named carborundum), either clamped between two flat metal contacts or mounted in fusible alloy in a metal cup, with a contact made by a hardened steel point pressed firmly against it with a spring. Carborundum, an artificial product of electric furnaces produced in 1893, required a heavier pressure than the cat whisker contact. The carborundum detector was popular because its sturdy contact did not require readjustment each time it was used, like the delicate cat whisker devices. Some carborundum detectors were adjusted to a sensitive contact at the factory and then sealed and did not require adjustment by the user. It was not sensitive to vibration and so was used in shipboard wireless stations where the ship was rocked by waves, and military stations where vibration from gunfire could be expected. Another advantage was that it was tolerant of high currents, and could not be "burned out" by atmospheric electricity from the antenna. Therefore it was the most common type used in commercial radiotelegraphy stations.
Carborundum is a semiconductor with a wide band gap of 3 eV, so to make the detector more sensitive a forward bias voltage of several volts was usually applied across the junction by a battery and potentiometer. The voltage was adjusted with the potentiometer until the sound was loudest in the earphone. The bias moved the operating point to the curved "knee" of the device's current-voltage curve, which produced the largest rectified current.
Patented and first manufactured in 1906 by Pickard, this was the first type of crystal detector to be commercially produced. Silicon required more pressure than the cat whisker contact, although not as much as carborundum. A flat piece of silicon was embedded in fusible alloy in a metal cup, and a metal point, usually brass or gold, was pressed against it with a spring. The surface of the silicon was usually ground flat and polished. Silicon was also used with antimony and arsenic contacts. The silicon detector had some of the same advantages as carborundum; its firm contact could not be jarred loose by vibration, so it was used in commercial and military radiotelegraphy stations.
The "Perikon" detector, invented 1908 by Pickard consisted of two crystals in metal holders, mounted face to face with their surfaces touching, forming a crystal-to-crystal contact. One crystal was zincite (zinc oxide, ZnO), the other was a copper iron sulfide, either bornite (Cu5FeS4) or chalcopyrite (CuFeS2). In Pickard's commercial detector (see picture), multiple zincite crystals were mounted in a fusible alloy in a round cup (on right), while the chalcopyrite crystal was mounted in a cup on an adjustable arm facing it (on left). The chalcopyrite crystal was moved forward until it touched the surface of one of the zincite crystals. When a sensitive spot was located, the arm was locked in place with the setscrew. Multiple zincite pieces were provided because the fragile zincite crystal could be damaged by excessive currents and tended to "burn out" due to atmospheric electricity from the wire antenna or currents leaking into the receiver from the powerful spark transmitters used at the time. This detector was also sometimes used with a small forward bias voltage of around 0.2V from a battery to make it more sensitive.
Although the zincite-chalcopyrite "Perikon" was the most widely used crystal-to-crystal detector, other crystal pairs were also used. Zincite was also used with carbon, galena, and tellurium. Silicon was used with arsenic and antimony crystals.
How it works
The contact between two dissimilar materials at the surface of the detector's semiconducting crystal forms a crude semiconductor diode, which acts as a rectifier, conducting electric current in only one direction and resisting current flowing in the other direction. In a crystal radio, it was connected between the tuned circuit, which passed on the oscillating current induced in the antenna from the desired radio station, and the earphone. Its function was to act as a demodulator, rectifying the radio signal, converting it from alternating current to a pulsing direct current, to extract the audio signal (modulation) from the radio frequency carrier wave. The audio frequency current produced by the detector passed through the earphone causing the earphone's diaphragm to vibrate, pushing on the air to create sound waves. This diagram shows a simplified explanation of how it works:
- (A) This graph shows the amplitude modulated radio signal from the receiver's tuned circuit, which is applied as a voltage across the detector's contacts. The rapid oscillations are the radio frequency carrier wave. The audio signal (the sound) is contained in the slow variations (modulation) of the size of the waves. If this signal were applied directly to the earphone, it could not be converted to sound, because the audio excursions are the same on both sides of the axis, averaging out to zero, resulting in no net motion of the earphone's diaphragm.
- (B) This graph shows the current through the crystal detector which is applied to the earphone and bypass capacitor. The crystal conducts current in only one direction, stripping off the oscillations on one side of the signal, leaving a pulsing direct current whose amplitude does not average zero but varies with the audio signal.
- (C) This graph shows the current which actually passes through the earphone. A bypass capacitor across the earphone terminals smooths the waveform, removing the radio frequency carrier pulses, leaving the audio signal. When this varying current passes through the earphone voice coil, it creates a varying magnetic field which pulls on the earphone diaphragm, causing it to vibrate and produce sound waves.
The crystal detector was the most successful of many detector devices that were used in the early days of radio. It replaced earlier electrolytic, magnetic, and particularly coherer detectors in radio receivers around 1906. Later, when AM radio transmission was developed to transmit sound, around World War I, crystal detectors proved able to receive this transmission as well.
The "unilateral conduction" of crystals, as it was then called, was discovered by Ferdinand Braun, a German physicist, in 1874 at the University of Würzburg, before radio had been invented. Bengali-Indian scientist Jagadish Chandra Bose was the first to use a crystal to detect radio frequency waves, in his pioneering experiments with microwaves in 1894, applying for a patent on a galena detector in 1901. The crystal detector was developed as a practical device for use in wireless communication mainly by G. W. Pickard. His first detector, which used a silicon crystal, was patented in 1906. At nearly the same time, Henry Harrison Chase Dunwoody, a retired general in the U.S. Army Signal Corps, patented the silicon carbide (carborundum) detector, an artificial substance created accidentally during attempts by Edward Acheson to create diamonds. Pickard tested more than 30,000 combinations of crystal and wire contacts and developed several types of detectors that saw wide use. One variation consisted of a pair of different crystals with their faces touching, such as zincite touching bornite or chalcopyrite. Pickard named this the Perikon detector, from "PERfect pIcKard cONtact". Other detectors patented by Pickard employed the common crystal iron pyrite.
From the earliest wireless telegraphy days of radio, well into the age of commercial AM broadcasting, unamplified radio receivers were powered only by the radio energy they picked up through their antennas. By 1907 the crystal detector had replaced other detectors in the early unamplified radiotelegraphy receivers, and the crystal radio became the dominant type of radio receiver. Manufactured and homemade by the millions, it helped introduce radio to the public, contributing to the development of radio from an experimental hobby to an entertainment medium around 1920. After about 1920, receivers using crystal detectors were largely superseded by the first amplifying receivers, which used vacuum tubes. These did not require the fussy adjustments that crystals required, were more sensitive, and also were powerful enough to drive loudspeakers. Nevertheless, the expense of the early vacuum tubes and the batteries needed to run them meant that the crystal detector remained in commercial and military use for almost a decade more. However, by the late 1920s, radios using crystal detectors were relegated to use by hobbyists and youth groups and have been used by them as educational devices to the present day.
The temperamental, unreliable action of the crystal detector was a barrier to its acceptance as a standard component in commercial radio equipment and was one reason for its rapid replacement by vacuum tubes after 1920. Frederick Seitz, a later semiconductor researcher, wrote:
Such variability, bordering on what seemed the mystical, plagued the early history of crystal detectors and caused many of the vacuum tube experts of a later generation to regard the art of crystal rectification as being close to disreputable.
Historically, many minerals and compounds have been used as crystal detectors, the most important being silicon, iron pyrite ("fool's gold", iron disulfide, FeS2), galena, molybdenite (MoS2), and silicon carbide (carborundum, SiC). Some were used with gold or graphite cat whiskers. The term "cat whisker detector" has been popularly applied to any crystal detector that incorporates a small gauge, resilient, formed length of wire to contact the crystal with only a small amount of force. Another type had a crystal-to-crystal junction instead of a cat whisker, with two crystals mounted facing each other. One crystal was moved forward on an adjustable mount until the crystal faces touched. The most common of these was a zincite–chalcopyrite junction trade-named Perikon, but zincite–bornite (ZnO-Cu5FeS4), silicon–arsenic, and silicon–antimony junctions were also used. To increase sensitivity, some of these junctions such as silicon carbide were biased by connecting a battery and potentiometer across them to provide a small constant forward voltage across the junction.The goal of researchers was to find junctions that were not as sensitive to vibration and unreliable as galena and pyrite. Some of these other junctions, particularly carborundum, were stable enough that they were equipped with a more permanent spring-loaded contact rather than a cat whisker. For this reason, carborundum detectors were preferred for use in large commercial wireless stations and military and shipboard stations that were subject to vibration from waves and gunnery exercises. Another quality desired was the ability to withstand high currents without damage, because in communication stations the fragile detector junction could be "burned out" by atmospheric electric charge from the antenna or high radio frequency current leaking into the receiver from the powerful spark-gap transmitter during transmissions. Carborundum detectors, which used large-area contacts, were also particularly robust in this regard.
The oxide layers that form on many ordinary metal surfaces have semiconducting properties, and detectors for crystal radios have been improvised from a variety of everyday objects such as rusty needles and corroded pennies. The foxhole radio was a crystal radio receiver improvised by soldiers during World War II without access to conventional sets. It used a razor blade and a safety pin or pencil lead to form a demodulating junction. Much patience was required to find an active detecting site on the blade.
The point-contact semiconductor detector was subsequently resurrected around World War II because of the military requirement for microwave radar detectors. Vacuum-tube detectors do not work at microwave frequencies[why?]. The small area of the point contact minimized minority carrier storage and capacitance, making these diodes fast enough to function at radar frequencies. Silicon and germanium point-contact diodes were developed. Wartime research on p-n junctions in crystals paved the way for the invention of the point-contact transistor in 1947.
The germanium diodes that became widely available after the war proved to be as sensitive as galena and did not require any adjustment, so germanium diodes replaced crystal detectors in the few crystal radios still being made, largely putting an end to the manufacture of this antique radio component. Although crystal detectors are obsolete, modern point-contact silicon detectors are still commercially produced. Thus, the point-contact method used to make these first semiconductor diodes 100 years ago is still being used today.
- Barretter detector
- Electrolytic detector
- List of historic technological nomenclature
- Point-contact transistor
- Reginald Fessenden
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- For example, Advanced Semiconductor Inc. (North Hollywood, California, USA) is selling Si point-contact detectors which will cover from UHF (ultra high frequency) to 16 GHz.
|Wikimedia Commons has media related to Cat's-whisker detector.|
- Crystal and Solid Contact Rectifiers 1909 publication describes experiments to determine the means of rectification (PDF file).
- Radio Detector Development from 1917 The Electrical Experimenter
- The Crystal Experimenters Handbook 1922 London publication devoted to point-contact diode detectors (PDF file courtesy of Lorne Clark via earlywireless.com)
- U.S. Patent 906,991 - Oscillation detector (multiple metallic sulfide detectors), Clifford D. Babcock, 1908
- U.S. Patent 912,613 - Oscillation detector and rectifier ("plated" silicon carbide detector with DC bias), G.W. Pickard, 1909
- U.S. Patent 912,726 - Oscillation receiver (fractured surface red zinc oxide (zincite) detector), G.W. Pickard, 1909
- U.S. Patent 933,263 - Oscillation device (iron pyrite detector), G.W. Pickard, 1909
- U.S. Patent 1,118,228 - Oscillation detectors (paired dissimilar minerals), G.W. Pickard, 1914