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 electrical contacts between a crystal and a metal was discovered in 1874 by Karl Ferdinand Braun. Crystal detectors were first used to receive radio waves in 1898 by Braun and 1894 by Jagadish Chandra Bose in his microwave experiments. 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, 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 develops direct current that carries the audio frequencies that were originally carried on the alternating current radio frequency signal.
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.
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.
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. The crystal radio was the most widely used of these. 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 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.
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 silicon detector was the first crystal detector type that widely took the place of coherers and electrolytic detectors in wireless receivers. A much larger metal point size and greater force is usually used with the silicon crystal detector than with some other minerals. The silicon detector is more sensitive than the carborundum detector, but less sensitive than the galena detector.
The junction consists of silicon on one side and the rounded or pointed end of a metal cylinder or screw on the other. It is preferred that the contact point be of platinum. In a common implementation of this detector, the crystal is placed, without fastening, upon a flat metal surface that is centered under the rounded or pointed contact. The crystal may be readily re-positioned under the point by sliding on the flat surface. Better sensitivity is obtained if the crystal is set into low melting point alloy to form the non-rectifying connection to the crystal.
In many implementations, the silicon crystal is polished to a smooth, flat face on the side that the adjustable point will contact.
The pointed contact is brought to bear against the surface of the silicon crystal while the operator listens for the desired sound from the headphones. Different locations on the surface of the crystal are tried until one is found that produces the desired sound.
The physical construction of the galena detector commonly includes a mounting base of insulating material, two binding posts or plugs for electrical connection, a metal cup that holds a galena crystal, and a metal bracket or hardware that retains a moveable metal structure equipped with a knob and a thin wire to contact the crystal with controllable spring pressure.
A galena mineral crystal forms the semiconductor side of the junction. Galena (PbS, lead sulfide), is a naturally occurring ore of lead. It is a semiconductor with a small bandgap of about 0.4 eV and is used without treatment directly as it is mined. However, not all galena crystals would function in a detector; 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 is mounted in a metal cup, which forms one side of the circuit. The electrical contact between the cup and the crystal has to be good, because this contact must not act as a second rectifying junction, which will prevent the device from functioning. To make good contact with the crystal, it is either clamped with setscrews or mounted in low melting point alloy metal. Because the relatively high melting temperature of tin–lead solder can damage many crystals, a low-melting-point (well under 200 °F (93 °C)) alloy such as Wood's metal is used. One surface is left exposed to allow contact with the cat whisker.
The "cat whisker" (also "catwhisker" or "catswhisker"), a springy piece of thin metal wire, forms the metal side of the junction. Phosphor bronze wire of about 30 gauge is commonly used because it has the optimal amount of springiness. It is mounted on an adjustable arm with an insulated handle so that the entire exposed surface of the crystal can be probed from many directions to find the most sensitive spot. Cat whiskers in simple detectors may be straight or curved, but most manufactured cat whiskers include a coiled section in the middle that serves as a spring. The crystal requires just the right gentle pressure by the wire; too much pressure causes the device to conduct in both directions.
The tip of the wire contacting the surface of the crystal forms an unstable point-contact metal–semiconductor junction, forming a Schottky barrier diode. This junction conducts electric current in only one direction and resists current flowing in the other direction. In a crystal radio, its function is to rectify 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 metal whisker is the anode, and the crystal is the cathode; current flows from the whisker into the crystal but not in the other direction.
Only certain sites on the crystal surface function as rectifying junctions. The device is very sensitive to the exact geometry and pressure of contact between wire and crystal. It is therefore made adjustable, and a usable point of contact is found by trial and error before each use. The wire is suspended from a moveable arm and is dragged across the crystal face by the operator until the device begins functioning. In a crystal radio, the operator tunes the radio to a strong local station if possible and then adjusts the cat whisker until the station or static is heard in the radio's earphones. This requires some skill and a great deal of patience; even then, a good contact is easily lost by the slightest vibration. An alternative method of adjustment is to use a battery-operated buzzer to generate a test signal. The spark produced by the buzzer's contacts functions as a weak radio transmitter whose radio waves are received by the detector. When a rectifying spot has been found on the crystal, the buzz is heard in the earphones. The buzzer is then turned off.
- Barretter detector
- Electrolytic detector
- List of historic technological nomenclature
- Point-contact transistor
- Reginald Fessenden
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- although at the microwave frequencies he used these detectors did not function as rectifying semiconductor diodes like later crystal detectors, but as a thermal detector called a bolometer. Lee, Thomas H. (2004). Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits, Vol. 1. Cambridge University Press. pp. 4–5. ISBN 0521835267.
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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.
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|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