Crystal detector
          ================
       
          From Wikipedia, the free encyclopediaJump to navigation Jump to searchThis
          article is about historical crystal detectors. For modern crystal
          detectors, see [1]Diode § Radio demodulation.[2] [2]Galena cat whisker
          detector used in early crystal radio[3] [3]Precision crystal detector
          with [4]iron pyrite crystal, used in commercial wireless stations,
          1914. The crystal is inside the metal capsule under the vertical
          needle (right). The leaf springs and thumbscrew allow fine adjustment
          of the pressure of the needle on the crystal.
       
          A crystal detector is an obsolete [5]electronic component used in some
          early 20th century [6]radio receivers that consists of a piece of
          crystalline [7]mineral which [8]rectifies the [9]alternating current
          radio signal.[1] It was employed as a [10]detector ([11]demodulator)
          to extract the audio [12]modulation signal from the modulated carrier,
          to produce the sound in the earphones.[2][3] It was the first type of
          [13]semiconductor diode,[2][4] and one of the first [14]semiconductor
          electronic devices.[5] The most common type was the so-called cat
          whisker detector, which consisted of a piece of crystalline mineral,
          usually [15]galena ([16]lead sulfide), with a fine wire touching its
          surface.[1][5][6]
       
          The "asymmetric conduction" of electric current across electrical
          contacts between a crystal and a metal was discovered in 1874 by [17]Karl
          Ferdinand Braun.[7] Crystals were first used as radio wave detectors
          in 1894 by [18]Jagadish Chandra Bose in his [19]microwave experiments.[2][8][9]
          Bose first patented a crystal detector in 1901.[10] The crystal
          detector was developed into a practical radio component mainly by [20]G.
          W. Pickard,[5][11][12] who began research on detector materials in
          1902 and found hundreds of substances that could be used in forming
          rectifying junctions.[3][13] The physical principles by which they
          worked were not understood at the time they were used,[14] but
          subsequent research into these primitive point contact [21]semiconductor
          junctions in the 1930s and 1940s led to the development of modern [22]semiconductor
          electronics.[1][5][15][16]
       
          The [23]unamplified radio receivers that used crystal detectors were
          called [24]crystal radios.[17] The crystal radio was the first type of
          radio receiver that was used by the general public,[15] and became the
          most widely used type of radio until the 1920s.[18] It became obsolete
          with the development of [25]vacuum tube receivers around 1920,[1][15]
          but continued to be used until World War II and remains a common
          educational project today thanks to its simple design.
       
          Contents
          --------
       
            * 1 How it works
       
            * 2 Types
       
                * 2.1 Cat whisker detector
       
                * 2.2 Carborundum detector
       
                * 2.3 Silicon detector
       
                * 2.4 Crystal-to-crystal detectors
       
            * 3 History
       
                * 3.1 Braun's experiments
       
                * 3.2 Bose's experiments
       
                * 3.3 Pickard: first commercial detectors
       
                * 3.4 Use during the wireless telegraphy era
       
                * 3.5 Crystodyne: negative resistance diodes
       
                * 3.6 Discovery of the light emitting diode (LED)
       
                * 3.7 Use during the broadcast era
       
                * 3.8 Development of the theory of semiconductor rectification
       
                * 3.9 The first modern diodes
       
            * 4 See also
       
            * 5 References
       
            * 6 External links
       
          How it works[[26]edit]
          ----------------------
       
          [27] [27]Diagram showing how a crystal detector works
       
          The contact between two dissimilar materials at the surface of the
          detector's semiconducting crystal forms a crude [13]semiconductor
          diode, which acts as a [8]rectifier, conducting [28]electric current
          well in only one direction and resisting current flowing in the other
          direction.[3] In a [24]crystal radio, it was connected between the
          [29]tuned circuit, which passed on the oscillating current induced in
          the [30]antenna from the desired radio station, and the earphone. Its
          function was to act as a [11]demodulator, [8]rectifying the radio
          signal, converting it from [9]alternating current to a pulsing [31]direct
          current, to extract the [32]audio signal ([12]modulation) from the
          [33]radio frequency [34]carrier wave.[3][5] An AM demodulator which
          works in this way, by rectifying the modulated carrier, is called an
          envelope detector. The [35]audio frequency current produced by the
          detector passed through the [36]earphone causing the earphone's [37]diaphragm
          to vibrate, pushing on the air to create [38]sound waves. The earphone
          was typically a piezoelectric crystal type, so sensitive that the
          radio receiver could operate without an electrical power supply, using
          only energy from the incident radio wave to drive the earphone
          directly, with no electronic amplification. This diagram shows a
          simplified explanation of how it works:[7][19][20]
       
                (A) This graph shows the [39]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
                [33]radio frequency [34]carrier wave. The [32]audio signal (the
                sound) is contained in the slow variations ([12]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, which would result 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 passes through the
                earphone. A bypass [40]capacitor across the earphone terminals,
                in combination with the intrinsic forward resistance of the
                diode, creates a [41]low-pass filter that smooths the waveform
                by removing the radio frequency carrier pulses and leaving the
                audio signal. When this varying current passes through the
                earphone piezoelectric crystal, it causes the crystal to deform
                (flex), deflecting the earphone diaphragm; the varying
                deflections of the diaphragm cause it to vibrate and produce
                sound waves ([42]acoustic waves). If instead a voice-coil type
                headphone is used, the varying current from the low-pass filter
                flows through the voice coil, generating a varying magnetic
                field which pulls and pushes the earphone diaphragm, again
                causing it to vibrate and produce sound.
       
          [43] Circuit of a simple crystal radio. The crystal detector D is
          connected between the tuned circuit L,C1 and the earphone E. C2 is the
          bypass capacitor.[44] Pictorial diagram from 1922 showing the circuit
          of a cat whisker crystal radio. This common circuit did not use a
          tuning [40]capacitor, but used the capacitance of the antenna to form
          the [29]tuned circuit with the coil.
       
          Crystal radios had no [23]amplifying components to increase the
          loudness of the radio signal; the sound power produced by the earphone
          came solely from the radio waves of the radio station being received,
          intercepted by the antenna. Therefore, the sensitivity of the detector
          was a major factor determining the sensitivity and reception range of
          the receiver, motivating much research into finding sensitive
          detectors.
       
          In addition to its main use in crystal radios, crystal detectors were
          also used as radio wave detectors in scientific experiments, in which
          the DC output current of the detector was registered by a sensitive
          [45]galvanometer, and in test instruments such as [46]wavemeters used
          to calibrate the frequency of [47]radio transmitters.[21]
       
          Types[[48]edit]
          ---------------
       
          The crystal detector consisted of an electrical contact between the
          surface of a [49]semiconducting [50]crystalline [7]mineral and either
          a metal or another crystal.[3][5] Since at the time they were
          developed no one knew how they worked, crystal detectors evolved by
          trial and error. The construction of the detector depended on the type
          of crystal used, as it was found different minerals varied in how much
          contact area and pressure on the crystal surface was needed to make a
          sensitive rectifying contact.[3][22] Crystals that required a light
          pressure like [15]galena were used with the wire cat whisker contact;
          [51]silicon was used with a heavier point contact, while [52]silicon
          carbide ([53]carborundum) could tolerate the heaviest pressure.[3][22][23]
          Another type used two crystals of different minerals with their
          surfaces touching, the most common being the "Perikon" detector. 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[[54]edit]
       
          [55] Galena cat whisker detector from a 1920s crystal radio[56] Cat
          whisker detector using iron pyrite crystal[57] Galena detector in a
          cheap 1930s crystal radio[58] Popular form in portable radios, with
          the crystal protected inside a glass tube
       
          Patented by [17]Karl Ferdinand Braun[2] and [20]Greenleaf Whittier
          Pickard[6] in 1906, this was the most common type of crystal detector,
          mainly used with [15]galena[24][25] 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").[3][5][23][26] The contact between the tip of the wire
          and the surface of the crystal formed a crude unstable point-contact
          [59]metal–semiconductor junction, forming a [60]Schottky barrier diode.[5][27]
          The wire whisker is the [61]anode, and the crystal is the [62]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.[5][22] 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.[5][7][14] Therefore, a
          usable point of contact had to be found by trial and error before each
          use.[5] The wire was suspended from a moveable arm and was dragged
          across the crystal face by the user until the device began
          functioning.[22] 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 [63]radio noise (a static hissing noise) was
          heard in the radio's earphones.[28] This required some skill and a lot
          of patience.[7] An alternative method of adjustment was to use a
          battery-operated [64]buzzer connected to the radio's ground wire or
          [65]inductively coupled to the tuning coil, to generate a test signal.[28][29]
          The spark produced by the buzzer's contacts functioned as a weak [47]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 detector consisted of two parts mounted next to each other on a
          flat nonconductive base:
       
                Crystal
       
          [66] [66]Galena crystals sold for use in crystal detectors, Poland,
          1930s 
       
                A [50]crystalline [7]mineral formed the semiconductor side of
                the junction. The most common crystal used was [15]galena ([16]lead
                sulfide, PbS, varieties were sold under the names "Lenzite"[22]
                and "Hertzite"),[5][24][25] a widely occurring ore of [67]lead,
                although other crystalline minerals were also used, the more
                common ones were [4]iron pyrite (iron sulfide, FeS2, "fool's
                gold", also sold under the trade names "Pyron"[30] and "Ferron"[22]),[3][24][26]
                [68]molybdenite ([69]molybdenum disulfide, MoS2),[22][24][26]
                and [70]cerussite ([71]lead carbonate, PbCO3)[24] Not all
                specimens of a crystal would function in a detector, often
                several crystal pieces had to be tried to find an active one.[22]
                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.[31] To make good contact with the
                crystal, it was either clamped with setscrews or embedded in
                [72]solder. Because the relatively high melting temperature of
                tin-lead [72]solder can damage many crystals, a [73]fusible
                alloy with a low melting point, well under 200 °F (93 °C), such
                as [74]Wood's metal was used.[5][22][24] 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. [75]Phosphor bronze wire of
                about 30 [76]AWG / 0.25 mm diameter was commonly used because it
                had the right amount of springiness.[28][30][32] 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
                homemade detectors usually had a simple curved shape, but most
                professional cat whiskers had a coiled section in the middle
                that served as a spring.[33] The crystal required just the right
                gentle pressure by the wire; too much pressure caused the device
                to conduct in both directions.[5] 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.
       
          Carborundum detector[[77]edit]
       
          [78] Professional carborundum detector used in radiotelegraphy
          stations[79] Carborundum detector marketed to radio hobbyists, 1911
       
          Invented in 1906 by [80]Henry H. C. Dunwoody,[34][35] this consisted
          of a piece of [52]silicon carbide (SiC, then known by the trade name
          carborundum), either clamped between two flat metal contacts,[5][22][26]
          or mounted in [73]fusible alloy in a metal cup with a contact
          consisting of a hardened steel point pressed firmly against it with a
          spring.[36] Carborundum, an artificial product of electric furnaces
          produced in 1893, required a heavier pressure than the cat whisker
          contact.[3][5][22][36] The carborundum detector was popular[24][36]
          because its sturdy contact did not require readjustment each time it
          was used, like the delicate cat whisker devices.[3][22][26] Some
          carborundum detectors were adjusted at the factory and then sealed and
          did not require adjustment by the user.[3] 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.[5][22] Another advantage was that it was
          tolerant of high currents, and could not be "burned out" by
          atmospheric electricity from the antenna.[3] Therefore, it was the
          most common type used in commercial radiotelegraphy stations.[36]
       
          Silicon carbide is a semiconductor with a wide [81]band gap of 3 eV,
          so to make the detector more sensitive a forward [82]bias voltage of
          several volts was usually applied across the junction by a battery and
          [83]potentiometer.[22][26][36][35] The voltage was adjusted with the
          potentiometer until the sound was loudest in the earphone. The bias
          moved the [84]operating point to the curved "knee" of the device's
          [85]current–voltage curve, which produced the largest rectified
          current.[22]
       
          [86] Original Pickard silicon detector 1906[87] Silicon-antimony
          detector used in naval wireless stations 1919. The silicon crystal is
          mounted on an adjustable stage that can be moved in two dimensions by
          micrometer knobs (right) to find sensitive spot.
       
          Silicon detector[[88]edit]
       
          Patented and first manufactured in 1906 by Pickard,[11][35] this was
          the first type of crystal detector to be commercially produced.[12]
          Silicon required more pressure than the cat whisker contact, although
          not as much as carborundum.[22] A flat piece of [51]silicon was
          embedded in [73]fusible alloy in a metal cup, and a metal point,
          usually [89]brass or [90]gold, was pressed against it with a spring.[26][37]
          The surface of the silicon was usually ground flat and polished.
          Silicon was also used with [91]antimony[22] and [92]arsenic[30]
          contacts. The silicon detector was popular because it had much the
          same advantages as carborundum; its firm contact could not be jarred
          loose by vibration, but it did not require a bias battery, so it saw
          wide use in commercial and military radiotelegraphy stations.[22]
       
          Crystal-to-crystal detectors[[93]edit]
       
          [94] [95] (left) "Perikon" zincite-chalcopyrite detector, ca. 1912,
          manufactured by Pickard's firm, Wireless Specialty Apparatus Co.
          (right) Another form of crystal-to-crystal contact detector, made as a
          sealed plugin unit, ca. 1919
       
          Another category was detectors which used two different crystals with
          their surfaces touching, forming a crystal-to-crystal contact.[5][26]
          The "Perikon" detector, invented 1908 by Pickard[38] was the most
          common. Perikon stood for "PERfect pIcKard cONtact".[5] It consisted
          of two crystals in metal holders, mounted face to face. One crystal
          was [96]zincite ([97]zinc oxide, ZnO), the other was a copper iron
          sulfide, either [98]bornite (Cu5FeS4) or [99]chalcopyrite (CuFeS2).[22][26]
          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.[22][36]
       
          Although the zincite-chalcopyrite "Perikon" was the most widely used
          crystal-to-crystal detector, other crystal pairs were also used.
          Zincite was used with carbon, galena, and [100]tellurium. Silicon was
          used with [92]arsenic,[30] [91]antimony[22] and [100]tellurium
          crystals.
       
          History[[101]edit]
          ------------------
       
          [102] [102]The graphic symbol used for solid-state diodes originated
          as a drawing of a point contact crystal detector.[39][[103]original
          research?]
       
          During the first three decades of radio, from 1888 to 1918, called the
          [104]wireless telegraphy or "spark" era, primitive [47]radio
          transmitters called [105]spark gap transmitters were used, which
          generated radio waves by an [106]electric spark.[17][40] These
          transmitters were unable to produce the [107]continuous sinusoidal
          waves which are used to transmit [108]audio (sound) in modern AM or FM
          radio transmission.[41] Instead spark gap transmitters transmitted
          information by [104]wireless telegraphy; the user turned the
          transmitter on and off rapidly by tapping on a [109]telegraph key,
          producing pulses of radio waves which spelled out text messages in
          [110]Morse code. Therefore, the [6]radio receivers of this era did not
          have to [111]demodulate the radio wave, extract an [32]audio signal
          from it as modern receivers do, they merely had to detect the presence
          or absence of the radio waves, to make a sound in the earphone when
          the radio wave was present to represent the "dots" and "dashes" of
          Morse code.[1] The device which did this was called a [10]detector.
          The crystal detector was the most successful of many detector devices
          invented during this era.
       
          The crystal detector evolved from an earlier device,[42] the first
          primitive radio wave detector, called a [112]coherer, developed in
          1890 by [113]Édouard Branly and used in the first radio receivers in
          1894–96 by Marconi and [114]Oliver Lodge.[5][40] Made in many forms,
          the coherer consisted of a high resistance electrical contact,
          composed of conductors touching with a thin resistive surface film,
          usually oxidation, between them.[40] Radio waves changed the
          resistance of the contact, causing it to conduct a DC current. The
          most common form consisted of a glass tube with electrodes at each
          end, containing loose metal filings in contact with the electrodes.[1][5]
          Before a radio wave was applied, this device had a high [115]electrical
          resistance, in the megohm range. When a radio wave from the antenna
          was applied across the electrodes it caused the filings to "cohere" or
          clump together and the coherer's resistance fell, causing a DC current
          from a battery to pass through it, which rang a bell or produced a
          mark on a paper tape representing the "dots" and "dashes" of Morse
          code. Most coherers had to be tapped mechanically between each pulse
          of radio waves to return them to a nonconductive state.[17][40]
       
          The coherer was a very poor detector, motivating much research to find
          better detectors.[5] It worked by complicated thin film surface
          effects, so scientists of the time didn't understand how it worked,
          except for a vague idea that radio wave detection depended on some
          mysterious property of "imperfect" electrical contacts.[5] Researchers
          investigating the effect of radio waves on various types of
          "imperfect" contacts to develop better coherers, invented crystal
          detectors.[42]
       
          Braun's experiments[[116]edit]
       
          The "unilateral conduction" of crystals was discovered by [17]Karl
          Ferdinand Braun, a German physicist, in 1874 at the [117]University of
          Würzburg.[2][8][43] He studied [118]copper pyrite (Cu5FeS4), [4]iron
          pyrite (iron sulfide, FeS2), galena (PbS) and copper antimony sulfide
          (Cu3SbS4).[44] This was before radio waves had been discovered, and
          Braun did not apply these devices practically but was interested in
          the [119]nonlinear [120]current–voltage characteristic that these
          sulfides exhibited. Graphing the current as a function of voltage
          across a contact made by a piece of mineral touched by a wire cat
          whisker, he found the result was a line that was flat for current in
          one direction but curved upward for current in the other direction,
          instead of a straight line, showing that these substances did not obey
          [121]Ohm's law. Due to this characteristic, some crystals had up to
          twice as much resistance to current in one direction as they did to
          current in the other. In 1877 and 1878 he reported further experiments
          with [122]psilomelane, (Ba,H
          2O)
          2Mn
          5O
          10. Braun did investigations which ruled out several possible causes
          of asymmetric conduction, such as [123]electrolytic action and some
          types of [124]thermoelectric effects.[44]
       
          Thirty years after these discoveries, after Bose's experiments, Braun
          began experimenting with his crystalline contacts as radio wave
          detectors.[2] In 1906 he obtained a German patent on a galena cat
          whisker detector, but was too late to obtain patents in other
          countries.
       
          Bose's experiments[[125]edit]
       
          [126] Bose's galena detector from his 1901 patent. This version was
          deliberately made to look and function like a human eyeball, with a
          lens focusing millimeter waves on the galena contact.[127] Bose's
          millimeter wave spectrometer, 1897. The galena detector is inside the
          horn antenna (F). The battery (V) creates a current through the
          detector measured by the galvanometer (G)
       
          The first person to use crystals for radio wave detection was Indian
          physicist [18]Jagadish Chandra Bose of the [128]University of Calcutta
          in his landmark 60 GHz [19]microwave optics experiments from 1894 to
          1900.[45][46] Like other scientists since Hertz, Bose was
          investigating the similarity between radio waves and light by
          duplicating classic [129]optics experiments with radio waves.[47] He
          first used a [112]coherer consisting of a steel spring pressing
          against a metal surface with a current passing through it.
          Dissatisfied with this detector, around 1897 Bose measured the change
          in resistivity of dozens of metals and metal compounds exposed to
          microwaves.[46][48] He experimented with many substances as contact
          detectors, focusing on [15]galena.
       
          His detectors consisted of a small galena crystal with a metal point
          contact pressed against it with a thumbscrew, mounted inside a closed
          [130]waveguide ending in a [131]horn antenna to collect the
          microwaves.[46] Bose passed a current from a battery through the
          crystal, and used a [45]galvanometer to measure it. When microwaves
          struck the crystal the galvanometer registered a drop in resistance of
          the detector. At the time scientists thought that radio wave detectors
          functioned by some mechanism analogous to the way the eye detected
          light, and Bose found his detector was also sensitive to visible light
          and ultraviolet, leading him to call it an artificial retina. He
          patented the detector 30 September 1901.[8][10] This is often
          considered the first patent on a semiconductor device.
       
          Pickard: first commercial detectors[[132]edit]
       
          [133] [133]"Microphone" coherer detector from 1909 similar to one
          Pickard discovered rectification with, widely used in the first
          receivers. It consists of a steel needle resting on two carbon blocks.
          A semiconducting layer of corrosion on the steel may have been
          responsible for the rectification.
       
          [20]Greenleaf Whittier Pickard may be the person most responsible for
          making the crystal detector a practical device. Pickard, an engineer
          with the American Wireless Telephone and Telegraph Co. invented the
          rectifying contact detector,[49][50] discovering [8]rectification of
          radio waves in 1902 while experimenting with a [112]coherer detector
          consisting of a steel needle resting across two carbon blocks.[12][13][50]
          On 29 May 1902 he was operating this device, listening to a
          radiotelegraphy station. Coherers required an external current source
          to operate, so he had the coherer and telephone earphone connected in
          series with a 3 cell [134]battery to provide power to operate the
          earphone. Annoyed by background "frying" noise caused by the current
          through the carbon, he reached over to cut two of the battery cells
          out of the circuit to reduce the current[12][13]
       
            The frying ceased, and the signals, though much weakened, became
            materially clearer through being freed of their background of
            microphonic noise. Glancing over at my circuit, I discovered to my
            great surprise that instead of cutting out two of the cells I had
            cut out all three; so, therefore, the telephone diaphragm was
            being operated solely by the energy of the receiver signals. A
            contact detector operating without local battery seemed so
            contrary to all my previous experience that ... I resolved at once
            to thoroughly investigate the phenomenon.[12][13]
       
          The generation of an audio signal without a DC bias battery made
          Pickard realize the device was acting as a rectifier. During the next
          four years, Pickard conducted an exhaustive search to find which
          substances formed the most sensitive detecting contacts, eventually
          testing thousands of minerals,[8] and discovered about 250 rectifying
          crystals.[5][12][13] In 1906 he obtained a sample of fused [51]silicon,
          an artificial product recently synthesized in electric furnaces, and
          it outperformed all other substances.[12][13] He patented the silicon
          detector 30 August 1906.[8][11] In 1907 he formed a company to
          manufacture his detectors, Wireless Specialty Products Co., and the
          silicon detector was the first crystal detector to be sold
          commercially.[12] Pickard went on to produce other detectors using the
          crystals he had discovered; the more popular being the [4]iron pyrite
          "Pyron" detector and the [96]zincite–[99]chalcopyrite
          crystal-to-crystal "Perikon" detector in 1908,[38] which stood for "PERfect
          pIcKard cONtact".[5]
       
          Use during the wireless telegraphy era[[135]edit]
       
          [136] [136]Marconi Type 106 crystal receiver made from 1915 to around
          1920. Detector is visible at lower right. Until the triode began to
          replace it in World War I the crystal detector was cutting-edge
          technology.
       
          [137]Guglielmo Marconi developed the first practical wireless
          telegraphy transmitters and receivers in 1896, and radio began to be
          used for communication around 1899. The coherer was used as detector
          for the first 10 years, until around 1906.[18] During the [104]wireless
          telegraphy era prior to 1920, there was virtually no [138]broadcasting;
          radio served as a point-to-point text messaging service. Until the
          [139]triode vacuum tube began to be used around [140]World War I,
          radio receivers had no [23]amplification and were powered only by the
          radio waves picked up by their antennae.[12] Long distance radio
          communication depended on high power transmitters (up to 1 MW), huge
          wire antennas, and a receiver with a sensitive detector.[12]
       
          Crystal detectors were invented by several researchers at about the
          same time.[5] Braun began to experiment with crystal detectors around
          1899,[2] around when Bose patented his galena detector.[8] Pickard
          invented his silicon detector in 1906. Also in 1906 [80]Henry Harrison
          Chase Dunwoody,[51] a retired general in the U.S. Army Signal Corps,
          patented the [52]silicon carbide ([53]carborundum) detector,[34][35]
          Braun patented a galena cat whisker detector in Germany,[52] and [141]L.
          W. Austin invented a silicon–tellurium detector.
       
          Around 1907 crystal detectors replaced the [112]coherer and [142]electrolytic
          detector to become the most widely used form of radio detector.[18][53]
          Until the triode vacuum tube began to be used during World War I,
          crystals were the best radio reception technology, used in
          sophisticated receivers in wireless telegraphy stations, as well as in
          homemade crystal radios.[54] In transoceanic radiotelegraphy stations
          elaborate inductively coupled crystal receivers fed by mile long wire
          antennas were used to receive transatlantic telegram traffic.[55] Much
          research went into finding better detectors and many types of crystals
          were tried.[31] The goal of researchers was to find rectifying
          crystals that were less fragile and sensitive to vibration than galena
          and pyrite. Another desired property was tolerance of high currents;
          many crystals would become insensitive when subjected to discharges of
          atmospheric electricity from the outdoor wire antenna, or current from
          the powerful spark transmitter leaking into the receiver. Carborundum
          proved to be the best of these;[36] it could rectify when clamped
          firmly between flat contacts. Therefore, carborundum detectors were
          used in shipboard wireless stations where waves caused the floor to
          rock, and military stations where gunfire was expected.[5][22]
       
          In 1907–1909, [143]George Washington Pierce at Harvard conducted
          research into how crystal detectors worked.[12][44] Using an [144]oscilloscope
          made with Braun's new [145]cathode ray tube, he produced the first
          pictures of the waveforms in a working detector, proving that it did
          rectify the radio wave. During this era, before modern [146]solid-state
          physics, most scientists believed that crystal detectors operated by
          some [147]thermoelectric effect.[35] Although Pierce didn't discover
          the mechanism by which it worked, he did prove that the existing
          theories were wrong; his oscilloscope waveforms showed there was no
          [148]phase delay between the voltage and current in the detector,
          ruling out thermal mechanisms. Pierce originated the name crystal
          rectifier.
       
          Between about 1905 and 1915 new types of radio transmitters were
          developed which produced [107]continuous sinusoidal waves: the [149]arc
          converter (Poulsen arc) and the [150]Alexanderson alternator. These
          slowly replaced the old [151]damped wave spark transmitters. Besides
          having a longer transmission range, these transmitters could be [152]modulated
          with an [32]audio signal to transmit sound by [39]amplitude modulation
          (AM). It was found that, unlike the coherer, the rectifying action of
          the crystal detector allowed it to [111]demodulate an AM radio signal,
          producing audio (sound).[17] Although other detectors used at the
          time, the [142]electrolytic detector, [153]Fleming valve and the
          triode could also rectify AM signals, crystals were the simplest,
          cheapest AM detector.[17] As more and more radio stations began
          experimenting with transmitting sound after World War I, a growing
          community of radio listeners built or bought crystal radios to listen
          to them.[17][56] Use continued to grow until the 1920s when vacuum
          tube radios replaced them.[17][56]
       
          Crystodyne: negative resistance diodes[[154]edit]
       
          [155] [155]Negative resistance diode [156]oscillator constructed by
          [157]Hugo Gernsback in 1924 to Losev's instructions. The zincite point
          contact diode which serves as the active device is labeled (9).
       
          Some semiconductor diodes have a property called [158]negative
          resistance which means the current through them decreases as the
          voltage increases over a part of their [120]I–V curve. This allows a
          diode, normally a [159]passive device, to function as an [23]amplifier
          or [156]oscillator. For example, when connected to a [160]resonant
          circuit and biased with a DC voltage, the negative resistance of the
          diode can cancel the positive resistance of the circuit, creating a
          circuit with zero AC resistance, in which spontaneous oscillating
          currents arise.
       
          This property was first observed in crystal detectors around 1909 by
          [161]William Henry Eccles[57][58] and Pickard.[13][59] They noticed
          that when their detectors were biased with a DC voltage to improve
          their sensitivity, they would sometimes break into spontaneous
          oscillations.[59] However these researchers just published brief
          accounts and didn't pursue the effect.
       
          The first person to exploit negative resistance practically was
          self-taught Russian physicist [162]Oleg Losev, who devoted his career
          to the study of crystal detectors. In 1922 working at the new [163]Nizhny
          Novgorod Radio Laboratory he discovered negative resistance in biased
          [96]zincite ([97]zinc oxide) point contact junctions.[59][60][61][62][63]
          He realized that amplifying crystals could be an alternative to the
          fragile, expensive, energy-wasting vacuum tube. He used biased
          negative resistance crystal junctions to build solid-state [23]amplifiers,
          [164]oscillators, and amplifying and regenerative [6]radio receivers,
          25 years before the invention of the transistor.[57][61][63][64] Later
          he even built a [165]superheterodyne receiver.[63] However his
          achievements were overlooked because of the success of vacuum tubes.
          His technology was dubbed "Crystodyne" by science publisher [157]Hugo
          Gernsback[64] one of the few people in the West who paid attention to
          it. After ten years he abandoned research into this technology and it
          was forgotten.[63]
       
          The negative resistance diode was rediscovered with the invention of
          the [166]tunnel diode in 1957, for which [167]Leo Esaki won the 1973
          [168]Nobel Prize in Physics. Today, negative resistance diodes such as
          the [169]Gunn diode and [170]IMPATT diode are widely used as [19]microwave
          oscillators in such devices as [171]radar speed guns and [172]garage
          door openers.
       
          Discovery of the light emitting diode (LED)[[173]edit]
       
          In 1907 British Marconi engineer [174]Henry Joseph Round noticed that
          when direct current was passed through a [52]silicon carbide
          (carborundum) point contact junction, a spot of greenish, bluish, or
          yellowish light was given off at the contact point.[65] Round had
          constructed a [175]light emitting diode (LED). However he just
          published a brief two paragraph note about it and did no further
          research.[66]
       
          While investigating crystal detectors in the mid-1920s at Nizhny
          Novgorod, [162]Oleg Losev independently discovered that biased
          carborundum and zincite junctions emitted light.[65] Losev was the
          first to analyze this device, investigate the source of the light,
          propose a theory of how it worked, and envision practical
          applications.[65] He published his experiments in 1927 in a Russian
          journal,[67] and the 16 papers he published on LEDs between 1924 and
          1930 constitute a comprehensive study of this device. Losev did
          extensive research into the mechanism of light emission.[63][65][68]
          He measured rates of evaporation of benzine from the crystal surface
          and found it was not accelerated when light was emitted, concluding
          that the luminescence was a "cold" light not caused by thermal
          effects.[63][68] He theorized correctly that the explanation of the
          light emission was in the new science of [176]quantum mechanics,[63]
          speculating that it was the inverse of the [177]photoelectric effect
          discovered by [178]Albert Einstein in 1905.[65][69] He wrote to
          Einstein about it, but did not receive a reply.[65][69] Losev designed
          practical carborundum electroluminescent lights, but found no one
          interested in commercially producing these weak light sources.
       
          Losev died in World War II. Due partly to the fact that his papers
          were published in Russian and German, and partly to his lack of
          reputation (his upper class birth barred him from a college education
          or career advancement in [179]Soviet society, so he never held an
          official position higher than technician) his work is not well known
          in the West.[65]
       
          Use during the broadcast era[[180]edit]
       
          [181] [181]Family listening to the first radio broadcasts on a crystal
          radio in 1922. Since crystal radios cannot drive loudspeakers they
          must share earphones.[182] After 1920, the crystal radio became a
          cheap alternative radio for youth and the poor.[183] Cartridge
          carborundum detector (top) with bias battery used in vacuum tube radio
          from 1925
       
          In the 1920s, the amplifying [139]triode [25]vacuum tube, invented in
          1907 by [184]Lee De Forest, replaced earlier technology in both radio
          transmitters and receivers.[70] AM [185]radio broadcasting
          spontaneously arose around 1920, and radio listening exploded to
          become a hugely popular pastime. The initial listening audience for
          the new broadcasting stations was probably largely owners of crystal
          radios.[17] But lacking amplification, crystal radios had to be
          listened to with earphones, and could only receive nearby local
          stations. The amplifying vacuum tube radios which began to be
          mass-produced in 1921 had greater reception range, did not require the
          fussy adjustment of a cat whisker, and produced enough audio output
          power to drive [186]loudspeakers, allowing the entire family to listen
          comfortably together, or dance to Jazz Age music.[17]
       
          So during the 1920s vacuum tube receivers replaced crystal radios in
          all except poor households.[8][17][71] Commercial and military
          wireless telegraphy stations had already switched to more sensitive
          vacuum tube receivers. Vacuum tubes temporarily put an end to crystal
          detector research. The temperamental, unreliable action of the crystal
          detector had always been a barrier to its acceptance as a standard
          component in commercial radio equipment[1] and was one reason for its
          rapid replacement. Frederick Seitz, an early semiconductor researcher,
          wrote:[14]
       
            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.
       
          The crystal radio became a cheap alternative receiver used in
          emergencies and by people who couldn't afford tube radios:[8]
          teenagers, the poor, and those in developing countries.[56] Building a
          crystal set remained a popular educational project to introduce people
          to radio, used by organizations like the [187]Boy Scouts.[17] The
          galena detector, the most widely used type among amateurs,[5] became
          virtually the only detector used in crystal radios from this point on.[24][25]
          The carborundum junction saw some use as a detector in early vacuum
          tube radios because it was more sensitive than the triode [188]grid-leak
          detector. Crystal radios were kept as emergency backup radios on
          ships. During [189]World War II in Nazi-occupied Europe the radio saw
          use as an easily constructed, easily concealed clandestine radio by
          Resistance groups.[56] After World War II, the development of modern
          semiconductor diodes finally made the galena cat whisker detector
          obsolete.[56]
       
          Development of the theory of semiconductor rectification[[190]edit]
       
          [14]Semiconductor devices like the crystal detector work by [191]quantum
          mechanical principles; their operation cannot be explained by [192]classical
          physics. The birth of [176]quantum mechanics in the 1920s was the
          necessary foundation for the development of [193]semiconductor physics
          in the 1930s, during which physicists arrived at an understanding of
          how the crystal detector worked.[72] The German word halbleiter,
          translated into English as "[194]semiconductor", was first used in
          1911 to describe substances whose conductivity fell between [195]conductors
          and [196]insulators, such as the crystals in crystal detectors.[73]
          [197]Felix Bloch and [198]Rudolf Peierls around 1930 applied quantum
          mechanics to create a theory of how electrons move through a crystal.[73]
          In 1931, [199]Alan Wilson created quantum [200]band theory which
          explains the electrical conductivity of solids.[72][73] [201]Werner
          Heisenberg conceived the idea of a [202]hole, a vacancy in a crystal
          lattice where an electron should be, which can move about the lattice
          like a positive particle; both electrons and holes conduct current in
          semiconductors.
       
          A breakthrough came when it was realized that the rectifying action of
          crystalline semiconductors was not due to the crystal alone but to the
          presence of impurity atoms in the crystal lattice.[74] In 1930 [203]Bernhard
          Gudden and Wilson established that electrical conduction in
          semiconductors was due to trace impurities in the crystal, a "pure"
          semiconductor did not act as a semiconductor, but as an [196]insulator
          (at low temperatures).[72] The maddeningly variable activity of
          different pieces of crystal when used in a detector, and the presence
          of "active sites" on the surface, was due to natural variations in the
          concentration of these impurities throughout the crystal. Nobel
          Laureate [204]Walter Brattain, coinventor of the transistor, noted:[74]
       
            At that time you could get a chunk of silicon... put a cat whisker
            down on one spot, and it would be very active and rectify very
            well in one direction. You moved it around a little bit-maybe a
            fraction, a thousandth of an inch-and you might find another
            active spot, but here it would rectify in the other direction.
       
          The "metallurgical purity" chemicals used by scientists to make
          synthetic experimental detector crystals had about 1% impurities which
          were responsible for such inconsistent results.[74] During the 1930s
          progressively better refining methods were developed,[8] allowing
          scientists to create ultrapure semiconductor crystals into which they
          introduced precisely controlled amounts of trace elements (called
          [205]doping).[74] This for the first time created semiconductor
          junctions with reliable, repeatable characteristics, allowing
          scientists to test their theories, and later making manufacture of
          modern [206]diodes possible.
       
          The theory of rectification in a metal-semiconductor junction, the
          type used in a cat whisker detector, was developed in 1938
          independently by [207]Walter Schottky[75] at [208]Siemens & Halske
          research laboratory in Germany and [209]Nevill Mott[76] at [210]Bristol
          University, UK.[72][73][74] Mott received the 1977 [168]Nobel Prize in
          Physics. In 1949 at [211]Bell Labs [212]William Shockley derived the
          [213]Shockley diode equation which gives the nonlinear exponential
          [85]current–voltage curve of a crystal detector, observed by
          scientists since Braun and Bose, which is responsible for
          rectification .[72]
       
          [214] [214]1N23 silicon diode. Grid 1/4 inch.
       
          The first modern diodes[[215]edit]
       
          The development of [19]microwave technology during the 1930s run up to
          [189]World War II for use in military [216]radar led to the
          resurrection of the point contact crystal detector.[8][50][74]
          Microwave radar receivers required a [119]nonlinear device that could
          act as a [217]mixer, to mix the incoming microwave signal with a [218]local
          oscillator signal, to shift the microwave signal down to a lower [219]intermediate
          frequency (IF) at which it could be amplified.[74] The vacuum tubes
          used as mixers at lower frequencies in [165]superheterodyne receivers
          could not function at microwave frequencies due to excessive
          capacitance. In the mid-1930s [220]George Southworth at [211]Bell Labs,
          working on this problem, bought an old cat whisker detector and found
          it worked at microwave frequencies.[8][74] [221]Hans Hollmann in
          Germany made the same discovery.[8] The [222]MIT Radiation Laboratory
          launched a project to develop microwave detector diodes, focusing on
          silicon, which had the best detecting properties.[8] By about 1942
          point-contact silicon crystal detectors for radar receivers such as
          the 1N21 and 1N23 were being mass-produced, consisting of a slice of
          [223]boron-doped silicon crystal with a [224]tungsten wire point
          pressed firmly against it. The cat whisker contact did not require
          adjustment, and these were sealed units. A second parallel development
          program at [225]Purdue University produced [226]germanium diodes.[8]
          Such [227]point-contact diodes are still being manufactured, and may
          be considered the first modern diodes.
       
          After the war, [228]germanium diodes replaced galena cat whisker
          detectors in the few crystal radios being made. Germanium diodes are
          more sensitive than silicon diodes as detectors, because germanium has
          a lower forward voltage drop than silicon (0.4 vs 0.7 volts). Today a
          few galena cat whisker detectors are still being made, but only for
          antique replica crystal radios or devices for science education.
       
          See also[[229]edit]
          -------------------
       
            * [112]Coherer
       
            * [230]Barretter detector
       
            * [142]Electrolytic detector
       
            * [231]Magnetic detector
       
            * [232]List of historic technological nomenclature
       
            * [233]Point-contact transistor
       
            * [234]Reginald Fessenden
       
          References[[235]edit]
          ---------------------
       
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          Phillips, Vivian J. (1980). [330]Early Radio Wave Detectors. London:
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            * ^ The 1918 edition of the US Navy's manual of radio stated: "There
              are two types of detectors now in use: the Audion [triode] and the
              crystal or rectifying detector. Coherers and microphones [another
              type of coherer detector] are practically obsolete... but the use
              of Audions...is increasing."
       
          Robison, Samuel Shelburne (1918). [360]Manual of Wireless Telegraphy
          for the Use of Naval Electricians, 4th Ed. Washington DC: United
          States Naval Institute. p. 156.
       
            * ^ The 1920 "British Admiralty Handbook of Wireless Telegraphy"
              stated that: "Crystal detectors are being replaced by [triode]
              valve detectors which are more stable, easier to adjust, and
              generally more satisfactory". The 1925 edition said valves were "replacing
              the crystal for all ordinary purposes"
       
          Phillips, Vivian J. (1980). [330]Early Radio Wave Detectors. London:
          Institute of Electrical Engineers. pp. [330]212. [237]ISBN [302]978-0906048245.
       
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            * ^ a b c d e f g h [364]Michael Riordan, Lillian Hoddeson (1998)
              Crystal Fire: The Invention of the Transistor and the Birth of the
              Information Age, p. 89-93
       
            * ^ Schottky, W. "Halbleitertheorie der Sperrsschicht."
              Naturwissenschaften Vol. 26 (1938) pp. 843. Abstract in English as
              "Semiconductor Theory of the Blocking Layer" in Sze, S.M.
              Semiconductor Devices: Pioneering Papers. (World Scientific
              Publishing Co., 1991) pp. 381
       
            * ^
       
          Mott, Neville F. (1 May 1939). [365]"The theory of crystal rectifiers".
          Proceedings of the Royal Society of London, Series A. 171 (944):
          27–38. [254]doi:[366]10.1098/rspa.1939.0051. [367]JSTOR [368]97313.
          Retrieved 3 August 2018. reprinted in Alexandrov, A. S. (1995). [369]Sir
          Neville Mott: 65 Years in Physics. World Scientific. pp. 153–179.
          [237]ISBN [370]978-9810222529.
       
          External links[[371]edit]
          -------------------------
       
          Wikimedia Commons has media related to [372]Cat's-whisker detector.
       
            * [373]Crystal and Solid Contact Rectifiers 1909 publication
              describes experiments to determine the means of rectification ([374]PDF
              file).
       
            * [375]Radio Detector Development from 1917 [376]The Electrical
              Experimenter
       
            * [377]The Crystal Experimenters Handbook 1922 London publication
              devoted to point-contact diode detectors (PDF file courtesy of
              Lorne Clark via earlywireless.com)
       
            * 
       
          Gardner, Arthur C. (August 1945). [378]"Rectifying Crystals" (PDF).
          Radio. 29 (8): 48–50, 68–69.
       
          Patents
       
            * [379]U.S. Patent 906,991 - Oscillation detector (multiple metallic
              sulfide detectors), Clifford D. Babcock, 1908
       
            * [380]U.S. Patent 912,613 - Oscillation detector and rectifier
              ("plated" silicon carbide detector with DC bias), G.W. Pickard,
              1909
       
            * [299]U.S. Patent 912,726 - Oscillation receiver (fractured surface
              red zinc oxide (zincite) detector), G.W. Pickard, 1909
       
            * [381]U.S. Patent 933,263 - Oscillation device (iron pyrite
              detector), G.W. Pickard, 1909
       
            * [382]U.S. Patent 1,118,228 - Oscillation detectors (paired
              dissimilar minerals), G.W. Pickard, 1914
       
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