Thermionic Devices

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Today, electronic equipment is based almost entirely on solid-state, semiconductor devices, such as diodes, transistors and integrated circuits1, but it was not like that in the past. The electronics industry was founded on devices known as thermionic valves2, and with it grew the radio and television industries. For example, the BBC produced the first television broadcast3 in 1929, fully 18 years before Shockley, Brattain and Bardeen constructed the first practical transistor4. This entry gives the Early History of the Thermionic Valve and some of its later developments, finally giving some of the applications for which thermionic devices still dominate.


This history is really rather remarkable, when one considers that Sir Joseph John Thomson did not discover the electron until 1915, and yet valves had been developed to make electronic amplifiers by 1911!


The Physics and Early History


Thermal random motion can impart sufficient energy to an electron that it can escape from a conductor. This effect is not normally noticeable, for two principal reasons :-


  1. The effect is strongly temperature dependent, and requires high temperatures in order that emission becomes a probable event


  2. If a conductor loses electrons, which are negatively charged, it then becomes positively charged, and so attracts any free electrons in the vicinity, including those which get emitted.


although the effect is sometimes useful as a thermoelectric means of generation. It works if the energy of the emitted electrons is high enough to overcome the electrostatic counter-forces.


In Britain, as early as 1873, Professor Guthrie conducted experiments which showed that, in a vacuum, an electrically charged red hot iron sphere lost that charge if it was negative, but did not do so if the charge was positive. This observation of thermionic emission was almost certainly not understood at the time.

Thomas Alva Edison (1847-1931) was greatly interested in electric lighting, which was in its infancy in 1883 when he discovered 'Etheric Force', now known as the 'Edison effect'. He found that an electrically isolated plate in the vicinity of the filament of an electric lamp, which he called a 'damper', could be used to vary the current in that filament. Edison was granted US Patent No 307,031 for the device in that same year, but thereafter he lost interest in it - he hoped that it would prevent gradual blackening of the glass experienced with those early, carbon filament lamps.


Edison knew that current would flow from the plate to the filament when the plate was positive (with respect to the filament), and no current flowed when the plate was negative. He was intrigued by this - but could find no practical use for it. He demonstated it widely, in particular to Professor Sir John Ambrose Fleming (1849-1945), professor of electrical engineering at University College, London.


The Thermionic Diode


On 16 November 1904, Fleming applied for the British patent of the thermionic diode, which he called an 'Oscillation Valve', and the application was granted on 13 November 1905, as British Patent No. 803,684. The device was intended to be used as a detector of radio frequency waves, but it was not a commercial success. Primitive point contact semiconductor diodes were more sensitive, and much cheaper to manufacture. The diode is of historic importance, because all other thermionic devices are elaborations of this basic device. There was also a time at which the thermionic diode was sufficiently well developed, and cheap5, that all other forms of detector were temporarily eclipsed.


The basic diode comprises two electrodes (the origin of its name), which are


  • The cathode, which must be heated to a high temperature. A filament, comprising a fine wire, is heated to a high temperature by the passage of current through it. This filament may be used as the cathode, or may be surrounded by an electrode which is the cathode, which is electrically separate from but in close thermal contact with the filament. The cathode is very hot - anything from a dull red heat to a bright white heat - say 800°C to 2000°C


  • The anode, which is not intentionally heated, which is a plate6 in close proximity to, but electrically isolated from, the cathode.


The entire electrode assembly must be kept in a high vacuum, with the electrodes isolated from one another. Typically, this meant they were housed in a glass container - called the envelope - and so were known to the electronics trade as 'bottles' in the UK, or 'tubes' in the US7.


The Thermionic Triode


The next development, and by far the most important, was the addition of a third electrode, taking the form of a grid between the anode (or plate) and cathode. When the anode is held at a positive potential with respect to the cathode, electrons emitted from the cathode can pass through the grid as they flow towards the anode. This device, originally called an 'Audion', was invented by Lee de Forest and patented (in the US) on


All thermionic devices are elaborations of one basic device – the thermionic diode – which uses the fact that a metallic conductor will emit electrons when sufficiently heated.


Thermionic devices used to be many and varied, but all suffer from gradual deterioration when in use, and sometimes abrupt failure too. They have been almost completely superceded, although a few devices are still in common use


  • The cathode ray tube (or CRT) is still widely used as a television and computer display device - but it can only be a matter of time before these disappear, superceded by various flat screen technologies.


  • The magnetron remains dominant for the generation of medium and high powers at very high but fixed frequencies, so practically every microwave oven contains them.


  • At high frequency and very high power, thermionic devices still have enough advantages to be used in the high power parts of some broadcast and radar transmitters.


  • Collidge tubes are usually used to generate the X-rays used in both medical imaging and industrial measurement applications.

1
commonly referred to as 'chips'
2
vacuum tubes or electronic tubes outside the UK
3
an experimental broadcast, with 30 line definition
4
a germanium point-contact type
5
especially if parts of the ubiquitous 'double diode triode'
6
in the US it is usually called the 'plate', whereas in the UK it is usually called the 'anode'
7
and in the UK, a 'tube' meant a cathode ray tube

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