Thermionic Diodes
Thermionic diodes are thermionic-valve devices (also known as vacuum tubes, tubes, or valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope. Early examples were fairly similar in appearance to incandescent light bulbs.
In thermionic-valve diodes, a current through the heater filament indirectly heats the thermionic cathode, another internal electrode treated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals; these substances are chosen because they have a small work function. (Some valves use direct heating, in which a tungsten filament acts as both heater and cathode.) The heat causes thermionic emission of electrons into the vacuum. In forward operation, a surrounding metal electrode called the anode is positively charged so that it electrostatically attracts the emitted electrons. However, electrons are not easily released from the unheated anode surface when the voltage polarity is reversed. Hence, any reverse flow is negligible.
In a mercury-arc valve, an arc forms between a refractory conductive anode and a pool of liquid mercury acting as cathode. Such units were made with ratings up to hundreds of kilowatts, and were important in the development of HVDC power transmission. Some types of smaller thermionic rectifiers sometimes had mercury vapor fill to reduce their forward voltage drop and to increase current rating over thermionic hard-vacuum devices.
Until the development of semiconductor diodes, valve diodes were used in analog signal applications and as rectifiers in many power supplies. They rapidly ceased to be used for most purposes, an exception being some high-voltage high-current applications subject to large transient peaks, where their robustness to abuse made them the best choice. As of 2012 some enthusiasts favoured vacuum tube amplifiers for audio applications, sometimes using valve rather than semiconductor rectifiers.
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