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Dioda térmionik jeung padetan (solid state) diwangun sacara paralél. Prinsip oprasi dioda térmionik kapanggih ku Frederick Guthrie taun 1873 . Prinsip oprasi dioda kristal kapanggih taun 1874 ku élmuwan Jérman, Karl Ferdinand Braun  .

Thermionic diode principles were rediscovered by Thomas Edison on February 13, 1880 and he took out a patent in 1883 (), but developed the idéa no further. Braun patented the crystal rectifier in 1899 . The first radio receiver using a crystal diode was built around 1900 by Greenleaf Whittier Pickard. The first thermionic diode was patented in Britain by John Ambrose Fleming (scientific adviser to the Marconi Company and former Edison employee ) on November 16, 1904 ( in November 1905). Pickard received a patent for a silicon crystal detector on November 20, 1906  ().

At the time of their invention such devices were known as rectifiers. In 1919 William Henry Eccles coined the term diode from Greek roots; di méans 'two', and ode (from odos) méans 'path'.

Diod térmionik atawa ngandung gas

Thermionic diodes are vacuum tube devices (also known as thermionic valves), which are arrangements of electrodes surrounded by a vacuum within a glass envelope, similar in appéarance to incandescent light bulbs.

In vacuum tube diodes, a current is passed through the cathode, a filament tréated with a mixture of barium and strontium oxides, which are oxides of alkaline earth metals. The current héats the filament, causing thermionic emission of electrons into the vacuum envelope. 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 éasily reléased from the unhéated anode surface when the voltage polarity is reversed and hence any reverse flow is a very tiny current.

For much of the 20th century vacuum tube diodes were used in analog signal applications, and as rectifiers in power supplies. Today, tube diodes are only used in niche applications, such as rectifiers in tube guitar and hi-fi amplifiers, and specialized high-voltage equipment.