After the development by Lee de Forest of the triode vacuum tube in 1906, it was realized that the upper frequency at which the device could be used was limited by the spacing between internal components.
•
In 1920, Heinrich Barkhausen and Karl Kurz at the Technische Hochschule in Dresden, Germany used the velocity modulation theory in developing the retarded-field triode that could provide UHF operation.
Based on the West Coast, he was in direct competition with Lee de Forest, a leading manufacturer of triodes sold under the Audion brand.
TRIAC (triode for alternating current), an electronics component
In 1937, Alan Blumlein originated a configuration between a "triode strapped" tetrode and normal tetrode, that connects the extra grid of a tetrode to a tap from the OPT part way between the anode voltage and the supply voltage.