Triode were invented, Philipp Lenard used the principle of grid control while conducting photoelectric experiments in 1902. The first vacuum tube used in radio was the thermionic diode or Fleming valve, invented by John Ambrose Fleming in 1904 as a detector for radio receivers. It was an evacuated glass bulb containing two electrodes, a heated filament and a plate (anode). Triodes came about in 1906 when American engineer Lee De Forest and Austrian physicist Robert von Lieben independently patented tubes that added a third electrode, a grid, between the filament and plate to control current. Von Lieben's partially-evacuated three-element tube, patented
Triode in mass-marketed consumer electronics, having been overtaken by less expensive transistor-based solid-state devices. However, more recently, vacuum tubes have been making somewhat of a comeback. Triodes continue to be used in certain high-power RF amplifiers and transmitters. While proponents of vacuum tubes claim their superiority in areas such as high-end and professional audio applications, empirical evidence shows that the solid-state MOSFET is virtually identical in performance. In triode datasheets, characteristics linking the anode current (I) to anode voltage (V) and grid voltage (V) are usually given. From here, a circuit designer can choose the operating point of the particular triode.