The simplest vacuum tube, thediode(i.e.Fleming valve), was invented in 1904 byJohn Ambrose Fleming. It contains only a heated electron-emitting cathode and an anode. Electrons can flow in only one direction through the device—from the cathode to the anode. Adding one or morecontrol gridswithin the tube allows the current between the cathode and anode to be controlled by the voltage on the grids.
In the 1940s, the invention ofsemiconductor devicesmade it possible to producesolid-statedevices, which are smaller, more efficient, reliable, durable, safer, cooler, and more economical than thermionic tubes. Beginning in the mid-1960s, thermionic tubes were being replaced by thetransistor. However, thecathode-ray tube(CRT) remained the basis for television monitors andoscilloscopesuntil the early 21st century.
Thermionic tubes are still employed in some applications, such as themagnetronused in microwave ovens, certain high-frequencyamplifiers, as well as high end audio amplifiers, which many audio enthusiasts prefer for their "warmer"tube sound, and amplifiers for electric musical instruments such as guitars (for desired effects, such as "overdriving" them to achieve a certain sound or tone).
Not all electronic circuit valves / electron tubes are vacuum tubes.Gas-filled tubesare similar devices, but containing a gas, typically at low pressure, which exploit phenomena related toelectric discharge in gases, usually without a heater.