In 1906, Lee de Forest invented the audione, later called the triode, an improvement on Fleming’s tube. Despite inventing the first triode, which acted as an amplifier and changed the face of the broadcasting industry, Lee de Forest faced many setbacks.

By this time, De Forest had focused on what we now call radio and began making improvements to the diode vacuum tubes that time available diodes. They could correct signals, but not amplify them, and were also unable to respond to small changes in electromagnetic radiation.

In 1906, De Forest proposed a three-element electronic valve similar to the two-element device used earlier, using the simple technique of adding a third electrode between the two existing electrodes.

In 1913, William D. Coolidge invented the Coolidge tube, the first practical X-ray tube. In 1920, RCA began commercial production of the first vacuum tube.