Wikipedia
The Fetron was a range of solid-state plug-compatible replacements for vacuum tubes (valves).
Fetrons were manufactured by Teledyne Semiconductor from 1967; primarily as a low-maintenance and low-power swap-in to replace vacuum tubes which were becoming increasingly obsolete and difficult to source with the widespread use of solid-state electronics. They were used in large numbers in telephone exchanges. Vacuum tubes use significant amounts of power to maintain operating temperature, and large installations had substantial air-conditioning requirements; replacing them with fetrons was expected to reducing cooling and maintenance costs.
A typical fetron consisted of a cascade configured pair of JFET transistors, some simple RC networks to control the devices characteristics, and a fuse. The device was mounted in a metal tube with a base which has the same pin-out as the vacuum tube that it replaced.