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.
[1][2] 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 reduce cooling and maintenance costs.
[3] A typical fetron consisted of a cascode configured pair of JFETs, some simple RC networks to control the device characteristics, and a fuse.
Western Electric produced the conceptually similar Hybrid Integrated Network devices, which were used in their own telephone equipment.