Yes, FET is a Scrabble word!

Letter values in Scrabble US: F 4, E 1, T 1.

Is it a valid word?

FET is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a transistor in which most current flows in a channel whose effective resistance can be controlled by a transverse electric field

Also: field-effect transistor.