Field Effect Transistor (FET) - Invention | Basic/Original/First Patent

The first solid state replacement for vacuum triode for amplifying electric current was patented by Julius Edgar Lilenfeild in the US on Oct 8, 1926. The Patent US1745175 was issued on Jan 28, 1930. The patent can be considered as the basic patent or original patent for field effect transistor (FET). 

It describes a 'method of and apparatus for controlling the flow of an electric current between two terminals of an electrically conducting solid by establishing a third potential between said terminals; and is particularly adaptable to the amplification of oscillating currents such as prevail, for example, in radio communication.'