The search for a reliable solid-state amplifier was envisioned earlier around 1920.
It is interesting to note that more common attempts were based on the field-effect principle, most likely because its concept was akin to the grid-control action of the thermionic valves, i.e. crystal device controlled by voltage instead of current.
In 1920, based in his pioneering researches the German physicist working in the USA, Julius E. Lilienfeld managed to obtein several patents for an amplifier design based on Copper Sulfide.

Reproduction of the patent nº 1,745,175, granted to the physicist J.E. Lilienfeld by the US Patent Office in January 1930, describing an early method and a solid-state apparates for controlling electric current later on invented by Bell Laboratories under the name of transistor.

However, since his experiments were conducted in near obscurity he was unable to draw serious attention to it. Around 1935 in England, one of the first patents was issued to the German inventor Oscar Heil for a field -effect crystal amplifier.
Basically its concept used a control electrode to regulate the current flow through a thin semiconductor layer made with several types of materials like Copper Oxide, Vanadium Pentoxide, Tellurium and Iodine. In reality the Heil proposal for such a kind of crystal amplifier was the forerunner of the insulated-gate field effect transistor, since the control electrode was isolated from the semiconductor substrate.
In later thirties, early researches in photoelectricity started by the german physicist R.W. Pohl gave birth to a slow-acting triode relied on a wire grid to control the flow of elctrons through a heated Potassium Bromide crystal.
Even considering it was just a laboratory work nonetheless it proved its theoretical feasibility to obtain signal amplification through a field-effect solid-state device.
As aforeseen, the researches to obtain the first solid-state amplifier operating by the field-effect principle used modulation current flowing through a chunck of semiconductor material by injection of chargers carriers through a gate or grid, which should be isolated from the slab.
However, the injected charge carriers should not affect the flow of current through the semiconductor slab because they got trapped in the material's surface. It is interesting to note that although the simplicity of the field-effect solid-state amplifier's operating concept, due to the misundersanding of the behavior of the layer formation, later explained by Bardeen, through his "theory of surface state", its practical development was destine to come only after the invention of a far complex semiconductor device, the bipolar transitor.
In the later fifties semiconductor associated industries had a substantialy expanse of the business. The perenial saga for technically improve, high performance and affordable cost semiconductor devices led the engineers to revive the field-effect transitor researches considering mainly its simple concept and low operation consumption.

To the left " FETRON" manufactured by" Teledyne" ; right the HIN or " Hybrid Integrated Network" , to left the " FETRON" a solid state component, however manufactured for " Western Electric".

In 1958, Stanislas Teszner, a polish scientist, working for a subsidiary of General Electric in France finally developed the first field-effect transistor without an isulated gate, made in Germanium alloy for operation in high frequency range and sold under the trade name of TECNITRON.
In the USA the company Teledyne made the first junction field-effect transitor in 1960. Later in 1967, the company launched in the market an interesting linear semiconductor system using this type of transitor that was sold under the tradename of FETRON.
In reality the FETRON comprised a set of junction-field-effect device cascode-connected. It was orginally developed to replace the enormous quantities of thermionic valves type pentodes and tetrodes used in the American telephone system due to their several operational disadvantages when compared with TRANSISTORS.
It can be seen that the first types junction-field-effect devices used the depletion region of a reverse-biased pn junction to control the effective cross section and so the condutivity of a crystal substrate.
Working together in the researche laboratories of RCA, Steve Hofstein and Frederic Heiman, using the new and updated Planar technology developed by Fairchild, they introduced a modification in the configuration of the junction-field-effect transistor.
Thus, a metal gate, insulated from the Silicon material by a layer of Silicon Oxide, replaced the reverse-biased junction control structure of the junction-field-effect transistor.
Finally the engineers succceded in building a reliable field-effect transitor with insulated gate according to the original concept proposed by Heil in 1935, giving birth to the MOSFET semiconductor, an acronym for metal-oxide-semiconductor field-effect transitor.
The MOS technology was a big step in the semiconductor manufacturing as through the same it was possible to build in a very small area in the crystal matrix several types of components like diodes, transistors, led to the new field of the microelectronics.