Hi,
Syl wrote:

You're right. It was Lilienfeld, 1924. Here's a link
with a copy of his 1930 solid state amplifier patent:
http://www.todaysengineer.org/May03/history.asp

That's a good reference. There was also an article by J.B. Johnson
(who knew a bit about solid-state physics) in Physics Today, May 1964.
According to Johnson, Lilienfeld's design could never have worked,
because of the materials specified (not the purity, but the materials
themselves). Lilienfeld made more substantial contributions to
field-emission theory. I used to correspond with a man who worked
under Lilienfeld around 1930 at Ergon Research Lab, Malden, Mass, a
spinoff of Amrad. He even sent me a couple of snapshots of the
Doctor. That work, and much of Lilienfeld's later effort, was on
capacitor electrolytes.

73, Alan