This one line (especially the part in parenthesis) hits directly on the heart of the matter: Lillienfeld likely never actually built his transistor (it would have almost certainly been impossible to build a working one due to material limitations), but he nevertheless described the concept. An invention doesn't become viable when someone thinks it up, it becomes viable when the material science is there to make it ha…
I've adopted a personal rule about "invention," loosely based on the patent system. Claiming to have had an "idea" is not enough. Ideas are a dime a dozen. But a patent requires a coherent description, novelty, and practicability. And disclosure. Reduction to practice is where I think the rubber meets the road. This is where you find out if someone actually understands the subject matter well enough to have invented…
On the other hand coming up with the idea of the solid state transistor in the mid 1920s is not as useless an invention as my 5 year old self's invention of the tv watch. On the contrary, it was a brilliant idea. There is no doubt a longer story about why it didn't go anywhere then, and the fact that it didn't go anywhere tends to negate the claim to "invention", but that's semantics. The idea itself was world-changing. I'm sure Shockley, Brattain, and Bardeen were aware of this earlier work and would have found it inspiring because if there was any way to achieve solid state transistors... (Imagine how WWII would have gone had one of the major powers had solid state transistors.)