Another link to some interesting experiments on building your own thin-film transistors.

And a Quarter Gets You Coffee » Homemade Thin-Film Transistor Experiments

Addendum: Of course any mention of building your own transistors would be remiss if it didn’t mention hacker-savant Jeri Ellsworth. She did an amazing job fabricating her own semiconductor devices, and in this video from Metalab, explains some of the basics.