I thought it would be fun to put some notes about silicon here.
Silicon can be bought in 4" wafers (small). Say 25 for $450. You want to start with doped silicon (as you won't be melting it down, and adding chemicals to the silicon) reference: https://www.electronicproducts.com/Analog_Mixed_Signal_ICs/Transistors_Diodes/DIY_transistor_fabrication_Are_you_up_to_the_challenge.aspx.
You need some basic equipment, but anyone with access to a lab or hackerspace might have enough. From previous reference:
Nitrogen tank Kiln Hydrofluoric acid (HF) Phosphosilicate film Prime-grade silicon wafer Vinyl sticker in place of photoresist for masks Color chart for identifying thickness of oxide layer
I have nitrogen, an oven only up to 450F (not high enough...), can obtain HF and wafers... However, silicon is not cheap. This hobby looks to be on the expensive side.
Is there a limit to what size a die can be? How about transistors that are 4" across...