It would be nice to pattern diodes and semiconductors on PCB without components as follows: etch circuit layout of a copper layer, mask the traces so they don't oxidize, then heat the PCB to have unmasked copper turn into Cu2O (cuprous oxide, a semiconductor).
Anyone seriously attempting this should make sure they understand solid state physics, and at a minimum understand diffusion length of charge carriers and the different type of contacts: Ohmic, Schottky ( for example https://lampz.tugraz.at/~hadley/psd/lectures20/contacts.pdf )
Performance will be horrible, but in some situations constructing and inspecting the device oneself can be paramount ( bootstrapping a secure computational platform, implementing formal verifier associated to a cryptocurrency, ... )