I know why silicon is used for ICs and transistors in general due to its doping features. But I don't know why we use silicon as a PCBs, all we need is to isolate pins from each other in a PCB, right? So, will any insulating material work? Such as wood?
PCB's are NOT made of silicon!
PCBs are made from coated paper (coated to make it somewhat fire resistant), this is the cheap option.
Better PCBs are made from epoxy coated fiberglass, a common material is FR4.
Reasons not to use silicon for PCBs:
Silicon is only a good isolator when it is very pure (no contaminants). For making semiconductors extremely pure Silicon manufactured. This is expensive to make but not much of it is needed per transistor / IC so the total material cost per device is quite low.
Silicon is very brittle it is simply unsuitable to make a strong PCB that can withstand some mechanical force.
Reasons not to use wood for PCBs:
Wood is flammable
Wood is not strong when it is thin.
Wood can bend over time due to aging and moisture.
Coated paper and FR4 do not have the disadvantages mentioned above. Also, it is easy to coat them with copper to make the connections. It is easy to drill holes in the material. These materials are cheap.
After some research, silicon pcb does exists and there is an industry doing them.
As this publication shows https://www.eejournal.com/article/20130723-siliconcirc/
"Silicon circuit boards (SiCBs) are similar to printed circuit boards (PCBs) but made with a silicon substrate rather than FR-4."
They allow pad pitch of 30-40 micron. Or 3 micron according to the second link.
The main purpose as advertised is ultra high density,for micro scale systems.
This company manufactures them http://www.sbmicrosystems.us/what/silicon-pcbs/
One can see the use, with specific miniaturized ICs into small devices as wearable glasses, health monitoring ring, or substrate interconnect for CPUs/cache memory, nano robotics...
It's a very specific use and they are not meant to replace Pcbs.
Never seen a PCB made from Silicon, for the reasons others have mentioned. Or for wood for that matter.
Materials I have used in designs include FR-4, Polyimide, Duroid (PTFE), Megtron, and ceramics.
Silicon as a substrate material actually does make some technical sense in that if you attaching bare Si chips to the substrate (using flip chip or bump pad technology), the fact that the CTE of the substrate matches the CTE of the chips means that you are much less likely to have cracked interconnects. But, except for some R&D activities, I've never seen that actually used in a product.
Would be interesting to hear anyone else's experience in this area. Maybe I should just make this into a new question.