I have watched several youtube videos on how transistors work but all the explanations use a classic resin coated transistor with three prongs you can solder to you project . What I just can’t seem to grasp is how the that can be translated to a solid silicon disk.
Physically, how are the three layers separated (npn/pnp); and more importantly, how do you wire up a billion of them?
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[NMOS Transistor Step-by-Step](https://youtu.be/w_znRopGtbE?si=iw36UC1CySDmGtsY)
I work in the semiconductor manufacturing industry and there are still some things I don’t understand but this video can give you an idea of how it is accomplished. A silicon wafer has hundreds on chips with thousands of transistors on them.
The doping layers aren’t physically separated. You wire them all up with many different layers of copper. If you want to really understand all this you’re gonna need to do your own research because it’s not a simple answer. I have a degree in this stuff and still couldn’t explain half of it very well. Manufacting semiconductors is arguably the most technically difficult thing humans have ever accomplished. It’s VERY VERY complex abd difficult to fully understand.
The term you’re looking for is **integrated circuit**. YouTube should have good stuff when you search that.
Briefly, the silicon is treated with different chemicals or etched away to create small transistors.
In simple terms, they are not wired up but they themselves are also the wires.
You take a big polished slice of a silicone crystal, called a wafer.
Then you go through multiple very complex processes, including etching away material and depositing thin films of other materials to create tiny (microscopic) structures. These structures are your transistors and wires as well as many other electronic or mechanical components.
How all of this is done can vary a lot and depends on the chips you want to produce.
If you want to see the tiny structures, here is an electron microscope zoom into a microprocessor:
The appearance of transistors in Integrated Circuits has changed fairly radically over the years. You asked about bipolar (NPN/PNP) transistors, but the vast majority these days are MOSFETs.
In the last several generations there’s been a transition from “planar” transistors to [FINFETs](https://www.engineersgarage.com/wp-content/uploads/2019/07/Image-Showing-Constructional-Difference-Between-Conventional-Plane-Transistor-and-FINFET.jpg). Instead of laying the gate over the channel (with the gate oxide separating them), the gate electrode wraps around the channel on 3 sides, giving better performance but being more difficult to manufacture. There have been experiments with wrapping it around all 4 sides, but that’s even more difficult to manufacture.
They are wired up by putting an insulator over the transistor, etching contact holes in it, filling those holes with tiny metal cylinders, putting another layer of insulator over that, etching trenches in it that go over the metal cylinders, then filling the trenches with metal. This makes thin, flat metal “wires” that connect to the cylinders and down to the transistors. (I’m simplifying everything quite a bit.) Because the topology gets rather difficult and you don’t want some wires connected, you do this in many layers, up to about a dozen or so.
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