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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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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