Eli5: How are they able to make transistors so extremely small?
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They use a process called photolithography where silicon wafers are etched using lasers. The silicon wafers are coated with a chemcial called a photoresist which is sensitive to UV light. The circuits are then projected on to the wafer uisng UV light which changes the electrical characteristics of that area of silicon
Because a transistor is just a switch controlled by an electronic signal, or can theoretically be made from just 3 atoms for the base, collector and emittor.
The real answer is probably: because it is highly worthwhile to do, that justifies throwing trillions of dollars onto this problem.
The technical problem is not how to make any particular one small (we could basically assemble them atom by atom), but how to pack millions of transistors into close proximity.
Every three years since about 1962, engineers have found a way to make transistors on a slab of silicon half as big as before, over and over again. This has been done by making the equipment have more accuracy and operate more cleanly to reduce defects. Reducing the transistor size this many times has gotten them from big enough to see, down to just a few dozen atoms across.
They use a process called “photolithography” to modify a piece silicon by adding carefully controlled impurities and plating on metal wires.
Pure silicon doesn’t conduct electricity, but by adding specific impurities you can alter its electrical properties. There are a variety of different types of transistor design, but one of the older ones is made by having two areas of silicon with “n” impurities, and a third area of silicon with “p” impurities in between. Connect a wire to each area of silicon and you have a transistor.
Photolithography works by using a projector (like a regular video or film projector) to project a design pattern onto a piece of silicon. The difference between a lithography projector and a cinema projector, is that a cinema projector is designed with a magnifying lens – but a lithography projector is designed with a minifying lens. This allows you to take a big design drawing and project it at tiny size onto the silicon.
The trick with photolithography is that before the design is projected on – the silicon is coated with a light activated varnish (called a photoresist). Liquid photoresist is painted on. Then the design is projected. Where the light hits the photoresist , it sets hard. Where the photoresist is in a dark part of the design, it stays liquid. Then the silicon is washed, and the liquid photoresist gets washed off, but the hard photoresist is left forming a barrier. Then impurities are added – the photoresist gets in the way, so only areas of silicon without photoresist are affected. Then there is a chemical wash which removes the hardened photoresist , going back to flat silicon.
The process is then repeated with different design patterns to add different impurities or to spray on metal wires.
Transistors are just a collection of p doped silicon, n doped silicon, and then more p doped. (Or npn depending on the type of transistor). It’s all about how we make them. We take a silicon wafer and add atoms or remove layers atoms at a time. We can also use lasers and projections to basically draw shapes on the wafers as small as the wavelength of light. These are structures so small we can’t even use light to see them.
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