When you open up a computer – or these days almost any electronics – you’ll see a bewildering array of wires and other mysterious tiny devices. Among them, of course, are the ubiquitous computer logic chips.

It’s well known that these wonders of technology are built on wafers of silicon. They’re fabricated using photolithography, which has parallels to digital photography. A mask allows the surface of the silicon to be selectively etched by exposing it to light in a way that’s a bit like how lithography cuts precise patterns into stone. Material can also be deposited in a manner similar to screen printing.

Chip construction might be complicated, but the fundamentals of the logic built into them is extremely simple and hasn’t changed since the birth of computers mid-last century.

We’ll skip the electronics, but the key point is that these circuits are selectively switched on and off, just like a light switch. Off means ‘No’ (False), and On means ‘Yes’ (True).

Circuits are constructed from handful of types of logic ‘gate’. Think of them as herding livestock through a yard from different pens. Most take two inputs to determine whether the output gate is open (True) or closed (False).

The gate is open if you have a cow OR a goat; a cow AND a goat; a cow and a goat – but not both (XOR), neither cow or goat (NOR and its inverse XNOR); not a cow and goat together (NAND); or anything that’s NOT a pig.
Don’t worry if you have to think about that. The point is, every logic circuit is constructed from these seven basic types.

Real circuits, of course, chain millions and billions of these together to give the vast complexity in a modern computer.

Chips contain incredibly small patterns, down to a few tens of nanometres in size. The electronics are now so small we’re on the verge of quantum effects where one component can spontaneously flip state or interfere with a neighbouring one.

To get around these limits, computers are becoming massively parallel – they ‘scale out’ by running many side by side. After that, the next big step will be quantum computing.

Response: Rod Taylor, Fuzzy Logic

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