# Microelectronics

The time it takes for information in an electrical circuit to be passed from one point in the circuit to another depends on the speed of the electromagnetic pulse in the circuit and the distance between the two places.

When you press a switch to switch on a light the light does not instantaneously. There is a tiny time lag during which the electric pulse is travelling from the stitch to the light rather like the movement of a mechanical pulse down a stretched helical spring. The further the light is from the switch the bigger is this delay. You may think this is nonsense because you are used to switching on lights that are quite close to you and they seem to come on immediately.

However this is because of the small distance and the terrific speed at which the electric pulse moves down the wire. In a copper wire it travels at about 200 000 km a second or about ? of the speed of light. This means that it will travel 200 metres in a millionth of a second!

In an electric circuit like those you might build in the lab at school with batteries and bulbs the electric pulse probably takes around a thousand millionth of a second to get from the battery to a bulb! That will seem pretty immediate.

There is little we can do to increase up the speed off this electrical pulse if we want to speed up the time of transmission of information round a circuit but we can shorten the time it takes to travel between two points on the circuit by making the circuit smaller.

If we imagined two people signalling to each other with flags the time it takes light to travel between the two places will affect the time it takes one of them to see the other's flag move. Bringing the two people closer together will shorten the time between the movement of one flag and when it is seen to move by the other person. (Figure 1)

Maybe a better analogy is a line of people passing a message down from one to the other by whispering it to the person nearest them. Obviously the people a few places down the line will 'get the message' much sooner than those at the other end of the line. (Figure 2)

It is just the same with an electrical circuit although it is harder to 'see' the electrical pulse travel down the wire. In the 1920's valves were designed to be used in radios, these valves got smaller and then in 1948 the transistor was invented. This was very much smaller than any valve and so the circuits could operate much more quickly.

So people started looking for ways to make electronic circuits smaller so that they could pass information more quickly and so make the circuit operate faster.

## Integrated circuits

This was the beginning of the development of the integrated circuit. These are circuits where all the components are made in a chip of solid material such as silicon and each component in the circuit is made by altering the nature of parts of the material. Each component is very small and so integrated circuits can operate quickly. The size of a transistor on an integrated circuit can now be as small as 1 micron (a thousandth of a millimetre) across! Some circuits may have up to 100 million components per square centimetre.

You have only to think about your computers or mobile phone. In the 1980s a personal computer had a speed of only 1kHz whereas now (2013) many of our home computers (including the one I am using now) operate at speeds of up to 3 GHz (3000 MHz) and research systems with speeds up to 100 GHz are being developed. The 'speed' of your computer usually refers to how quickly the CPU (Central processing unit) can operate.

(However the amount of RAM that a computer has, the clock speed of that RAM and the cache size are all really important factors in its overall performance)

The applications of integrated circuits in our lives today are enormous. They are used in computers, mobile phones, satellite communication, digital cameras, MP3 players, games consoles, washing machines, cars, televisions and so on.

Thinking back to our people with flags – if each person is required to do a separate job then to do ten jobs you need ten people. If these people are large they take up a large amount of room but of they are small they take up less room and the 'job' can be done in a smaller space. (Figure 3)

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