Published: 19 September, 2002

Silicon: Good semiconductor, and inexpensive

Why are computer chips written on silicon?

The key components in computer chips are millions of tiny transistors. A transistor can be understood as an electrically-controllable switch. The application of an electrical signal, a voltage, to one input of a transistor can turn on or off the flow of electricity going through a channel within the device. By connecting transistors together so that the electrical signal flowing out of one can control the currents in other transistors downstream, an interconnected system of transistors working together can be used to perform complex calculations.

In order for the electrical current inside a transistor to be switched between being on and off, the channel of the transistor must be made from a material whose electrical properties are changed by applying a voltage. For most materials, this doesn't happen. For example, metals like copper allow electrons to flow regardless of how much voltage is applied nearby. Insulators, like window glass or most plastics, don't possess any mobile electrons which can contribute to an electrical current. The best materials for making transistors are called semiconductors, and silicon is one example. Semiconductors are sort of halfway between insulators and good metal conductors. The number of mobile electrons in a semiconductor changes under the influence of small voltages, so that for some values of applied voltage there are electrons present which can flow through the transistor's channel, and for other applied voltages the transistor is insulating.

Silicon is only one out of many different semiconductor materials, and it is not the only one that is used in applications. For instance, a combination of the elements gallium and arsenic forms crystals which permit electrons to move faster than in silicon, so that this material is sometimes used when extreme speed is important. The reason that silicon is used in computers is because it is easier, and therefore less expensive, to make complicated circuits out of silicon than for any other material. Computer circuits require some parts to be made out of insulators in addition to the parts that are semi conducting. With silicon, it is easy to make a good insulator by adding some oxygen to produce silicon oxide. The efficiency with which computer circuits can be made is truly amazing. The average cost is now much less than one penny per 10,000 transistors.