Computer chips are written on silicon because it is inexpensive and a quality semiconductor. A good semiconductor responds to voltage to allow computers to make complex calculations. Silicon is also abundant in the natural world.
A computer chip contains many transistors or switches. The transistors control the flow of electricity through the chip following the complex instructions of computer code. Silicon and other semiconductors provide the right conductivity for tight control. Conductors, such as most metals, let the electricity flow straight through the material and control is not possible. Insulators, such as most plastics, do not allow electricity to travel through them. Semiconductors allow electricity to move in a controlled pattern to facilitate the creation of complex chip circuits.
Although other semiconductors exist, silicon is common and inexpensive. It is the second most abundant element in the Earth's crust, accounting for about 28 percent of it by mass. Computer chips are complex, multilayered devices with detailed circuitry that require the addition of some insulator into their circuits. Silicon combines with oxygen to make the insulator silicon oxide. This process is easy for chip manufacturers.
On the other hand, silicon is not the fastest semiconductor. In some high-speed applications, computer chips are manufactured from crystals formed of arsenic and gallium. This more expensive material is also more difficult to work with, but it does provide top speed when necessary.