20 November 2008


All integrated circuits are fabricated from semiconductors, substances whose ability to conduct electricity ranks between that of a conductor and that of a nonconductor, or insulator. Silicon is the most common semiconductor material. Because the electrical conductivity of a semiconductor can change according to the voltage applied to it, transistors made from semiconductors act like tiny switches that turn electrical current on and off in just a few nanoseconds (billionths of a second). This capability enables a computer to perform many billions of simple instructions each second and to complete complex tasks quickly.

The basic building block of most semiconductor devices is the diode, a junction, or union, of negative-type (n-type) and positive-type (p-type) materials. The terms n-type and p-type refer to semiconducting materials that have been doped—that is, have had their electrical properties altered by the controlled addition of very small quantities of impurities such as boron or phosphorus. In a diode, current flows in only one direction: across the junction from the p- to n-type material, and then only when the p-type material is at a higher voltage than the n-type. The voltage applied to the diode to create this condition is called the forward bias. The opposite voltage, for which current will not flow, is called the reverse bias. An integrated circuit contains millions of p-n junctions, each serving a specific purpose within the millions of electronic circuit elements. Proper placement and biasing of p- and n-type regions restrict the electrical current to the correct paths and ensure the proper operation of the entire chip.

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