A semiconductor material has an electrical conductivity value falling between that of a conductor, such as copper, and an insulator, such as glass. Its resistance decreases as its temperature increases, which is a behavior opposite to that of a metal. Their conductive properties can be usefully altered by the deliberate and controlled introduction of impurities ("doping") into the crystalline structure. When two regions doped differently on the same crystal, a semiconductor junction is created. The behavior of charge carriers including electrons, ions and electron holes in these junctions is the basis of diodes, transistors and all modern electronics.
Semiconductor devices may exhibit a range of useful properties such as passing the current more easily in one direction than the other, showing varying resistance and sensitivity to light or heat. Because the electrical properties of a semiconductor material can be modified by doping, or by the application of electric fields or light, devices made from semiconductors can be used for amplification, switching and power conversion.
Modern understanding of the properties of a semiconductor is based on quantum physics to explain the movement of charge carriers in a crystal lattice. Doping greatly increases the number of charge carriers within the crystal. When a doped semiconductor contains mainly free holes it is called "p-type", and when it contains mainly free electrons it is known as "n-type". Semiconductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of dopant type p and n. A single semiconductor crystal can have many p- and n-type regions; The p-n junctions between these regions are responsible for useful electronic behavior.
Although some pure elements and many compounds show semiconductor properties, silicon, germanium and gallium compounds are most commonly used in electronic devices. The elements near the so-called "metal ladder", where metalloids are located in the periodic table, are usually used as semiconductors.
Some of the properties of semiconductor materials were observed in the middle of the nineteenth century and the first decades of the twentieth century. The first practical application of semiconductors in electronics was the development in 1904 of the Cat's-whisker detector, a primitive semiconductor diode widely used in early radio receivers. Developments in quantum physics in turn enabled the development of the transistor in 1947 and the integrated circuit in 1958.
