 A semi-conductor material has an electrical conductivity value falling between that of a conductor, like copper, gold, etc. and an insulator, such as glass. Their resistance decreases as their temperature increases, which is behavior opposite to that of a metal. Their conducting properties may be altered in useful ways by the deliberate, controlled introduction of impurities doping into the crystal structure. Where two differently doped regions exist in the same crystal, a semi-conductor junction is created. The behavior of charged carriers which include electrons, ions and electron holes at these junctions is the basis of diodes, transistors and all modern electronics. Some examples of semi-conductors are silicon, germanium, and gallium-barceneide. After silicon, gallium-barceneide is the second most common semi-conductor used in laser diodes, solar cells, microwave frequency-integrated circuits, and others. Silicon is a critical element for fabricating most electronic circuits. Semi-conductor devices can display a range of useful properties such as passing current more easily in one direction than the other, showing variable resistance, and sensitivity to light or heat. Because the electrical properties of a semi-conductor material can be modified by doping, or by the application of electrical fields or light, devices made from semi-conductors can be used for amplification, switching, and energy conversion. The conductivity of silicon is increased by adding a small amount of pentahalent antimony, phosphorus, or arsenic or trivalent baron, gallium, endium atoms part in 108. This process is known as doping and resulting semi-conductors are known as doped or extrinsic semi-conductors. Apart from doping, the conductivity of a semi-conductor can equally be improved by increasing its temperature. This is contrary to the behavior of a metal in which conductivity decreases with increase in temperature. The modern understanding of the properties of a semi-conductor relies on quantum physics to explain the movement of charged carriers in a crystal lattice. Doping greatly increases the number of charged carriers within the crystal. When a doped semi-conductor contains mostly free holes it is called P-type and when it contains mostly free electrons it is known as N-type. This semi-conductor materials used in electronic devices are doped under precise conditions to control the concentration and regions of P, and N-type dopants. A single semi-conductor crystal can have many P, and N-type regions. The P-N junctions between these regions are responsible for the useful electronic behavior. Although some pure elements and many compounds displace semi-conductor properties, silicon, germanium, and compounds of gallium are the most widely used in electronic devices. Elements near the so-called metalloid staircase where the metalloids are located on the periodic table, are usually used as semi-conductors. Some of the properties of semi-conductor materials were observed throughout the mid-19th and first decades of the 20th century. The first practical application of semi-conductors in electronics was the 1904 development of the Katz-Wisker detector, a primitive semi-conductor diode used in early radio receivers. Developments in quantum physics in turn allowed the development of the transistor in 1947 and the integrated circuit in 1958.