Transistors form the basis of all modern electronic devices and systems, including the integrated circuits used in systems ranging from radio and television to computers. Transistors are solid-state electron devices made out of a category of materials called semiconductors. Semiconductors are a category of materials with an electrical conductivity that is between that of conductors and insulators. In the extrinsic or doped semiconductor, impurities are purposely added to modify the electronic characteristics. In the case of silicon, every silicon atom shares its four valence electrons with each of its four nearest neighbors in covalent bonds. If an impurity or dopant atom with a valency of three, such as boron, is substituted for silicon, three of the four valence electrons of the dopant atom will be held in covalent bonds. In a P-type semiconductor the hole concentration is generally many orders of magnitude larger than the electron concentration.