The use of n-type and p-type silicon is a foundation concept in the design of transistors. Pure silicon is not conductive. However, it can be made conductive by adding other elements to its crystalline structure, which then become known as "n-type" or "p-type" silicon. The interaction of n-type and p-type silicon to electric fields and to each other is used to make areas in a transistor change from conductive to non-conductive and vice versa (see crystalline semiconductor). See FET, MOSFET, bipolar transistor and doping. N-Type (Negative) N-type silicon is silicon that has been chemically combined (doped) with phosphorus gas to make it conductive. A silicon atom has four electrons in its outer shell and bonds tightly with four surrounding silicon atoms creating a crystal matrix with eight electrons in the outer shells. However, phosphorus has five electrons, and when combined, the fifth electron becomes a "free" electron that moves easily within the crystal when a voltage is applied. Because the charge carriers are electrons, n-type refers to a negative charge. P-Type (Positive) In contrast, p-type silicon is silicon doped with boron gas that turns it into a conductive material that readily accepts electrons when voltage is applied. Boron has only three electrons in its outer shell and can bond with only three of the four surrounding silicon atoms. This leaves one silicon atom with a vacant location in its outer shell, called a "hole," that readily accepts an electron. Because the charge carriers are holes, p-type silicon is said to have a positive charge.