Semiconductor

A Semiconductor 

Generally, when using semiconductors one must distinguish between "N-type" and "P-type" materials. "N-type" refers to a semiconductor material with an excess of electrons; "P-type" to a material with a deficit of electrons (or an excess of "holes").

The semiconductor type dictates the operational modes of a semiconductor device. For example, in an NPN-Transistor, composed of a layer of P-type silicon sandwiched on either side by N-type silicon, the P-type layer provides an insulator by way of its electron deficit. Thus, when no external voltage is applied, the NPN transistor is said to be "off". However, applying a voltage between the middle (base) layer and one of the sides fills the electron holes, effectively providing a continuous conductor across the N layers. In this state, the transistor is "on".