A semiconductor is a solid substance that has a conductivity between that of an insulator and that of most metals, either due to the addition of an impurity or because of temperature effects. Devices made of semiconductors, notably silicon, are essential components of most electronic circuits. Good examples of semiconductor materials are germanium, selenium, and silicon. The doping agents used are of two types, resulting in two types of extrinsic semiconductor. An extrinsic semiconductor which has been doped with electron donor atoms is called an n-type semiconductor because the majority of charge carriers in the crystal are negative electrons. In p-type semiconductors, holes are the majority carriers and electrons are the minority carriers. P-type semiconductors are created by doping an intrinsic semiconductor with acceptor impurities.