In physics, a hole is an electric charge carrier with a positive charge, equal in magnitude but opposite in polarity to the charge on the electron. Holes and electrons are the two types of charge carriers responsible for current in semiconductor materials. A hole is the absence of an electron in a particular place in an atom. Although it is not a physical particle in the same sense as an electron, a hole can be passed from atom to atom in a semiconductor material. Electrons orbit the nucleus at defined energy levels called bands or shells. A hole forms in an atom when an electron moves from the so-called valence band (the shell outside the closed shells that is partially or completely filled with electrons) into the conduction band (the outer "cloud" from which electrons most easily escape from, or are accepted by, the atom). Both electrons and holes are present in any semiconductor substance. Electrons flow from minus to plus, and holes "flow" from plus to minus. The more abundant charge carriers are called majority carriers; the less abundant are called minority carriers. In N-type semiconductor material, electrons are the majority carriers and holes are the minority carriers. In P-type semiconductor material, the opposite is true. In the processing of semiconductors, the number of charge carriers can be increased by a process known as doping, which consists of adding minute amounts of elements called impurities. Certain impurities, when added to a semiconducting element such as silicon, increase the number of electrons and produce an N-type material; other impurities increase the number of holes and produce a P-type material. Both N-type and P-type material are important in the manufacture of solid-state electronic components