Doping and Charge Carriers

For a semiconductor material to have the properties necessary in order to function as electronic components, impurities are usually added. The impurities cause the material to conduct currents in certain ways. The addition of an impurity to a semiconductor is called doping. Sometimes the impurity is called a dopant.

Donor Impurities

When an impurity contains an excess of electrons, the dopant is called a donor impurity. Adding such a substance causes conduction mainly by means of electron flow, as in an ordinary metal such as copper or aluminum. The excess electrons are passed from atom to atom when a voltage exists across the material. Elements that serve as donor impurities include antimony, arsenic, bismuth, and phosphorus. A material with a donor impurity is called an N-type semiconductor, because electrons have negative (N) charge.

Acceptor Impurities

If an impurity has a deficiency of electrons, the dopant is called an acceptor impurity. When a substance such as aluminum, boron, gallium, or indium is added to a semiconductor, the material conducts by means of hole flow. A hole is a missing electron—or more precisely, a place in an atom where an electron should be, but isn’t. A semiconductor with an acceptor impurity is called a P-type semiconductor, because holes have, in effect, a positive (P) charge.

Majority and Minority Carriers

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Pictorial representation of hole flow. Solid black dots represent electrons, moving in one direction. Open circles represent holes, moving in the opposite direction.
Charge carriers in semiconductor materials are either electrons, each of which has a unit negative charge, or holes, each of which has a unit positive charge. In any semiconductor substance, some of the current takes the form of electrons passed from atom to atom in a negative to-positive direction, and some of the current occurs as holes that move from atom to atom in a positive-to-negative direction.
 
Sometimes electrons account for most of the current in a semiconductor. This is the case if the material has donor impurities, that is, if it is of the N type. In other cases, holes account for most of the current. This happens when the material has acceptor impurities, and is thus of the P type. The dominating charge carriers (either electrons or holes) are called the majority carriers. The less abundant ones are called the minority carriers. The ratio of majority to minority carriers can vary, depending on the way in which the semiconductor material has been manufactured.
 
Above figure is a simplified illustration of electron flow versus hole flow in a sample of N-type semiconductor material, where the majority carriers are electrons and the minority carriers are holes. The solid black dots represent electrons. Imagine them moving from right to left in this illustration as they are passed from atom to atom. Small open circles represent holes. Imagine them moving from left to right in the illustration. In this particular example, the positive battery or power-supply terminal (or “source of holes”) would be out of the picture toward the left, and the negative battery or power-supply terminal (or “source of electrons”) would be out of the picture toward the right.