Semiconductor Technology from A to Z

Doping means introducing foreign atoms into a semiconductor crystal in order to specifically alter its conductivity. Two of the most important substances used to dope silicon are boron (3 valence electrons = trivalent) and phosphorus (5 valence electrons = pentavalent). Others include aluminium and indium (trivalent), as well as arsenic and antimony (pentavalent).

The dopant elements are incorporated into the lattice structure of the semiconductor crystal; the number of outer electrons determines the type of doping. Elements with 3 valence electrons are used for p-type doping, while pentavalent elements are used for n-type doping. The conductivity of a deliberately impurified silicon crystal can be increased in this way by a factor of 10⁶.

The pentavalent dopant element has one more outer electron than the silicon atoms. Four of its outer electrons can bond with a silicon atom each, while the fifth remains freely mobile and serves as a charge carrier. This unbound electron requires far less energy to be raised from the valence band into the conduction band than the electrons responsible for the intrinsic conductivity of silicon. The dopant element that releases an electron is referred to as an electron donor (from Latin donare = to give).

By releasing negative charge carriers, the dopant elements become positively charged and remain fixed in the lattice; only the electrons are able to move. Doped semiconductors whose conductivity is based on free (negative) electrons are called n-conducting or n-doped. While holes (along with an equal number of electrons) can be spontaneously generated within the crystal at any time, the number of free electrons introduced by the donors now predominates, which is why these are referred to as majority charge carriers. Holes, on the other hand, are called minority charge carriers.

n-type doping with phosphorus

Arsenic is used as an alternative to phosphorus, since its diffusion coefficient is lower. This means that the dopant diffuses less during subsequent process steps, so the doping remains in the location where it was originally introduced.

In contrast to the free electron produced by doping with phosphorus, trivalent dopant elements have exactly the opposite effect. They can accept an additional outer electron, thereby leaving a hole behind in the valence band of the silicon atoms. As a result, the electrons in the valence band become mobile. The holes move in the direction opposite to the movement of the electrons. When indium is used as the dopant element, the energy required for this is only 1 % of the energy needed to raise the valence electrons of silicon atoms into the conduction band.

By accepting an electron, the dopant element becomes singly negatively charged; such dopant atoms are called electron acceptors (from Latin acceptare = to accept). Here too, the dopant element remains fixed within the crystal lattice; only the positive charge moves. These semiconductors are referred to as p-conducting or p-doped because their conductivity is based on positive holes. Analogous to n-doped semiconductors, holes are the majority charge carriers here, while free electrons are the minority charge carriers.

p-type doping with boron

Externally, doped semiconductors are electrically neutral. The terms n-type and p-type doping refer only to the majority charge carriers.

N-doped and p-doped semiconductors behave in an approximately similar way with respect to current flow. As the number of dopant elements increases, so does the number of charge carriers in the semiconductor crystal. Even a very small amount of dopant elements is sufficient for this. Weakly doped silicon crystals contain only 1 foreign atom per 1,000,000,000 silicon atoms, whereas at the highest doping levels the ratio of foreign atoms to silicon atoms is, for example, 1 to 1,000.

In n-doped semiconductors, introducing a dopant element with five outer electrons makes an unbound electron available within the crystal, which can therefore be raised into the conduction band with comparatively little energy. As a result, n-doped semiconductors exhibit a donor level close to the conduction band edge, and the band gap that needs to be overcome is very small.

By introducing a trivalent dopant element, p-doped semiconductors provide a vacant site that can already be filled by an electron from the valence band with only a small amount of energy. In p-doped semiconductors, an acceptor level is therefore located close to the valence band edge.

Band diagram of doped semiconductors