Semiconductor diodes part 1

Semiconductor diodes

The crystalline materials are classified by their conductance to metals, insulators and semiconductors.

In metals, the valence electrons are shared by all metal ions, the electrons can move freely in the crystal and drifted by the effect of an electric field. The high concentration of free electrons makes the metal a good conductor.

In insulators, the electrons are localized to their parent atoms, molecules or ions. The electric field can not remove the electrons from the bonds, they can not take part in conduction.

The electrons of a semiconductor also belong to particular bonds but they can be relatively easily removed by thermal excitation even at room temperature and move freely in the semiconductor crystal. When an electron gets free from a bond, sooner or later its empty place will be filled with an electron from a neighbouring bond. This empty place will migrate in the crystal, like a bubble. This missing electron is called a free hole, and it is considered a free carrier, like the free electron, only with positive charge.

The number of free carriers in a semiconductor is very low (~10¹⁴ particles/cm³) and their number increases with increasing temperature. Therefore the conductivity of semiconductors increases with temperature in contrast with the metals.

There are elementary semiconductors like silicon and germanium, and compound semiconductors, like GaAs, InSb and other.

A pure semiconductor contains equal number of free electrons and holes, produced by thermal excitation. Such a crystal is called intrinsic semiconductor.

Each atom of elementary semiconductors like silicon and germanium is connected to its four neighbour by covalent bonds. The number of free carriers in silicon or germanium can be increased by adding group III (boron for example) or group V impurity (arsenic or phosphorus) atoms to the crystal. These impurities substitute for the silicon or germanium atoms.

Donor and acceptor atoms and free electrons and holes in a silicon crystal

A five-valence atom is connected by four bonds to its neighbours but its fifth valence electron is superficial, not needed to the bonds. It is only loosely bound to the parent atom and easily removed, contributing to the free electrons. Such a group -V impurity atom is called donor, and the semiconductor containing donor atoms is called n-type because the majority of the free carriers are electrons.

Substituting an atom by a three-valence atom like boron, one electron is missing to the four bounds. This place of the missing electron is filled by an electron, the empty place moves around in the crystal as a free hole. Such group-III impurity atoms are called acceptors, and the crystal having free holes as majority carriers is called p-type.

THE P-N JUNCTION

If one side of the silicon (germanium) crystal is p-type and the other is n-type, the boundary between these region is called p-n junction.

The free carriers at each side behave as gas, they diffuse to the other side, and their they recombine with the other type of carriers. While the bulk of both sides is electrically neutral, having equal number of negative acceptor ions and positive holes in the P sides, and equal number of positive donor atoms and free electrons in the N side, there are no free carriers in the vicinity of the junction. These region is called depleted region, and its contains localized negative acceptor ions at the P side and positive donor ions at the N side. This charge distribution produces an electric field near the junction which prevents other free carriers to cross the junction.

If we connect leads to both sides of the semiconductor piece we get a p-n junction diode.