p and n type semiconductors

SemiconductorsSemiconductors are also referred to as metalloids.

Metalloids occur at the division between metals and non-metals in the Periodic Table.

Metalloids such as silicon and germanium are covalent elements. They have a higher conductivity than typical non-metals but a much lower conductivity than that of metals.

The electrical resistance of semiconductors decreases as temperature increases i.e. the electrical conductivity increases as temperature increases.

In order for semiconductors to conduct electricity, electrons must be able to move from one atom to the next.

In a solid the electrons are arranged in bands. Energy is required to move electrons from the valence band to the conductive band. The gap between these bands determines whether a substance will conduct.

Insulators have large gaps between these two bands and so cannot conduct. In semiconductors the band gaps are smaller but few electrons have sufficient energy to move from the valance band to the conductive band.

At higher temperatures, the energy of the electrons increases allowing promotion of the electrons to the conductive band creating electron deficient holes called positive holes

The electrical conductivity of semiconductors also increases when they are exposed to light. The light energy again increases the energy of the electrons, allowing them to be promoted from the valance band to the conductive band. This is known as the photovoltaic effect.

Applying a voltage across the covalent structures of silicon and germanium causes the movement of positive holes and electrons in opposite directions through the lattice.

An electron from a neighbouring atom will replace the lost electron from its neighbour hence creating another positive hole (see handout).

Doping semiconductorsConduction of a semiconductor can be increased

by the deliberate addition of impurities (called dopants).

Adding a group 5 element such as phosphorus or arsenic, which have 5 outer electrons, introduces an extra electron into the lattice structure (Si and Ge have a valency of 4) and an n-type semiconductor is formed.

In an n-type semiconductor the main current carrier is surplus negative electrons.

Adding a group 3 element such as boron or aluminium, which only have three outer electrons, into the lattice structure causes positive holes to be formed and a p-type semiconductor is formed.

In a p-type semiconductor the main current carrier is positive holes.

p-n junctions

A p-n junction is formed between a layer of p-type and a layer of n-type semiconductor.

Solar cellsAt the p-n junction, some of the electrons from

the n-type semiconductor migrate to the p-type semiconductor and line up along the junction. The atoms in this region now have more electrons than protons and so have an overall negative charge which stops further electron migration.

The n-type region becomes positively charged while the p-type region becomes negatively charged.

When light energy reaches the p-n junction, atoms can be made to lose electrons, leaving positive holes behind. The electron will be pulled into the n-type layer by the small voltage at the junction, where it will fill a hole that it made when it first migrated.

The loss of the electron from the p-type layer will cause another freed electron from deeper within this layer to replace it leaving a positive hole behind.

Further migration of electrons occurs from p-type to n-type layer which becomes negative while the p-type layer becomes positive.

If a solar cell is connected to a circuit, electrons will flow from the n-type semiconductor to the p-type semiconductor via the external circuit as a current. Solar cells use this effect to turn sunlight into electricity