P-type and N-type Semiconductors
The properties of semiconductors change very significantly by adding very small amounts of impurity. If traces of impurities of materials having five electrons in the outer ring of their atoms are added they enter the crystal lattice sharing electrons with the silicon. However as they have one extra electron in the outer ring, one electron becomes free to move around the lattice. This enables a current to flow if a potential is applied across the material. As this type of material has a surplus of electrons in the lattice it is known as an N-type semiconductor. Typical impurities that are often used to create N-type semiconductors are phosphorous and arsenic.
It is also possible to place elements with only three electrons in their outer shell into the crystal lattice. When this happens the silicon wants to share its four electrons with another atom with four atoms. However as the impurity only has three, there is a space or a hole for another electron. As this type of material has electrons missing it is known as P-type material. Typical impurities used for P-type material are boron, and aluminium.