As the name suggests semiconductors are partially conductive materials, and lay somewhere between the conductivity of conductive metals and insulators.
N-type semiconductors are generally composed of silicon, or germanium, doped in antimony. The doping provides a free electron which increases the conductivity of the material.
The name derives from negative charge. This is because of electric current, where electrons break free from their atoms, and create a direction flow of electrons, with the aid of an electric field. Electrons flow towards vacancies in positively biased materials.
P-type semiconductors are also generally composed of silicon, or germanium. However, Instead of being doped in antimony, they are doped in an element such as indium. This has the opposite effect to that of doping in antimony, as indium has a vacancy in its outer shell of electrons, to which a free electron can easily occupy.
The letter ‘P’ represents the fact that this material has a positive charge.
Under normal circumstances, In N-type semiconductors free electrons flow away from the materiel. Whereas In P-type semiconductors, free electrons flow towards the material to occupy the vacancies.
In terms of conventional current, current flows from P-type (positive) semiconductors, and current flows towards N-type (negative) semiconductors.