The backbone of a semiconductor is its P-N Junction.
Semiconductors are prized for their unique place along the electrical conductivity spectrum. They benefit from being highly adaptable through a process called doping. This process creates the P-N junction. Through doping, and manipulating junctions on the conducting material, semiconductors display various useful properties – passing current in certain directions, showing variable resistance and being sensitive to light or heat.
Conduction in a semiconductor occurs because of the movement of free electrons and holes through the material. These free electrons and holes, called charge carriers, are adjusted with precise variability by adding impurities, atoms at a time, to the semiconductor.
Through doping, free holes or free electrons are added to the material, often concurrently in one device through several regions of varying concentrations and positions. P-type regions have free holes; N-type regions have free electrons. Where these regions meet is the P-N junction.
At the P-N junction, electrons from the N-type region that have reached the conduction band (or, they are excitable enough to travel when an electric field is applied) are free to diffuse across the junction and combine, or fill, the free holes in P-type region. Filling the holes leaves the P-type region with a negative ion, and a positive ion is left in the N-type region. Left alone, electrons would fill holes and no longer be able to cross the junction. By applying voltage, a forward bias occurs. This forward bias allows the current to flow uninterrupted because electrons are constantly forced out of the N-type region.