The n-type and p-type semiconductors respectively have electrons and holes as majority charge carriers. To form a p-njunction, the most convenient way is to introduce donor impurities on one side and acceptor impurities into the other side of a single semiconducting crystal.
There is greater concentration of electrons in the n-region of the crystal and of holes in the p-region. Because of this, electrons tend to diffuse to the p-region and holes to the n-region and recombine. Each recombination eliminates a hole and a free electron. This results in creation of positively and negatively charged ions near the junction in n and p regions, respectively. As these charges accumulate, they tend to act as shield preventing further movement of electrons and holes across the junction. Thus, after a few re-combinations, a narrow region near the junction is depleted in mobile charge carriers. It is about 0.5 µm thick and is called the depletion region or space-charge region.
Due to accumulation of charges near the junction, an electric field is established. This gives rise to electrostatic potential, known as barrier potential.This barrier has polarities. When there is no external electric field, this barrier prevents diffusion of charge carriers across the junction.
The barrier potential is characteristic of the semiconductor material. It is about 0.3 eV for Ge and about 0.7 eV for Si. The junction acts as a diode.
Semiconductor diodes are designated by two letters followed by a serial number. The first letter indicates the material: A is used for material with a band gap of 0.6 eV to 1.0 eV such as germanium. B is used for material with a band gap of 1.0 eV to 1.3 eV, such as silicon. The second letter indicates the main application: A signifies detection diode, B denotes a variable capacitance diode, E for tunnel diode, Y for rectifying diode and Z denotes Zener diode. The serial numbers specify power rating, peak reverse voltage, maximum current rating, etc.
For example, BY127 denotes a silicon rectifier diode and BZ148 represents a silicon Zener diode.