When an n type semiconductor is joined to a p type semiconductor such that the crystal structure remains continuous at the boundary, the junction is called the p-n junction and the resulting combination is called a p-n junction diode or semiconductor diode.
At high temperature one half p-type semiconductor is mixed with the half n-type semiconductor such way p-n junction made. We know that electrons are majority carriers in n type semiconductors and holes or positive charges are majority carrier in p type semiconductors. So, p type portion contains positive charges majority carrier and n type portion has electrons as majority carrier at p-n junction.
In a p-n junction if no potential difference is applied at the ends of the junction, some of the excess electrons in the n region will diffuse towards the p-region after breaking away from their parent atoms. The donor atoms in the n-region becomes positive ion on losing the electrons. The junction these positive ions remain fixed position. N-region is positively charged. In the same way at p-region the holes would diffuse with the electrons and becomes negative ions. So there is very small or this separation between two region. These accumulations of opposition charges on two sides of the junctions create an electric field across it called a potential barrier.
It is so called because the charges must overcome a barrier like potential to cross the junction. This region is also called potential difference region. Its general value is between 0.1eV to 0.3eV.
What is a Zener diode?
Zener diode is an important diode. It is used extensively electronic device. Many of electronic circuit require D.C. voltage to operate. But it is not always possible normally. The voltage can be stable if Zener diode is used. It gives regular voltage. A p-n junction diode meant to operate in the break down region is called a Zener diode. There is no difference between Zener diodes and normal diode except, it is properly doped. So as to have a sharp breakdown voltage. Zener diode is always connected in reverse mode so it is always reverse biased. For a semiconductor diode when reverse bias is increased then the critical voltage is gained where the reverse current increases sharply to a high value and the resistance of the diode reduces to zero. It is known as knee voltage or breakdown region. When the reverse current increases in the ordinary diode and cross the Zener voltage, it is destroyed or burnt. But Zener diode cannot burn or destroy for its construction. Zener diode is made of highly doped p and n type semiconductors.
A diagrammatic representation is shown in a figure. Zener diode is same as an ordinary diode except the bar is turned into Z-shape.
The working principle of Zener diode is to supply a constant voltage. When the reverse current increases Zener diode supplies constant voltage and the excess current passes through it. So, we can say Zener diode is a voltage regulator.