In some electronic circuits, fast operation of diodes is desired. A tunnel diode is such kind of semiconductor diode which is known for its very fast and reliable operation. This diode is made by the use of the process called tunnelling.
The pn junction of these diodes is not so wider, but they are heavily doped. This specific connection helps to align the conduction electrons and valance holes. Electrons are the charge carrier, and acts as the majority carrier for tunnel diode. Highly doped pn region aligns the n region conduction electrons and p region valance holes. This process is called as tunnelling.
There are two kind of operation in tunnel diode. In forward bias operation the proper tunnelling operation is achieved. When the forward bias voltage starts increasing then the conduction electrons tunnel through the p-n junction barrier. In this way conduction electrons and valance holes get aligned with each other. If voltage is increasing again then the flowing current is decreasing which is called negative resistance. In reverse bias operation tunnel diode is called back diode. In this operation electrons tunnel through the p-n junction barrier but in the reverse direction. Mostly tunnel diodes are configured as forward bias mode, and after the increase of the forward bias voltage up to a level, it behaves like normal diode. Then the conducting electrons travels through the pn junction.
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