A tunnel diode is a type of semiconductor device that allows current to flow in both directions. It is made from a p-n junction, where p-type and n-type materials are placed very close together. This unique structure enables quantum tunneling, allowing electrons to pass through the energy barrier even at low voltages. Tunnel diodes are known for their fast switching speeds and are often used in high-frequency applications. These diodes exhibit a negative resistance region in their current-voltage characteristics, which means that increasing voltage can lead to a decrease in current. This property makes them useful in oscillators and amplifiers. Tunnel diodes are commonly found in applications involving microwave technology and radio frequency circuits, contributing to advancements in electronics.