A tunnel diode, also known as an Esaki diode, is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki, Yuriko Kurose, and Takashi Suzuki when they were working at Tokyo Tsushin Kogyo, now known as Sony. Tunnel diodes have a heavily doped positive-to-negative (P-N) junction that is about 10 nm (100 Å) wide, which results in a broken band gap where conduction band electron states on the N-side are more or less aligned with valence band hole states on the P-side. They are usually made from germanium, but can also be made from gallium arsenide and silicon materials.

Tunnel diodes are used as very fast switching devices in computers, as well as in high-frequency oscillators and amplifiers. They can be used as a switch, amplifier, and oscillator, and since they show a fast response, they are used in applications that require high-speed switching. The negative resistance region is the most important and most widely used characteristic of the tunnel diode, and a tunnel diode biased to operate in the negative resistance region can be used as either an oscillator or an amplifier in a wide range of frequencies and applications. Tunnel diodes are also used extensively in high-speed switching circuits because of the rapid tunneling action that occurs with no transit time effect and therefore no signal distortion.