tunneldiodes

A tunnel diode, historically associated with the work of Leo Esaki in 1958, is a semiconductor diode that relies on quantum tunneling across a very thin, heavily doped p-n junction. This tunneling current creates a region of negative differential resistance in the current–voltage curve, enabling unusual high-speed operation at microwave frequencies.

Construction relies on heavy doping of both the p and n regions, which narrows the depletion layer and allows electrons to tunnel through the junction even at low forward biases. The resulting I–V characteristic shows a peak current (I_p) at a modest forward voltage, followed by a decline to a valley current (I_v) as voltage increases, and then a resumed rise.

Electrical behavior: negative differential resistance means that, for a certain voltage range, increasing voltage reduces current. This enables relaxation oscillators and wide-band microwave generation. The device is inherently fast and can operate at very high frequencies, but it typically handles low power and shows strong temperature dependence and noise.

Variants and relation to resonant tunneling diodes: The tunnel diode is different from resonant tunneling diodes, which use quantum wells and double barriers to produce NDR at discrete voltages with sharper features. Tunnel diodes are simpler and were among the first tunneling devices used for microwave electronics.

Applications and fabrication: They were widely used in the 1960s–70s for microwave oscillators, detectors, and fast switches. Modern use is limited to niche applications and education. Fabrication commonly employs silicon, germanium, or III–V materials, with very high dopant concentrations (on the order of 10^19–10^20 cm^-3) formed by diffusion or epitaxial growth.