- Tunnel diode
diodeor Esaki diode is a type of semiconductordiode which is capable of very fast operation, well into the microwavefrequency region, by using quantum mechanical effects.
It was invented in 1958 by
Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the electron tunnelingeffect used in these diodes.
These diodes have a heavily doped
p-n junctiononly some 10 nm (100 Å) wide. The heavy doping results in a broken bandgap, where conduction bandelectron states on the n-side are more or less aligned with valence bandhole states on the p-side.
Tunnel diodes were manufactured by
General Electricand other companies from about 1960, and are still made in low volume today. [ http://www.eetimes.com/special/special_issues/millennium/milestones/holonyak.html "Tunnel diodes, the transistor killers", EE Times, retrieved 2008 April 07 ] ] Tunnel diodes are usually made from germanium, but can also be made in gallium arsenideand siliconmaterials. They can be used as oscillators, amplifiers, frequency converters and detectors. [ Fink, page 7-35 ]
Forward bias operation
Under normal forward bias operation, as voltage begins to increase,
electronsat first tunnel through the very narrow p-n junction barrier because filled electron states in the conduction band on the n-side become aligned with empty valence band hole states on the p-side of the pn junction. As voltage increases further these states become more misaligned and the current drops — this is called " negative resistance", because current decreases with increasing voltage. As voltage increases yet further, the diode begins to operate as a normal diode, where electrons travel by conduction across the pn junction, and no longer by tunneling through the pn junction barrier. Thus the most important operating region for a tunnel diode is the negative resistance region.
Reverse bias operation
When used in the reverse direction they are called back diodes and can act as fast
rectifierswith zero offset voltage and extreme linearity for power signals (they have an accurate square law characteristic in the reverse direction).
reverse biasfilled states on the p-side become increasingly aligned with empty states on the n-side and electrons now tunnel through the pn junction barrier in reverse direction — this is the Zener effect that also occurs in zener diodes.
In a conventional semiconductor diode, conduction takes place while the pn junction is forward biased and blocks current flow when the junction is reverse biased. This occurs up to a point known as the 'reverse breakdown voltage' when conduction begins (often accompanied by destruction of the device). In the tunnel diode, the dopant concentration in the p and n layers are increased to the point where the reverse breakdown voltage becomes zero and the diode conducts in the reverse direction. However, when forward-biased, an odd effect occurs called 'quantum mechanical tunnelling' which gives rise to a region where an "increase" in forward voltage is accompanied by a "decrease" in forward current. This
negative resistanceregion can be exploited in a solid state version of the dynatron oscillatorwhich normally uses a tetrode thermionic valve(or tube).
The tunnel diode showed great promise as an oscillator and high-frequency threshold (trigger) device since it would operate at frequencies far greater than the tetrode would, well into the microwave bands. Applications for tunnel diodes included local oscillators for
UHFtelevision tuners, trigger circuits in oscilloscopes, high speed counter circuits, and very fast rise time pulse generator circuits. The tunnel diode can also be used as low-noise microwave amplifier. [ Donald G. Fink (ed), Electronic Engineers Handbook, McGraw Hill, New York, 1975, ISBN 0-07-02980-4 page 13-64 ] However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques. For many purposes a three-terminal device, such as a field-effect transistor, is more flexible than a device with only two terminals. Practical tunnel diodes operate at a few millamperes and a few tenths of a volt, making them low-power devices. [ L.W. Turner,(ed), Electronics Engineer's Reference Book, 4th ed. Newnes-Butterworth, London 1976 ISBN 0 408 00168 page 8-18 ] The Gunn diodehas similar high frequency capability and produces more power.
Resonant interband tunnel diode
Resonant tunnelling diode
Si/SiGe resonant tunnel diode
Si/SiGe resonant interband tunnel diode
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