Tunnel diode
A tunnel
diode or Esaki diode is a type of
semiconductor diode which is capable of very fast operation, well into the microwave region GHz, by utilizing
quantum mechanical effects.
It was named after Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the
electron tunneling effect used in these diodes.
These diodes have a heavily doped
p-n junction only some 10 nm wide. The heavy doping results in a broken bandgap, where
conduction band electron states on the n-side are more or less aligned with
valence band hole states on the p-side.
Encyclopedia
A
tunnel diode or
Esaki diode is a type of
semiconductor diode which is capable of very fast operation, well into the microwave region GHz, by utilizing
quantum mechanical effects.
It was named after Leo Esaki, who in 1973 received the Nobel Prize in Physics for discovering the
electron tunneling effect used in these diodes.
These diodes have a heavily doped
p-n junction only some 10 nm wide. The heavy doping results in a broken bandgap, where
conduction band electron states on the n-side are more or less aligned with
valence band hole states on the p-side.
Forward bias operation
Under normal forward bias operation, as voltage begins to increase,
electrons at first tunnel through the p-n junction barrier because electron states in the conduction band on the n-side become aligned with 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
rectifiers with zero offset voltage and extreme linearity for power signals.
Under
reverse bias at sufficiently high reverse voltage, electrons flow in the opposite direction, as now different electron states on each side of the pn junction become increasingly aligned and tunnel through the pn junction barrier in reverse direction — this is the Zener effect that also occurs in
zener diodes.
Technical comparisons
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 . 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 resistance region can be exploited in a solid state version of the dynatron oscillator which normally uses a tetrode
thermionic valve .
The tunnel diode showed great promise as an oscillator and high-frequency threshold device since it would operate at frequencies far greater than the tetrode would, in fact well into the
microwave bands. However, since its discovery, more conventional semiconductor devices have surpassed its performance using conventional oscillator techniques.
Tunnel diodes are also relatively resistant to
nuclear radiation, as compared to other diodes. This makes them well suited to higher radiation environments, such as those found in space applications.
See also
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