Tunnel diode
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A tunnel diode is a type of semiconductor diode which is capable of very fast switching speeds, up to 5 GHz, by utilizing quantum mechanical effects.
In a conventional semiconductor diode, conduction takes place while the PN junction is forward biased, conversely this device 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 impurities 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 (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, 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.