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 (100 Å) 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. (That is, they have an accurate square law characteristic in the reverse direction.)

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
A rough approximation of the VI curve for a tunnel diode, showing the negative differential resistance region A rough approximation of the VI curve for a tunnel diode, showing the negative differential resistance region

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 tunneling' 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