Anazor: Tunnel Diode

A tunnel diode is a pn junction that exhibits negative resistance between two values of forward voltages.

The two values of forward voltages are the peak point voltage and the valley point voltage.

Conventionally, an ordinary diode exhibits positive resistance when it is forward biased or reverse biased.
In the case where a semiconductor junction diode is heavily doped with impurities it exhibits negative resistance between the peak point voltage and valley point voltage, such a diode is a tunnel diode.
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Negative resistance simply means a situation where voltage is not directly proportional to current. In other words an increase in voltage leads to a decrease in current.

Basically, a tunnel diode is a pn junction with heavy doping of n - type and p - type semiconductor materials.

Now, this heavy doping gives rise to a large number of majority carriers which the results to many of this carriers not used during recombination thereby creating a very narrow depletion layer.

The Tunneling Effect:

As stated above the large number of majority carriers are not used in recombination which then causes much drifting in the p and n sections.
For the fact that many majority carriers are not used during recombination, it causes many valence electrons to have their energy levels raised to the conduction region.
Therefore, a very small applied voltage causes conduction to take place and since the depletion layer is very narrow, current easily pass through it.

Tunneling therefore, is the motion of valence electrons from the valence band to the conduction band with little or no applied voltage.
This simply implies that valence electrons easily travel through the forbidden gap.

As the voltage is applied to the tunnel diode, the current rapidly rises and this continues till the peak point voltage is attained.
When the peak point voltage is attained, as one applies voltage the current decreases and at this point the tunnel diode defies Ohm's law. This region is known as the Negative Resistance Region.
This negative resistance continues until the valley point is reached and when the valley point is reached the tunnel switches back to behaving according to Ohm's law. Therefore, as one applies voltage, the current increases too.

From the tunneling effect, one could easily observe that the tunnel diode when operated in the negative resistance can be used as an oscillator or switch.

Note that a tunnel diode has a high reverse current.