Tunnel Diode Theory & Operation

- as the name implies, the tunnel diode theory uses a tunnelling effect to provide a negative resistance region in its IV characteristic as the key to its operation.

While the tunnel diode is a semiconductor device using the same materials as other forms of diode and active devices, the very high levels of dopant used, cause the devices to operate in a very different manner.

The device theory shows that it does not act as a diode, but instead exhibits a negative resistance region in the forward direction.

The IV characteristic curve, combined with the very high speed of the diode means that the it can be used in a variety of microwave RF applications as an active device.

Tunnel diode theory basics

The characteristic curve for a tunnel diode shows an area of negative resistance. When forward biased the current in the diode rises at first, but later it can be seen to fall with increasing voltage, before finally rising again.

It is also interesting to note that current also flows in the reverse direction - the reverse breakdown voltage is actually zero and the diode conducts in the reverse direction. The characteristics near the original are virtually symmetrical.

The IV characteristic of the tunnel diode showing the important voltage turning points and the negative resistance region
Tunnel diode IV characteristic

The reason for this is that there are a number of different components to forming the overall curve.

  • Normal diode current:   This is the 'normal' current that would flow through a PN junction diode.
  • Tunnelling current:   This is the current that arises as a result of the tunnelling effect.
  • Excess current:   This is a third element of current that contributes to the overall current within the diode. It results from what may be termed excess current that results from tunnelling though bulk states in the energy gap, and means that the valley current does not fall to zero.

The different current contributions or components that make up the overall current passed by the tunnel diode
Tunnel diode current components

These three main components sum together to provide the overall level of current passed by the tunnel diode.

Tunnelling mechanism & theory

Tunnelling is an effect that is caused by quantum mechanical effects when electrons pass through a potential barrier. It can be visualised in very basic terms by them "tunnelling" through the energy barrier.

The tunnelling only occurs under certain conditions. It occurs within tunnel diodes because of the very high doping levels employed.

At reverse bias, the electrons tunnel from the valence band in the p-type material to the conduction band in the n-type material, and the level of the current increase monotonically.

For the forward bias situation there are a number of different areas. For voltages up to Vpe, electrons from the conduction band find increasing availability of empty states in the valence band and the level of current increases up to a point where the current equals Ipe.

Once this point is reach, it is found that number of empty states available for electrons with the level of energy they are given by the increased voltage level starts to fall. This means that the current level falls in line with this. The overall current level falls away relatively swiftly, dropping to near zero.

As the current from the tunnelling effect falls, so the diffusion current, which is the same action as occurs in a normal PN junction diode starts to increase and steadily becomes the dominant mechanism.

Tunnel diode characteristics

The diagram towards the top of the page shows the tunnel diode IV characteristic. This has a form of 'N' shaped curve. With an area of negative resistance between the peak voltage, Vpe and the valley voltage Vv.

The values for these voltages depend upon the diode material and also upon its individual characteristics.

Tunnel Diode Properties for Different Materials
Parameter Germanium Gallium Arsenide Silicon
Vpe (mV) 40 - 70 90 -120 80 - 100
Vv (mV) 250 - 350 450 -600 400 - 500
Ipe/Iv 10 -15 10 - 20 3 - 5

One of the useful figures of merit for a tunnel diode characteristic is the peak to valley current ratio, Ipe / Iv. From the values in the table it can be seen that silicon has a very low value and as a result, this means that it is not normally one of the best options for a tunnel diode.

By Ian Poole

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