Other diodes: Diode types
The basics of tunnel diode theory enable an understanding of the oeprtion of the diode to be gained.
The tunnel diode characteristics and operation depend upon some of the subtle differences between a normal PN junction and structure of the tunnel diode itself.Essentially it is the very high doping levels used in the tunnel diode its unique properties and characteristics.
Tunnel diode theory shows that it does not act as a normal diode, but instead exhibits a negative resistance region in the forward direction.
The I-V characteristic curve, combined with the very high speed of the diode mean that the it can be used in a variety of microwave RF applications as an active device.
Tunnel diode theory basics
The key to understanding tunnel diode theory is the characteristic curve in which there is a negative slope – this indicates an area of negative resistance. An area of negative resistance means that if the voltage is increased, the current actually falls – the opposite to Ohms Law.
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 origin of the graph are virtually symmetrical.
The characteristic curve for the tunnel diode is made up from several different elements:.
- Normal diode current: This is the normal or expected 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 three constituents of the tunnel diode current sum together to give the overall characteristic curve that is often seen in explanations of tunnel diode theory.
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 Characteristics for Different Materials|
|Parameter / Characteristic||Germanium||Silicon||Gallium Arsenide|
|Vpe (mV)||40 - 70||80 - 100||90 -120|
|Vv (mV)||250 - 350||400 - 500||450 -600|
|Ipe/Iv||10 -15||3 - 5||10 - 20|
One of the key indicators for the performance of tunnel diodes is the peak to valley current ratio: Ipe / Iv. This gives a theoretical indication of the performance of the tunnel diode. Using the table it can be seen that silicon has a very low value for Ipe / Iv and accordingly theory aligns with practice and it is found that the performance of silicon tunnel diodes is not as good as that of germanium and gallium arsenide and other combinations.
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