Summary of the Tunnel Diode

The tunnel diode was found many microwave applications because semiconductor devices of the day could not reach these frequencies. Although not widely used today, it is still sometimes mentioned and it is a fascinating device.

The tunnel diode was discovered by a Ph.D. research student named Esaki in 1958 while he was investigating the properties of heavily doped germanium junctions for use in high speed bipolar transistors. In the course of his research he produced some heavily doped junctions and as a result found that they produced an oscillation at microwave frequencies as a result of the tunnelling effect. It was subsequently found that other materials including gallium arsenide also produced the same effect.

Structure
The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Also the depletion region, the area between the p-type and n-type areas, where there are no carriers is very narrow. Typically it is in the region of between five to ten nano-metres – only a few atom widths.

As the depletion region is so narrow this means that if it is to be used for high frequency operation the diode itself must be made very small to reduce the high level of capacitance resulting from the very narrow depletion region.

Mode of operation
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. The reason for this is that there are a number of different components to forming the overall curve. The main two are the normal diode current across the junction, and the current arising from the tunnelling effect. It is this last component that is of interest in a tunnel diode.

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 barrier.

The tunnelling only occurs under certain conditions. This means that it peaks when a certain voltage is placed across the junction. This results in the current increasing to a point beyond that which would be expected for a standard pn junction. As the voltage across the diode is increased the effect reduces and the current through the device falls. This results in a negative resistance region on the curve of te diode that can be used to provide gain.

Advantages and disadvantages
One of the main reasons for the early success of the tunnel diode was its high speed of operation and the high frequencies it could handle. This resulted from the fact that while many other devices are slowed down by the presence of minority carriers, the tunnel diode only uses majority carriers, i.e. holes in an n-type material and electrons in a p-type material. The minority carriers slow down the operation of a device and as a result their speed is slower. Also the tunnelling effect is inherently very fast.

The device is rarely used these days and this results from its disadvantages. Firstly they only have a low tunnelling current and this means that they are low power devices. While this may be acceptable for low noise amplifiers, it is a significant drawback when they are sued in oscillators as further amplification is needed and this can only be undertaken by devices that have a higher power capability, i.e. not tunnel diodes. The third disadvantage is that they are problems with the reproducibility of the devices resulting in low yields and therefore higher production costs.

Applications
Although the device appeared promising some years ago, it was soon replaced by other semiconductor devices like IMPATT diodes for oscillator applications and FETs when used as an amplifier. Nevertheless it is an interesting device.