Tunnel Diode Tutorial

- a summary, tutorial or reference giving the essentials and basics of the tunnel diode found in many microwave applications.

The tunnel diode is a form of very fast semiconductor diode that can operate well into the microwave radio frequency region.

It differs from other forms of semiconductor diode in that it uses a quantum mechanical effect called tunnelling. This provides the tunnel diode with a negative resistance region in its IV characteristic curve that enables it to be used as an oscillator and as an amplifier.

Although they are not as widely used as some devices today, these devices do have their place within RF technology. They were used in television receiver front end oscillators and oscilloscope trigger circuits, etc. They have been shown to have a very long life and can offer a very high level of performance when used as an RF pre-amplifier. However today, their applications are often limited because more traditional three terminal devices can offer a better level of performance in many areas.


Tunnel diode development

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 Esaki produced some heavily doped junctions for high speed bipolar transistors and as a result he found that they produced an oscillation at microwave frequencies as a result of the tunnelling effect.

Then in 1973, Esaki received the Nobel prize for Physics for his work on the tunnel diode.

After the work by Esaki, other researchers demonstrated that other materials also showed the tunnelling effect. Holonyak and Lesk demonstrated a Gallium Arsenide device in 1960, and others demonstrated Indium tin, and then in 1962 the effect was demonstrated in materials including Indium Arsenide, Indium Phosphide and also Silicon.


Tunnel diode circuit symbol

Despite the operation of the tunnel diode. its circuit symbol is based on that for the standard diode, but has 'tails' added to the bar element of the symbol to differentiate it from other forms of PN junction diode.

The circuit symbol for the tunnel diode showing the slightly different arrangement on the bar to differentiate from other forms of diode
Tunnel diode circuit symbol


Advantages and disadvantages

The tunnel diode is not as widely used these days as it was oat one time. With the improvement in performance of other forms of semiconductor technology, they have often become the preferred option. Nevertheless it is still worth looking at a tunnel diode, considering its advantages and disadvantages to discover whether it is a viable option.

Advantages

  • Very high speed:   The high speed of operation means that the tunnel diode can be used for microwave RF applications.
  • Longevity:   Studies have been undertaken of the tunnel diode and its performance has been shown to remain stable over long periods of time, where other semiconductor devices may have degraded.

Disadvantages

  • Reproducibility:   It has not been possible to make the tunnel diode with as reproducible performance to the levels often needed.
  • Low peak to valley current ratio:   The negative resistance region and the peak to valley current is not as high as is often be required to produce the levels of performance that can be attained with other devices.

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 tunnel diode 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 tunnel diode 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 the tunnel diode is a useful device for certain applications.

Applications for the tunnel diode included uses as an oscillator, although it was also used as an amplifier and a mixer.

One of the major advantages of the tunnel diode which is currently beginning to be experienced is its longevity. Once manufactured its performance remains stable over long periods of time despite its use. Other devices might degrade slightly over time.

By Ian Poole


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