Tunnel Diode Structure & Fabrication

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

The basic structure of the tunnel diode is similar to that of an standard PN junction.

However there are a few key differences that enable the tunnel diode to operate differently to the ordinary PN junction.

Even so, the basic processes and fabrication techniques used with main-line semiconductor technology can be used for the tunnel diode.

Tunnel diode structure basics

The tunnel diode is similar to a standard p-n junction in many respects except that the doping levels are very high. Densities of the order of 5x10^19 cm^-3 are common.

A further difference is that 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, which equates to a width of only a few atoms.

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.

In terms of the material used for these diodes, the favoured semiconductor is germanium. Although other materials can be used and have been used, germanium has the advantage that it has a small energy gap that allows for more efficient tunnelling.

Tunnel diode fabrication structures

Tunnel diode structures generally fall into one of three basic structures:

  • Ball alloy:   This type of tunnel diode format is fabricated as a mesa structure. To achieve this form of structure, the fabrication technique involves bringing an alloy containing the required dopants into contact with a heavily doped substrate. The temperature used is around 500°C at which point the dopants quickly melt and diffuse into the substrate. The overall structure geometry is then defined by etching the diode to the required proportions.

    The tunnel diode device physical structure for the ball alloy method of fabrication
    Tunnel diode ball alloy structure

  • Pulsed bond:   This is a relatively straightforward structure to create, although careful process control is required during the fabrication process. The diode is created by using a wire coated with an alloy containing the required dopants. This is pressed hard onto the heavily doped substrate, and then a voltage pulse is applied. The effect of this is that the junction forms by a process of local alloying.

    The tunnel diode device physical structure for the pulsed bond method of fabrication
    Tunnel diode pulsed bond structure

    Despite this, there are drawbacks to this process because it can only produce a small junction, and the exact properties, including the area of the junction cannot be controlled tightly.
  • Planar structure:   Planar technology can be used to create the diode. Using this approach for the fabrication process, the heavily doped n+ substrate is masked off by an insulating layer to leave a small area exposed. This exposed area is then open to become the active area of the diode.

    The tunnel diode device physical structure for the planar method of fabrication
    Tunnel diode planar structure

    The doping for the area can be introduced by one of a number of means. It can be introduced by diffusion, alloying or epitaxial growth. Alternatively it is possible to grow an epitaxial layer over the whole surface and then etch away those areas that are not required to leave a mesa structure.

All three structures enable high performance diodes to be obtained.

Although these are three popular structures for tunnel diodes, new developments are occurring using different materials and also involving new structures that offer a greater variety of characteristics, or they may be tailored to the needs of a particular material that may be used.

By Ian Poole

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