Varactor Abrupt & Hyperabrupt Diodes

- essentials of the varactor abrupt and hyperabrupt diodes, what these terms mean and the key essentials of these forms of varactor diode.

Some varactor diodes may be referred to as abrupt and hyper-abrupt types. The terms of abrupt varactor diode, or hyperabrupt varactor diode refers to the properties of the varactor diode junction/

The abrupt varactor and hyperabrupt varactor diodes differ in the ways that thee junctions are fabricated. This gives significant differences in the performance of the two types of varactor diode.

Abrupt and hyperabrupt varactor basics

Often circuits using varactor diodes need a specific type of performance. In particular the capacitance voltage, C-V curve may need to be of a particular shape or have a particular relationship.

By controlling the doping in the manufacturing process, it is possible to obtain the required profile for the PN junction and in this way control the C-V characteristic for the varactor diode.

As might be expected, the names of the different varactor diodes refer to the profile of the PN junction itself and this provides for very different values and properties for some of the parameters.

Abrupt varactors and hyperabrupt varactors have different properties as detailed below.

Abrupt varactor diodes

Abrupt varactor diodes are the more commonly used for of diode. As the abruptness of the junction is governed by the doping concentration and also the profile, this is controlled during the manufacture. For an abrupt varactor diode the doping concentration is held constant, i.e constant doping level as far as reasonably possible.

The abrupt varactor exhibits an inverse square law C-V function. This provides for an inverse fourth law frequency dependence. In applications where a linear dependence is required, a lineariser is needed. This takes additional circuitry that may be an additional burden for some applications, not only in terms of circuitry, but also the slower response speed caused by the lineariser.

Hyperabrupt varactor diodes

Hyperabrupt junctions provide a C-V curve that has an inverse square law curve over at least some of the characteristic. This provides a narrow band linear frequency variation.

In addition to this the hyperabrupt junction gives a much greater capacitance change for the given voltage change.

The advantages of the hyperabrupt varactor come at a cost as there is a substantial reduction in Q when compared to abrupt varactor diode. As a result hyperabrupt diodes are generally only used at lower microwave frequencies - up to a few GHz at most.

By Ian Poole

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