Other diodes: Diode types
Some varactor diodes may be categories as abrupt and hyperabrupt types.
The terms abrupt varactor diode and hyperabrupt varactor diode refer to the properties of the varactor diode junction. The form of junction in the diodes has a major impact on the properties and performance.
Often when selecting a varactor diode for a particular application, it may be necessary to select whether an abrupt or hyperabrupt type is needed to meet the requirements of the circuit. As there are trade-offs for using one type of varactor to the other, it is good to have an understanding of the terminology.
Basics of abrupt & hyperabrupt varactor diodes
One of the key parameters for a varactor diode is the capacitance voltage curve. It is this curve that is one of the most important when selecting a particular type of varactor diode.
It is possible to obtain the required profile for the PN junction and hence the required C-V characteristic by controlling the doping profile.
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 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.
The choice of abrupt or hyperabrupt varactor diode can have a major impact upon the performance of the circuit. Often it is a balance between properties like Q and the bandwidth over which the diode needs to operate. By investigating the requirements and possible solutions, it is normally possible to obtain the required performance in terms of Q and bandwidth.
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