Other diodes: Diode types
Some varactor diodes may be categorised as abrupt and hyperabrupt types - these terms may sometimes be seen in the varcator data sheets..
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.
Although many varactor diodes will just be selected and bought simply as varactor or varicap diodes, in some instances there will be mention of the terms abrupt varactor diode and hyper-abrupt varactor diodes.
Basics of abrupt & hyperabrupt varactor diodes
When selecting varactor diodes two of the major values that are of importance are the absolute capacitance value as well as the capacitance variation property.
The capacitance and the capacitance change characteristic are both functions of the doping profile of the diode when the basic diode silicon chip is manufactured as part of an overall silicon wafer.
BY altering the doping profile of the actual junction it is possible to change the characteristics of the diode and its absolute capacitance and capacitance range.
It is possible to determine the capacitance of the diode for any given voltage, although this needs to be used with care dependent upon the type of diode:
C is the capacitance for a given applied reverse voltage,
V is the applied voltage,
C0 is the voltage at zero volts,
γ is the slope of the curve of log C vs log V,
Φ is the diode potential which is 0.7V for silicon and 1.3 volts for gallium arsenide.
Abrupt varactor diodes
Abrupt varactor diodes are the more commonly used form of varactor. 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 and follows the equation in the section above well.
Typically for an abrupt varactor, the value of gamma is taken to be 0.5, although to be more exact a value of 0.47 is more accurate.
Hyperabrupt varactor diodes
In addition to this the hyperabrupt junction gives a much greater capacitance change for the given voltage change.
It can be seen from the diagram that the doing profile for the the hyperabrupt diode has the N+ and P++ areas and the central n area has an almost constant doping level close to the N+ area, but towards the P++ region, the doping level is increased so that there is a much sharper change from N to P++ than there is for the abrupt diode.
The modelling of the capacitance change that the hyperabrupt produces is quite complicated. The slope of the log C vs log (V + φ) varies with the applied voltage. This means that the capacitance change in terms of gamma is an approximation.
If the range over which the applied voltage is relatively narrow, then the capacitance equation can be used with an average value of gamma for that range.
If a wider voltage range is required, then it is normal to match sections of the curve if linearisation is needed. This may be needed for circuits like tracking filters. Here the position of the filter needs to be known for a given voltage and this will require the voltage capacitance curve to be mapped.
Although hyperabrup varactor diodes seem to offer significant advantages over abrupt diodes, their use comes at a cost, not only in the fact that they are more expensive. When using a hyperabrupt diode there is a substantial reduction in Q when compared to abrupt versions As a result hyperabrupt diodes are generally only used at lower microwave frequencies - up to a few GHz at most, although with technology improving, the upper frequencies are increasing.
Comparing the two types of diode:
When selecting which type of diode to use, it is necessary to look at the performance and capabilities of both types of device.
Abrupt varactor diodes
- Abrupt varactor diodes are typically made from silicon.
- Abrupt varactor diodes have a capacitance ratio (CR) of around 2:1.
- Abrupt varactor diodes have a Q factor of around 100 at 1 GHz.
- Abrupt varactor diodes are relatively inexpensive and easy to manufacture.
Hyperabrupt varactor diodes
- Hyperabrupt varactor diodes are typically made from gallium arsenide (GaAs) or indium gallium arsenide (InGaAs).
- Hyperabrupt varactor diodes have a CR of around 5:1 or higher.
- Hyperabrupt varactor diodes have a Q factor of around 1000 at 1 GHz
- Hyperabrupt varactor diodes are more expensive and more difficult to manufacture than abrupt varactor diodes.
When designing a circuit using abrupt or hyperabrupt varactor diodes, there are a number of factors to consider, including:
Capacitance ratio (CR): The CR of the varactor diode will determine the tuning range of the circuit.
Q factor: The Q factor of the varactor diode will determine the efficiency of the circuit.
Cost: Hyperabrupt varactor diodes are more expensive than abrupt varactor diodes for a variety of reasons including manufacturability and the cost of processes and materials.
Manufacturability: Hyperabrupt varactor diodes are more difficult to manufacture than abrupt varactor diodes. This could have an impact on the availability of these diodes, especially for the more sophisticated and high performance ones that may be manufactured to order.
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.
There can also be issues with using abrupt diodes where large capacitance ranges are needed as this can require very high voltages. This can mean that high voltage abrupt varactors are needed along with the possibility of separate supplies for the driver. Using a hyperabrupt can sometimes overcome these issues.
By investigating the requirements and possible solutions, it is normally possible to obtain the required performance in terms of Q and bandwidth and voltage range.
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