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Varactor diode
Varactor diode tutorial includes:
• Varactor diode tutorial
• Varactor abrupt & hyperabrupt junctions
• Varactor specifications & parameters
• Varactor circuits
See also: Other types of diodes
• Varactor abrupt & hyperabrupt junctions
• Varactor specifications & parameters
• Varactor circuits
See also: Other types of diodes
Varactor diodes or varicap diodes are semiconductor devices that are widely used in the electronics industry and are used in many applications where a voltage controlled variable capacitance is required. Although the terms varactor diode and varicap diode can be used interchangeably, the more common term these days is the varactor diode.
Although ordinary PN junction diodes exhibit the variable capacitance effect and these diodes can be used for this applications, special diodes optimised to give the required changes in capacitance. Varactor diodes or varicap diodes normally enable much higher ranges of capacitance change to be gained as a result of the way in which they are manufactured. There are a variety of types of varactor diode ranging from relatively standard varieties to those that are described as abrupt or hyperabrupt varactor diodes.
Varactor diode applications
Varactor diodes are widely used within the RF design arena. They provide a method of varying he capacitance within a circuit by the application of a control voltage. This gives them an almost unique capability and as a result varactor diodes are widely used within the RF industry.
Although varactor diodes can be used within many types of circuit, they find applications within two main areas:
- Voltage controlled oscillators, VCOs: Voltage controlled oscillators are used for a variety of applications. One major area is for the oscillator within a phase locked loop - this are used in almost all radio, cellular and wireless receivers. A varactor diode is a key component within a VCO.
- RF filters: Using varactor diodes it is possible to tune filters. Tracking filters may be needed in receiver front end circuits where they enable the filters to track the incoming received signal frequency. Again this can be controlled using a control voltage. Typically this might be provided under microprocessor control via a digital to analogue converter.
Varactor diode basics
The varactor diode or varicap diode consists of a standard PN junction, although it is obviously optimised for its function as a variable capacitor. In fact ordinary PN junction diodes can be used as varactor diodes, even if their performance is not to the same standard as specially manufactured varactors.
The basis of operation of the varactor diode is quite simple. The diode is operated under reverse bias conditions and this gives rise to three regions. At either end of the diode are the P and N regions where current can be conducted. However around the junction is the depletion region where no current carriers are available. As a result, current can be carried in the P and N regions, but the depletion region is an insulator.
This is exactly the same construction as a capacitor. It has conductive plates separated by an insulating dielectric.
The capacitance of a capacitor is dependent on a number of factors including the plate area, the dielectric constant of the insulator between the plates and the distance between the two plates. In the case of the varactor diode, it is possible to increase and decrease the width of the depletion region by changing the level of the reverse bias. This has the effect of changing the distance between the plates of the capacitor.
Varactor diode symbol
As the primary function of a varactor diode is as a variable capacitor, its circuit symbol represents this. Sometimes they may be shown as ordinary diodes, whereas more usually the varactor diode circuit symbol shows the bar as a capacitor, i.e. two lines.
Varactor diodes are always operated under reverse bias conditions, and in this way there is no conduction. They are effectively voltage controlled capacitors, and indeed they are sometimes called varicap diodes, although the term varactor is more widely used these days.
Varactor diodes, or as they are sometimes called, varicap diodes are a particularly useful form of semiconductor diode. Finding uses in many applications where electronically controlled tuning of resonant circuits is required, for items such as oscillators and filters, varactor diodes are an essential component within the portfolio of the electronics design engineer. However to be able to use varactor diodes to their best advantage it is necessary to understand features of varactor diodes including the capacitance ratio, Q, gamma, reverse voltage and the like. If used correctly, varactor diodes provide very reliable service particularly as they are a solid state device and have no mechanical or moving elements as in their mechanical variable capacitor counterparts.
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.
When choosing a varactor diode, the varactor specifications need to be carefully determined to assess whether it will meet the circuit requirements.
While there will be many varactor diode specifications that are the same as those applied to other types of diode, including signal diodes, etc, there are many other varactor specifications that are crucial to the performance of the varactor in any variable capacitance role.
Many of the different varactor parameters will be detailed in the varactor specification sheets that may be accessed in the manufacturers literature.
Capacitance range and capacitance ratio
The actual capacitance range which is obtained depends upon a number of factors. One is the area of the junction. Another is the width of the depletion region for a given voltage.
It is found that the thickness of the depletion region in the varactor diode is proportional to the square root of the reverse voltage across it. In addition to this, the capacitance of the varactor is inversely proportional to the depletion region thickness. From this it can be seen that the capacitance of the varactor diode is inversely proportional to the square root of the voltage across it.
Diodes typically operate with reverse bias ranging from around a couple of volts up to 20 volts and higher. Some may even operate up to as much as 60 volts, although at the top end of the range comparatively little change in capacitance is seen.
One of the key parameters for a varactor diode is the capacitance ratio. This is commonly expressed in the form Cx / Cy where x and y are two voltages towards the ends of the range over which the capacitance change can be measured.
For a change between 2 and 20 volts an abrupt diode may exhibit a capacitance change ratio of 2.5 to 3, whereas a hyperabrupt diode may be twice this, e.g. 6.
However it is still necessary to consult the curves for the particular diode to ensure that it will give the required capacitance change over the voltages that will be applied. It is worth remembering that there will be a spread in capacitance values that are obtainable, and this must be included in any calculations for the final circuit.
Reverse breakdown
The reverse breakdown voltage of a varactor diode is of importance. The capacitance decreases with increasing reverse bias, although as voltages become higher the decrease in capacitance becomes smaller. However the minimum capacitance level will be determined by the maximum voltage that the device can withstand. It is also wise to choose a varactor diode that has a margin between the maximum voltage it is likely to expect, i.e. the rail voltage of the driver circuit, and the reverse breakdown voltage of the diode. By ensuring there is sufficient margin, the circuit is less likely to fail.
It is also necessary to ensure that the minimum capacitance required is achieved within the rail voltage of the driver circuit, again with a good margin as there is always some variation between devices.
Diodes typically operate with reverse bias ranging from around a couple of volts up to 20 volts or possibly higher. Some may even operate up to as much as 60 volts, although at the top end of the range comparatively little change in capacitance is seen. Also as the voltage on the diode increases, it is likely that specific supplies for the circuits driving the varactor diodes will be required.
Maximum frequency of operation
There are a number of items that limit the frequency of operation of any varactor diode. The minimum capacitance of the diode is obviously one limiting factor. If large levels of capacitance are used in a resonant circuit, this will reduce the Q. A further factor is any parasitic responses, as well as stray capacitance and inductance that may be exhibited by the device package. This means that devices with low capacitance levels that may be more suitable for high frequencies will be placed in microwave type packages. These and other considerations need to be taken into account when choosing a varactor diode for a new design.
As a particular varactor diode type may be available in a number of packages, it is necessary to choose the variant with the package that is most suitable for the application in view.
Varactor Q
An important characteristic of any varactor diode is its Q. This is particularly important in a number of applications. For oscillators used in frequency synthesizers it affects the noise performance. High Q diodes enable a higher Q tuned circuit to be achieved, and in turn this reduces the phase noise produced by the circuit. For filters the Q is again very important. A high Q diode will enable the filter to give a sharper response, whereas a low Q diode will increase the losses.
The Q is dependent upon the series resistance that the varactor diode exhibits. This resistance arises from a number of causes:
- the resistance of the semiconductor in the areas outside the depletion region, i.e. in the region where the charge is carried to the "capacitor plates".
- some resistance arising from the lead and package elements of the component
- some contribution from the die substrate
The Q or quality factor for the diode can be determined from the equation below:
Where:
Cv = the capacitance at the measured voltage
R = the series resistance
Cv = the capacitance at the measured voltage
R = the series resistance
From this it can be seen that to maximise the Q it is necessary to minimise the series resistance. Varactor diode manufacturers typically use an epitaxial structure to minimise this resistance.
When designing the circuit, the Q of the circuit can be maximised by minimising the capacitance.
There are many aspects to using varactor diodes in RF electronic circuits. The configuration to the varactor circuits can affect their operation.
In view of the fact that RF circuits are not always easy to optimise, it is necessary to ensure the varactor circuits utilise the best methods of driving varactor diodes as well as the most successful circuits.
Driving varactor diodes
The varactor diode requires the reverse bias to be applied across the diode in a way that does not affect the operation of the tuned circuit of which it is part. Care must be taken to isolate the bias voltage from the tuning circuit so that the RF performance is not impaired.
Typically the cathode is earthed or run at the DC common potential. The other end can then have the bias potential applied. The bias circuitry needs to be isolated for RF signals from the tuned circuit to prevent any degradation of the performance. Either a resistor or an inductor can be used for this as the diodes operate under reverse bias and present a high DC resistance.
Inductors can operate well under some situations as they provide a low resistance path for the bias. However they can introduce spurious inductance and under some circumstances they may cause spurious oscillations to occur when used in an oscillator. Resistors may also be used. The resistance must be high enough to isolate the bias circuitry from the tuned circuit without lowering the Q. They must also be low enough to control the bias on the diode against the effects of the RF passing through the diode. A value of 10 kohms is often a good starting point.
The varactor diodes may be driven in either a single or back to back configuration. The single varactor configuration has the advantage of simplicity. The back-to-back configuration overcomes the problem of the RF modulating the tuning voltage as the effect is cancelled out - as the RF voltage rises, the capacitance on one diode will increase and the other decrease. The back-to-back configuration also halves the capacitance of the single diode as the capacitances from the two diodes are placed in series with each other. It should also be remembered that the series resistance will be doubled and this will affect the Q.
When designing a circuit using varactor diodes, care must be taken to ensure that the diodes do not become forward biased. Sometimes, especially when using low levels of reverse bias, the signal in the RF section of the circuit may be sufficient over some sections of the cycle to overcome the bias and drive the diode into forward conduction. This leads to the generation of spurious signals and other nasty unwanted effects.
Surface Mount Technology (SMT)
In recent years there has been a drammatic change from the use of leaded components to surface mount technology. These SMT components make the manufacturing process much easier and faster.
Passive components
Semiconductor basics
Transistors and diodes are in widespread use today. Many millions are used each day apart from those that are incorporated in integrated circuits.
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