Varactor diodes

- essentials about the varactor or varicap diode including abrupt and hyperabrupt varactor diodes with details about their specifications and how to use them.

from the website: http://www.radio-electronics.com

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. Accordingly they are used in many RF circuits including voltage controlled oscillators, and filters. 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 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 is quite simple. It 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.

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 diode circuit symbol

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.

Abrupt and hyperabrupt varactor diodes

Some varactor diodes may be referred to as abrupt and hyper-abrupt types. The term refers to the junction where the change between P and N types is either abrupt, or very / hyperabrupt. With a very sharp junction, these diodes offer a relatively large percentage change in capacitance. They are particularly useful when oscillators or filters need to be swept over a large frequency range.

The abruptness of the junction within the varactor diode is governed by the doping concentration. When the doping density in the active region does not change, i.e. constant doping level) the varactor is classed as an abrupt varactor diode. However, if the doping density does change as a function of distance, then the varactor is classed as a hyperabrupt varactor diode.

The class of diode, i.e. whether it is an abrupt varactor diode or a hyperabrupt varactor diode affects the tuning slope or gamma of the diode. A varactor with an abrupt junction will have a gamma of around 0.5, whereas the gamma for a hyperabrupt varactor diode is greater than 0.5.

Abrupt varactor diodes will generally offer a high level of Q, whereas the hyperabrupt varactor diodes offer a very large change in capacitance for a relatively small voltage change. This makes them ideal for use in battery powered equipment where the available voltages are small.

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 the re will be a spread in capacitance values that are obtainable, and this must be included in any calculations for the final circuit.

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.

Varactor diode equivalent circuit

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:

Q = 1 / 2 pi C_v R

Where:
C_v = 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.

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.

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.

Typical circuit using a varactor diode for tuning

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.

Applying varactor tuning voltage via resistor and inductor

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.

Varactor back-to-back drive

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.

Summary

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.