# Varactor diode specifications

### In this section

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.

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 Cv R

Where:
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.

By Ian Poole

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