Varactor Diode

Varactor 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 circuits including voltage controlled oscillators, and filters. Sometimes these varactor diodes are also known as varicap or vari-cap diodes.

Their operation is based on the fact that a reverse biased PN junction acts as a small variable capacitor. Varying the reverse voltage changes the capacitance. Although any ordinary diode may be used in this way, diodes manufactured specifically for this purpose can offer controlled and higher levels of capacitance.

Concept
The varactor diode employs a standard PN junction. As the name suggests these consist of an area of P type material and a region of N type material. The N type region has a surfeit of electrons, whereas the P type region has a shortage and what are termed holes are generated. These are empty “slots” available for an electron within the crystal lattice of the semiconductor. Both electrons and holes can move around the lattice and in this way current flows if there is a general drift in one particular direction.

In the region where the P and N type semiconductor adjoin it is found that the electrons and holes combine and there are no carriers available to give rise to any current flow. It is this gap that is used as the dielectric between the two plates of the capacitor. The two plates being formed by the boundary of where the carriers are available to conduct electricity. As the capacitance of a capacitor is related to the distance between the two plates, it is possible to change the capacitance by varying the width of the depletion region.

If a voltage is applied across the device the width of the depletion layer changes. The greater the level of reverse bias that is placed across the diode, the greater the depletion region becomes and the further apart the “plates” become, and the smaller the capacitance across the diode. If a forward bias is placed across the diode, the depletion region reduces and eventually conduction takes place.

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 vari-cap diodes, although the term varactor is more widely used these days.

Parameters
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. This is governed by the doping concentration and it is normally adjusted to give a relatively abrupt junction which results in a greater capacitance change.

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.

Specifications
The most important characteristics of the diode are its capacitance and the range of capacitance that can be achieved. Normally two voltage points are specified, one at the top of the range and the other near the bottom at the minimum useable voltage. It is obviously important to select a diode which combines the correct capacitance range for the available tuning voltage range. The higher voltage specified is normally the maximum reverse bias and this should not be exceeded otherwise breakdown may occur.

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.

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 / hyper abrupt. 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.

Some very interesting information about varactor diodes is given in

http://www.hanssummers.com/radio/varicap/index.htm