Summary of the varactor or varicap 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.
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