Varactor diode
A varicap diode, varactor diode or tuning diode is a type of diode used in electronic circuits. It is principally used as a voltage-controlled capacitor, and its diode function is secondary. It is operated reverse-biased so no current flows through it, but since the width of the depletion region varies with the applied bias voltage, the capacitance of the diode can be made to vary. Generally, the depletion region width is proportional to the square root of the applied voltage; and capacitance is inversely proportional to the depletion region width. Thus, the capacitance is inversely proportional to the square root of applied voltage.
All diodes exhibit this phenomenon to some degree, but specially made varactor diodes exploit the effect to boost the capacitance and variability range achieved - most diode fabrication attempts to achieve the opposite.
Note that not all varactors are formed by diodes. In CMOS processes, varactors can be formed by placing a heavily positively-doped region (called an N+ implant) inside a lightly positively-doped region (called an NWELL.) The capacitance of these junctions behaves similarly to that of an NMOS transistor, which has an N+ implant inside a lightly negatively-doped region (called a PWELL) - which also forms a P-N junction device.
Varactors are commonly used in voltage controlled oscillators as part of Phase locked loops and Frequency Synthesisers.
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