Variable Capacitance Diode Intro

Added Date:2007-06-13

In electronics, a varicap diode, varactor diode or tuning diode is a type of diode which has a variable capacitance that is a function of the voltage impressed on its terminals.

Applications

Varactors are commonly used in parametric amplifiers, parametric oscillators and voltage-controlled oscillators as part of phase-locked loops and frequency synthesisers. It is principally used as a voltage-controlled capacitor, and its rectifier function is secondary.

Operation

Varactors are operated reverse-biased so no current flows , but since the width of the depletion zone 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.

Non-diode varicaps

Not all varactors are formed by diodes. In CMOS processes, varactors can be formed by placing a heavily positively-doped region (called an P+ implant) inside a lightly negatively-doped region (called an n well.) The capacitance of these junctions behaves similarly to that of an NMOS transistor, which has an N+ implant inside a lightly positively-doped region (called a p well) - which also forms a P-N junction device.

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