Varactor Diode

The term "varactor" comes from the English expression for variable reactor and indicates semiconductor diodes, which utilize the dependence of the diode capacitance on the operating point of the diode. 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. Varactors are commonly used in voltage-controlled oscillators as part of phase-locked loops and frequency synthesisers. In addition to the small serial resistance of the diodes and the voltage dependence of the capacitance, the matching behavior is also essential.

The calculated capacitance C as a function of the reverse voltage U for a hyperabrupt diode (m=-1), for an abrupt diode (m=0) and a linear diode with m=1.	The theoretical impurity concentration profile for a one-sided p+/n junction with m=0 (abrupt), m=1 (linear) and m = -1 (hyperabrupt).

The illustration on the above left shows the theoretically computed voltage dependence of the capacitance of three different diodes. The supposed doping profiles for m = 0, 1 and -1 result in an abrupt, linear and hyperabrupt diode, respectively. The illustration on the right above represents the computed doping distribution within the range of 0 to 3 µm at the pn junction.

Hint:

There's an interactive simulation available on the "Learning by Simulations" Web site which allows to calculate the characteristics of a varactor diode.

In practice, such theoretical profiles can be obtained only with difficulty. Complicated diffusion processes cause non-constant concentration-dependent diffusion coefficients and therefore result in deviations from the ideal dopings. Diodes are usually manufactured with complex technologies such as multiple diffusion or multiple implantation.

For developing varactor diodes, a compromise and/or an optimization always takes place between C-U characteristic, reverse voltage, diode quality factor and capacitance ratio. Within an IC technology, integration is only attainable with very much difficulty due to the application of the additionally necessary processes and the high precision attained with hyperabrupt diodes. Here, active and passive components integrated on a special substrate will supply the crucial contribution for future miniaturization.

The cross section of a varactor diode shows the characteristic multiple doping.