Varactors normally exhibit a voltage-dependent capacitance and can be fabricated from a variety of semiconductor materials. Varactor frequency multipliers are extensively used to provide local oscillator power to sensitive millimeter- and submillimeter-wavelength receivers. The inherently limited bandwidth of varactors can be improved by employing a transmission line periodically loaded with varactors. Since the standard varactor model consists of a resistance in series with a nonlinear capacitance, the elastance is used rather than the capacitance. Based on whether the multiplication is due to a nonlinear resistance or a nonlinear reactance, one can differentiate between the varistor and varactor type of multipliers. However, real devices exhibit properties and parameters that are a mixture of the ideal varistor and the ideal varactor multiplier. The varactor is useful as a frequency converter because of its good noise properties, and because gain can be achieved.