A diode under reverse bias may exhibit a capacitance that varies inversely with the applied voltage. A component that behaves in this manner, e.g., as a variable capacitor, may be termed a varactor. The variable capacitance of the diode may be used for “tuning” electrical circuits. Generally, semiconductor varactors may have a wider tuning range (e.g., capacitance variance) and lower control voltage requirements than dielectric varactors realized on materials such as barium strontium titanate (BST). However, the semiconductor varactors may typically exhibit a lower capacitance per unit area than a dielectric varactor, thereby requiring a larger die area to implement a given capacitance.
Generally, a varactor may be considered a two-port device, e.g., having a single input terminal and a single output terminal. As such, varactors may be prone to self-modulation distortion resulting from applied radio frequency (RF) voltages. This self-modulation distortion may introduce nonlinearity into a circuit using the varactors. To reduce this nonlinearity to acceptable levels, a number of individual varactors may be coupled in series to divide the RF voltage across them. If the number of varactors in the series is n, then the die area on the circuit board required to realize a desired net capacitance may be increased by a factor of n2 if the varactors are co-planar to one another. If a relatively large number of varactors are used, then this circuit may make the required die area prohibitively large for use in modern devices.
In some cases such as oscillator or voltage-controlled oscillator (VCO) circuits in a mobile device, a high quality factor of greater than approximately 50 for the varactor may be desirable for increasing efficiency or reducing battery drain of the mobile device. Generally, the quality factor may be considered to be a measurement of the reactance of the varactor (e.g., the impedance presented to an RF signal propagating through the varactor) compared to the resistance of the varactor. However, if the collector or sub-collector of the varactor has a relatively high resistivity, such a quality factor may be difficult to achieve.