Varactors are electronic devices, typically although not exclusively semiconductor PN junction devices, whose capacitance varies as a function of the applied voltage. The change in varactor capacitance with applied voltage comes about, for example, when the depletion region of a reverse biased junction widens and narrows with the applied voltage. This change in capacitance with applied voltage can be used to advantage to provide tuning and other functions where a voltage variable capacitance is useful. Varactors are much used for this purpose.
However the usefulness of varactors depends upon a number of factors such as the breakdown voltage, the tuning ratio and the series resistance. If a sufficiently large reverse voltage is applied, the varactor will breakdown or fail in generally the same manner as any semiconductor device. Obviously the maximum tuning voltage that can be applied is limited by the breakdown voltage. Hence, the breakdown voltage needs to be higher than the operating voltage. The tuning ratio (TR=Cjv=0/Cjv=V) is defined as the ratio of the capacitance Cjv at one voltage, e.g., v=0 volts, to the capacitance at another voltage, e.g., v=V volts, and is a measure of the amount of capacitance variation that can be usefully obtained. For example, the higher the tuning ratio, the greater the tuning range of an oscillator whose frequency is controlled by a varactor capacitance Cjv. So, it is desirable that the tuning ratio TR be large. Further, even though the varactor is DC reverse biased and little or no significant DC current flows, it is still carrying AC current so the series resistance contributes to AC losses. Hence, the series resistance of the varactor diode is important since it affects the “Q”, or quality factor, of the varactor as a tuning element. The higher the series resistance, the lower the Q.
It is usually the case with present day varactors that changing the device design or construction to improve one or more of the breakdown voltage, tuning ratio, or series resistance can adversely affect the others. Thus, a need continues to exist for improved varactor structures and methods wherein one or more of these factors, especially the tuning ratio, can be improved without adverse effect on the other varactor properties.