Semiconductor junction varactors with high voltage-sensitivity

For many varactor applications, structures with a large relative capacitance variation would be desirable. Two such structures have been investigated, namely, the hyperabrupt junction and the "pagoda" structure in which junction area varies with bias. All junctions are assumed to be
p^{+}n
type with an
n^{+}
substrate, and the width of the
n
region is optimum, i.e., completely swept out just at breakdown voltage. The series resistance is assumed to be contributed by the
n
region alone and is equal to the value at zero bias. Structures having equal breakdown voltages are intercompared with usual step junction varactors with respect to two figures of merit: 1) the dynamic quality factor
\tilde{Q}
defined by Kurokawa and Uenohara for low-noise reactance amplification, and 2) the transducer gain g
T
derived by Hyltin and Kotzebue for varactor harmonic generation. It is found for both applications that improvement due to increased capacitance-voltage sensitivity is offset or more than offset by the concomitant increase in RC product. For example, in the case of low-noise amplification, the improved relative capacitance variation can boost
\tilde{Q}
by a factor of about 3 to 5, but the accompanying increase in RC product lowers Q by a factor of about 4 to 16. However, in some cases the resistance in the junction may not be the factor which limits circuit performance. It may for instance be dominated by constant resistances such as those in the semiconductor bulk, ohmic contacts, or external circuits. In such a case the improvement in voltage sensitivity may be desirable.