Evaluation of voltage-dependent series resistance of epitaxial varactor diodes at microwave frequencies

The series resistance of a high-quality varactor diode is primarily determined by the resistance of the semiconductor material close to the junction. With increasing reverse bias, the width of the space-charge region becomes greater, and the series resistance decreases, Theoretical models of graded and step junctions have been assumed, and calculations have been made of the series resistance as a function of bias. Epitaxial silicon diodes have been measured for series resistance as a function of bias by using the transmission loss method at 6 to 12 Gc/sec., with the diode mounted across a reduced-height waveguide. The variation of series resistance with bias agrees well with the theoretical calculations. By measurement of the 3-db bandwidth of the series resonance of the diode mounted in the reduced-height waveguide, the junction capacitance and the effective series inductance of the package also can be determined.