An analysis of the frequency dependence of the capacitance of abrupt P− N junction semiconductor devices

Abstract

This work is a theoretical and experimental analysis of the frequency dependence of the capacitance of abrupt P− N junctions. The theoretical analysis is based on small-signal conditions and is applicable to devices in which the impurity ratio, R = N A N D , is large. The total capacitance of a junction device is made up of several components. The transition-region capacitance and the complex injection admittance comprise the junction effects. They are electrically in parallel. There is in series with the junction parameters, a complex conduction impedance due to the conduction process in the bulk regions. The conduction impedance is the ratio of the a.c. voltage across the bulk regions divided by the total current through the device. The frequency-dependent components are the injection admittance and the conduction impedance. The transition-region capacitance is found to be independent of frequency. The total capacitance is calculated for two germanium diodes as a function of applied d.c. bias. Results are obtained for six frequencies in the range from 1 to 100 Mc. Typical results are presented. Experimental verification of these theoretical results is obtained with two specially fabricated diodes. A comparison of theoretical and experimental results shows very good agreement.