Frequency Dependence of Junction Capacitance

Abstract

Diode junction capacitance is usually assumed to be a frequency independent parameter, varying only with applied reverse bias. This frequency independence is true in the case of ideal junction diodes up to very high frequencies above which dispersion at the space-charge edge becomes appreciable. Traps or deep centres in a p-n junction will cause effects similar to the free carrier dispersion. The junction capacitance of diodes fabricated with material containing traps or deep impurity levels will show a strong frequency dependence at low frequencies. The paper gives the junction capacitance values observed on certain diodes as a function of frequency. The results have been analysed to obtain the total junction capacitance C_t as a series combination of a frequency independent capacitance C_o (which is voltage dependent), and a frequency dependent capacitance C^f. It is proposed in the paper that such a study of the capacitance frequency behaviour of a junction diode is a practical way of determining the suitability of a given fabrication procedure (i.e. starting material, method of handling and creation of junction) for the given objective.