A novel technique is introduced to study fast diffusing charged defects in semiconductors. It is based on the capacitance change induced by ion drift in a reverse biased Schottky barrier. It is shown that such charge movement yields exponential capacitance transients, which contain information about the defect concentration and mobility. The method is checked on Li-diffused samples, where the extracted diffusion coefficient are in good agreement with literature data. It is next applied to interstitial copper (Cui) in silicon. In the proposed experiment Cui gives rise to a well defined signal which enables us to investigate near room temperature defect reactions involving Cui. The diffusion data extracted from copper diffused and quenched silicon samples establishes the origin of the signal. Near room temperature precipitation kinetics of Cui are studied and energy barriers are extracted.