Voltage drop process in power Si diode switched to a conducting state by an impact-ionization wave, which is excited by overvoltage pulse with a subnanosecond rise time, has been investigated. In experiments, a reverse voltage pulse was applied to a diode with a diameter of 6 mm without preliminary reverse bias, which provided the average rate of voltage rise across the diode dU/dt in the range of 1-10 kV/ns. Numerical simulations showed that calculated and experimentally observed voltage waveforms are in good quantitative agreement in the case when an active area of the structure Sa , through which a switching current flows, increases with dU/dt value increasing. It was shown that at dU/dt < 2 kV/ns the active area tends to zero, and at dU/dt > 10 kV/ns it approaches the total area of the structure. Comparison with the results of similar studies shows that the increase in the active area of the structure with the increase in the dU/dt value does not depend on the material of the structure (silicon and gallium arsenide), the number of layers in the semiconductor structure (diodes and thyristors), and also on the value of the initial bias voltage.