Voltage drop process in high-power thyristors switched to a conducting state by an impact-ionization wave, which is excited by overvoltage pulse with a nanosecond rise time, has been investigated. In experiments the input voltage with a rise rate dU/dt in the range of 0.5 to 6 kV/ns was applied across the thyristor having 2 kV DC operating voltage. Numerical simulations show that calculated and experimentally observed times of the voltage drop are in good quantitative agreement in the case only when an active area of the structure, through which a switching current flows, depends on the value of dU/dt. The active area is increased with dU/dt increasing as well as with increasing of the silicon initial resistivity. The active area is approximated monotonously to the total area of the structure at dU/dt values of over 12 kV/ns.