To determine a concentration of defects and/or impurities in a semiconductor wafer, a first value of current is caused to flow in the semiconductor wafer having a substrate of semiconducting material. The semiconductor wafer is exposed to a pulse of light whereupon electron-hole pairs generated in the semiconductor wafer in response to the light pulse cause the current to increase to a second value. After termination of the light pulse, the rate of change of the current from the second value toward the first value is determined. A concentration of defects and/or impurities in the semiconductor wafer is determined as a function of the rate of change.