INVESTIGATION OF CARRIER TRANSPORT THROUGH SILICON-WAFERS BY PHOTOCURRENT MEASUREMENTS

Measurement of the ac photocurrent in metal/insulator/semiconductor capacitors can be used as a tool to measure minority-carrier diffusion and lifetime. The amplitude of the ac photocurrent generated at a silicon surface biased into inversion depends on the number of excess minority carriers present at that surface. By comparing this amplitude when intensity-modulated light is directed to each side of the same device, minority-carrier from the back to the front of the device can be characterized. An analytical model of this transport process predicts the dependence of the ac photocurrent on frequency and wafer thickness, and allows the determination of a value of the bulk lifetime free of the influence of surface recombination. Measurements under low-light intensity levels are presented on n-type silicon wafers with lifetimes in the 10-100 mus range. Lifetimes are found about a factor of 2 lower than those measured with noncontact photoconductive decay, at high-light intensity levels. This is expected due to the difference between high- and low-level minority-carrier injection. Fitting the data to the model also yields a value of 115 mum for the average depth at which carriers are generated and diffuse to the front with backside illumination at 940 nm.