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Minority carrier lifetimes in germanium and silicon Dyment, John Cameron


An investigation of minority carrier lifetimes in germanium and silicon semiconducting material has been undertaken. A comparison of optical and electrical injection methods, as reported in the literature is given. An optical arrangement, including a coaxiallized spark gap system, is described. Sample preparation is discussed with respect to surface treatments. A well etched surface (CP4 etched) reduces the surface effect to negligible proportions compared to the volume effect. However, a ground or sandblasted surface caused the surface term to be dominant. The effects of constant current, photovoltaic effect at contacts, response time, and electrical field sweep-out of carriers are discussed. A brief description of a suitable electrical injection circuit is given. Measured values of lifetime using this method are in good agreement with the optical measurements. The volume lifetimes of n-type germanium were obtained for resistivities of 0.01, 5, 19, and 50 Ω-cm material. The lifetimes found at room temperature were 10⁻³ sec) for the p-type silicon only. Germanium at 78°K showed a trapping effect similar to the silicon at room temperature. A diffusion constant of 50 cm² /sec for holes in n-type material was used to establish a value for the surface recombination velocity of 2 x 10⁴ cm/sec for a ground surface.

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