Diffusion Length Measurement by Means of Ionizing Radiation

Penetrating radiation in the form of high-energy electrons, heavy particles, and gamma rays may be used to determine minority-carrier diffusion lengths in semiconductor materials containing junctions by measuring the radiation-induced short circuit current. The electron-beam method yields an accurate absolute determination of diffusion length once the carrier-generation rate as a function of depth in the material has been measured. A series of such experiments is described for silicon solar cells utilizing electrons ranging in energy from 0.61 Mev to 1.16 Mev. A resultant maximum generation rate of 2.25 × 106± 5 per cent carriers/cm per incident 1 Mev electron is obtained at a depth of 0.096 gm/cm2. Measurements with 16.8 Mev and 130 Mev protons, and Co60gamma rays are found to be in good agreement with the electron-beam measurements. An experimental arrangement is described which yields rapid and accurate diffusion-length measurements of solar cells under conditions in which radiation damage is negligible.