The ambipolar diffusion coefficient in silicon: Dependence on excess-carrier concentration and temperature

Recent articles have been published claiming that the ambipolar diffusion coefficient of semiconductors, Da, is basically independent of the excess-carrier concentration, thus contradicting the conventional result of, for example, Fletcher. However, only a few experimentally verified papers on this subject are published. In the present work, a detailed experimental analysis of the ambipolar diffusion coefficient is presented regarding both temperature and injection-level dependence. The ambipolar diffusion coefficient was measured in the low-doped n base of a p-i-n type diode at different excess-carrier concentrations and temperatures using an open-circuit carrier decay (OCCD) method based on the free-carrier absorption (FCA) technique. This investigation was performed in the carrier-concentration range of 1015–2×1017 cm−3 and in the temperature range of 300–420 K, respectively. The ambipolar diffusion coefficient is experimentally found to behave in reasonable agreement with Fletcher’s theory, thus decreasing with increasing excess-carrier concentration.