CONDITIONS FOR NEGLECTING SPACE-CHARGE EFFECTS ON DISTRIBUTIONS OF POINT-DEFECTS AND IV RELATIONS

Local neutrality is normally assumed in calculating the distributions of point detects and the I-V relations in systems, in which the defect s are mobile, though the Poisson equation with a nonzero space charge is the exact condition to be used. The question when local neutrality holds, al least approximately, and can be used in the analysis, is exa mined here. It is shown that local neutrality prevails (approximately) as long as gradients in the concentration of the majority charged def ects are small. Measures for these gradients are given for representat ive models. It is shown that these gradients may be of different origi n. They can be caused by non uniform doping, electrical field, gradien t in chemical potential and a combination of these factors.