INVESTIGATIONS ON THE DIFFUSION OF MINORITY CARRIERS FROM A POINT ON SILICON

Abstract

The field-free diffusion of minority carriers injected at a point in silicon has been studied and the time from injection to the maximum of the collector signal due to the arriving carriers measured for various emitter–collector distances. Firstly, it was found that for 2-ohm-cm, 6-microsecond, n-type material the time to maximum was proportional approximately to the emitter–collector spacing raised to the power 1.2 for flat surfaces and to the power 1.6 for 3- and 6-degree wedge samples. The power 1.6 was also found for a 17-degree wedge of 120-ohm-cm, 100-microsecond, p-type silicon. Secondly, decay times following the maxima appeared to increase with emitter–collector spacing. No adequate two-surface theory is available but comparison with a one-surface theory revealed serious disagreements. It would appear that the results may be explained qualitatively by assuming that the effective lifetime is dependent on the excess carrier density and decreases as the emitter point is approached.