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UBC Theses and Dissertations

Bulk photo-effects in inhomogeneous semiconductors Cox, Clarence Donald


An observable e.m.f. exists in a semiconductor when a non-equilibrium carrier concentration is present in a region of electrostatic potential gradient. These two conditions may arise in a variety of ways, and the e.m.f.'s associated with various combinations of conditions are listed. One of these e.m.f.'s arises from a photo-generated non-equilibrium carrier concentration in regions of electrostatic potential gradient due to an inhomogeneous impurity distribution. The thesis is chiefly concerned with the extension of the theory of this bulk photo e.m.f. and its comparison with experiment. Previous work on the subject is reviewed and the theory is developed to cover all conditions of illumination in the region of an arbitrary impurity density gradient. An expression for the bulk photo e.m.f. is derived by two approaches, integration of the electrostatic potential gradient, and integration of the carrier quasi Fermi levels, over the illuminated region. The latter derivation is shown to be more general in its application, and is used to obtain an expression for the photo e.m.f. both in an illuminated p-n junction and in an illuminated bulk inhomogeneity. Using the general result, expressions for the e.m.f. are written for the extreme cases of weak and strong illumination in extrinsic and nearly intrinsic semiconductors. The relation between bulk photo e.m.f. and photoconductive resistance decrease is examined. Measurements were made of the bulk photo e.m.f. as a function of light intensity. The proportionality of the effect at low levels of illumination was verified. Observations of the photo e.m.f. patterns showed a maximum of e.m.f. at positions of maximum conductivity gradient. Since the bulk photo e.m.f. is a function of conductivity gradient, light probe measurements give a sensitive technique for the detection of inhomogeneous impurity distributions. With weak illumination, the measured ratio of photo e.m.f. to photoconductive resistance decrease was a constant independent of light intensity. These observations verified the theory and suggested a use for this ratio in quantitative measurements of conductivity gradient. The photo e.m.f. at strong illumination was shown to be dependent on the impurity distribution outside the region of incident light. The conditions under which the ratio of photo e.m.f. to photoconductive resistance decrease is constant at strong illumination are shown to be in agreement with the theoretical treatment. Measurements of bulk photo e.m.f. as a function of temperature show a qualitative agreement with theory at low temperatures (extrinsic range). At high temperatures (intrinsic range) the results show a close agreement with the theoretically predicted behaviour.

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