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Traditionally, semiconductor reliability has been estimated from the life tests or accelerated stress tests at the completion of manufacturing processes. Recent research, however, has been directed to reliability estimation during the early production stage through a relation model of yield and reliability. Because the relation model depends on the assumed density distribution of manufacturing defects, we investigate the effect of the defect density distributions on the predicted reliability, for a single-area device without repair and for a two-area device with repair, respectively. We show that for any device, reliability functions preserve an ordering of yield functions. It is also pointed out that the repair capability improves only yield but not reliability, resulting in a large value of the factor that scales from yield to reliability. In order to achieve a reliable device, therefore, we suggest to improve yield and to perform the device test such as burn-in if the scaling factor is large.