Silicon planar photo-transistorsin hermetic pig-tail metallic package for airborne system application failed the acceptance test because of low responsivity. No significant difference between the electrical characteristics of good and defective devices could be detected by systematic investigation of focusing lens, anti-reflecting coating, optical alignment, and by internal electrical microprobing. EBIC inspection instead revealed that all defective photo-transistors were affected by an anomalous region with a very high photo-carrier generation rate. This effect was produced by the misalignment of the emitter metallization which partially exposed the emitterbase junction. Partial exposition of the emitter diffusion to impinging radiation generates an induced photocurrent in the emitter-base junction which has opposite direction with respect to the photocurrent induced in the base-collector junction. The extra photocurrent in the baseemitter junction reduces the emitter-base forward polarization and, as direct consequence, the phototransistor responsivity is lower.