In order to interpret the EBIC (electron beam induced current) photographs of scanning electron microscope, it is found nacessary to take the effect of photo injected carriers from forward biased pn junction into consideration. In this paper, results of constrast observation on a series of EBIC photographs of a trasistor at different levels of applied voltage are discussed. Based on the Bbers-Moll model for transistors together with some additional assumptions, we have been able to derive equations capable of describing the saturation current, effects of photo injected carriers from forward biased and reverse biased pn junctions. The theoretical analyses are qualitatively in agreement with the experimental results.