In addition to Emission Microscopy, Electro-Optical Probing (EOP) technique has shown its efficiency in the world of failure analysis since the last decades and its ability to measure waveforms on various devices. The different physical parameters that affect and modify the reflected laser probing beam through active silicon components are just beginning to be well known. All the physical and environmental parameters contribute to the modulation of the reflected laser probing beam onto structures under test. However, the origins of EOP signals from bipolar transistors junctions used in a lot of analog devices are not well known and not that much described in the literature. In this paper we present backside EOP signal measurements from bipolar devices. We compare these measurements on a failed transistor and on a reference transistor and we interpret the results according to a model previously developed for silicon devices (MOS and Bipolar)