This paper analyzes impact ionization phenomena in advanced polysilicon emitter bipolar transistors. Two intrinsic limitations affecting multiplication coefficient at high electric fields are discussed. Emission microscopy is adopted to directly investigate and observe current crowding effects at the basis of the first kind of instability, which takes place when the device is driven at constant emitter current I(E). The second kind of instability consists in the snap-back of the collector current I(C) when the device is driven at constant emitter-base voltage V(EB) and can be explained by a simple model which takes into account the voltage drop induced by negative base current on the base spreading resistance.