Explanation of current crowding phenomena induced by impact ionization in advanced Si bipolar transistors by means of electrical measurements and light emission microscopy

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 IE. The second kind of instability consists in the snap- back of the collector current Ic when the device is driven at constant emitter-base voltage VEB 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.