Carrier multiplication and avalanche breakdown in self-aligned bipolar transistors

Abstract

The extraction of the multiplication factor characterizing impact ionization in bipolar transistors is investigated in detail. In particular two d.c. measurement methods, superior in measurement accuracy in comparison with any method published previously, are presented. These methods clearly separate the voltage dependence of the multiplication factor from the voltage dependence of the base Gummel number. The methods presented are shown to yield consistent results and are verified by a comparison with multiplication data derived from breakdown measurements. They allow easy experimental characterization of carrier multiplication effects in bipolar transistors, and provide a valuable tool for the on-chip investigation of inhomogeneities in space charge layers and for the identification of failure mechanisms. The analytical approximation for the multiplication factor due to Miller is found to provide a good description of carrier multiplication in epitaxial base collector junctions in the low-level multiplication regime if BU cbo is substituted by the value BU cbo, m < BU cbo. The methods are applied but not restricted to self-aligned configurations and are of general interest for the precise characterization of impact ionization phenomena in reverse biased pn junctions.