Experimental method to extract AC collector-base resistance from SiGe HBT's

Abstract

A simple experimental technique is presented that enables the accurate extraction of the collector-base resistance r/sub /spl mu// that models the impact of neutral base recombination on the ac output resistance of bipolar transistors. Measurements of ratios of r/sub /spl mu// to the output resistance r/sub o/ arising from the Early effect for SiGe HBTs allows for the quantitative determination of the impact of neutral base recombination on analog circuits that are designed to have a high output resistance. It is found that SiGe HBT structures indicative of those currently being used commercially in advanced analog processes exhibit neutral base recombination that is significant enough to severely degrade the output resistance of analog circuits, even when the output transistors experience ac base voltage drive conditions. Finally, it is shown how extraction of the ratio of r/sub /spl mu// to r/sub o/ can be a useful tool to determine the impact of parasitic potential barriers formed by boron out-diffusion at the collector-base junction on device and analog circuit performance.