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A new method has been devised to measure directly the series collector resistance (r/SUB c/) of monolithic bipolar n-p-n transistors. The method uses the parasitic substrate p-n-p and the reverse n-p-n associated with each integrated n-p-n transistor to detect the internal collector-base voltage. The effects of temperature, conductivity modulation, and mobility on the collector resistance can be measured directly. Measurements on a range of devices indicate standard techniques such as the forced beta method measure only a fraction of the total collector resistance. The present technique yields results in good agreement with theoretically calculated values of r/SUB c/. This method is amenable to automated measurement systems.