Modeling of output resistance in SiGe heterojunction bipolar transistors with significant neutral base recombination

A simple compact model, suitable for circuit simulations, is derived which enables quantitative determination of the impact of neutral base recombination on the small signal ac output resistance of SiGe HBT's for arbitrary base ac drive conditions. The model uses existing SPICE parameters which are routinely extracted from bipolar transistors plus an additional model parameter which can be extracted from a proposed experimental technique involving output resistance measurements under base ac voltage and current drive conditions. The modeling approach also enables the forward and reverse base transit times to be related to transistor small signal ac output resistance by a simple analytic expression. The currently accepted expression for the r/sub /spl mu// parameter, which is used to model neutral base recombination in the ac hybrid-/spl pi/ equivalent circuit, is shown to be incorrect and is replaced by a new correct expression. Numerical simulations of a SiGe HBT structure which exhibits neutral base recombination are used to verify the validity of the model.