This paper describes the impact of the distributed RC effect in the intrinsic base region on the modeling of the output noise spectral density of a bipolar junction transistor. Simplified expressions to convert the distributed noise models to an improved hybrid-pi model with the new equivalent noise sources are proposed. The accuracy of the new model has been verified by circuit simulations. Based on the new model, a noise analysis is done. The simulation results are compared with one by the conventional model. From this, the sensitivity of the noise parameters to the distributed RC effect is studied. (C) 2003 Elsevier Ltd. All rights reserved.