A new model for the low frequency (LF) noise in the base current of polysilicon emitter bipolar junction transistors (BJTs) is proposed. This model is based on the carrier number fluctuation approach and satisfactorily accounts for the base current noise characteristics. The base current fluctuations are ascribed to surface and volume noise sources. The surface noise arises from the low frequency fluctuations of the surface generation–recombination base current component due to the dynamic trapping–detrapping of carriers into/from slow states located in the spacer oxide at the periphery of the emitter/base junction. The volume noise results from the intrinsic fluctuations of the diffusion base current due to the carrier number fluctuations in the emitter or at the emitter–base junction. This LF noise model has been applied to BJTs subjected to hot carrier stress after reverse biasing of the base–emitter junction. The evolution of the base current noise characteristics after stress clearly demonstrates that the hot carrier injection leads to a substantial increase of the surface base current noise component whereas the volume noise component as well as the collector current noise is nearly insensitive to the degradation‐induced defects. The application of the model provides an evaluation of the slow and fast trap densities generated as a function of the stress level. ©1996 American Institute of Physics.