The l/f noise in polysilicon emitter bipolar transistors is investigated. The main l/fnoisc source was found to be located in the oxide layer. This source causes both l/f noise in the base current SI, and l/f noise in the emitter series resistance S,. The l/f noise is ascribed to barrier height fluctuations of the oxide layer resulting in transparency fluctuations for both minority and majority carriers in the emitter, giving rise to Si, and S,,, respectively. This model predicts that Si, and Se are fully correlated. Our experimental results show a correlation factor in the range of 0.30.5. The deviation from full correlation is ascribed to local inhomogeneities in the oxide layer.