Low-frequency noise measurements are performed on classical Si bipolar transistors and on AlGaAs/GaAs heterojunction bipolar transistors (HBTs) mounted in a common-emitter configuration. Expressions for the spectral densities are derived taking into account a correlation between base and collector noise sourcesibandic. Values of emitter series resistances and of the base ideality factor are determined from these noise measurements. Then the spectral densities related toibandicas well as the cross-spectrum are extracted. In the case of classical Si transistors, the excess noise is attributed to the current noise sourceib. The effect of the base series resistance is shown on the white noise. For the HBTs, the white noise is not reached. The excess noise is attributed to the correlated current noise sourcesibandic. From the analysis of the current spectral densities with base current the increase of correlation with bias is revealed. The extracted current spectral densities provide the foundation for the calculation of the coherence function associated to base and collector currents. This coherence function is presented in part II. The shape of the curves shows correlation phenomena to be mainly related to generation-recombination components. Also in part II, the spectral densities associated with the correlated and uncorrelated part of the collector current are extracted and studied versus bias and geometry. ©1997 American Institute of Physics.