Noise correlation measurements in bipolar transistors .2. Correlation between base and collector currents

Low-frequency noise measurements were performed on heterojunction bipolar transistors (HBTs) and presented in part I of this article. These measurements revealed a partial correlation between base and collector noise current sources. With the help of the current spectral densities determined in part I, the coherence function is calculated and studied versus bias in this second part. Important values of the coherence function are obtained at the highest bias. Then the spectral densities S-c and S-nc associated to the correlated and uncorrelated parts of the collector current noise source are extracted and analysed. All spectra exhibit excess noise. A difference in shape is clearly observed because of an important generation-recombination component occurring around 1 kHz on the correlated part only. To compare the behavior of different transistors a normalized coupling coefficient is introduced. The correlation is seen to increase with the scaling-down of devices. The investigation of the coupling coefficient with collector current density leads to hypothesis concerning the origin of the generation-recombination (g-r) components. Some components are shown to originate in the intrinsic transistor, and others in the extrinsic one, depending on frequency and type of HBT. Mainly the noise correlation is due to leakage currents and/or recombinations in the base-collector junction. (C) 1997 American Institute of Physics.