Low-Frequency Noise in BJTs and HBTs: Influence of Internal Series Resistances

l/f Noise experiments are performed for npn GaAs/AlGaAs HBTs and npn polysilicon-emitter BJTs as a function of forward bias at room temperature. The experimental data are discussed with the help of new expressions for the l/f noise in bipolar transistors where the influence of internal series resistances has been taken into account. At low forward currents the l/f noise is determined by spontaneous fluctuations in the base and collector currents. At higher forward currents the internal series resistances and its l/f noise become important.