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The origin of 1∕f fluctuations in positive-negative-positive (PNP) polycrystalline silicon-emitter bipolar-junction transistors is described. The interfacial oxide (IFO) at the monosilicon–polycrystalline silicon interface is found to significantly affect the noise behavior. The low-frequency noise originates from two independent fluctuation mechanisms: in the diffusion and tunneling components of the base current noise power spectral density (SIB) and from the diffusion current and carrier number fluctuations in the collector current noise power spectral density (SIC). The Hooge noise parameters for electrons and holes are calculated from the diffusion fluctuation models for SIB and SIC, respectively. Noise measurements on devices with different sizes and different IFO thicknesses indicate that the fluctuations occur in the minority-carrier (electron) tunneling current component of SIB through the IFO. The thickness of the IFO is estimated using this noise model. The tunneling fluctuations dominate over the diffusion fluctuations for the smaller (0.7×0.7μm2) transistors, while the opposite is the case for the larger (0.7×100μm2) ones. The scaling effect on the noise performance of these transistors is discussed. The effect of the IFO on the dc characteristics and the noise behavior of the PNP transistors is compared to that of the negative-positive-negative (NPN) counterparts on the same wafer.

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