Low-frequency noise in polysilicon-emitter bipolar transistors

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Abstract
The authors present a review of low-frequency noise (LFN) in polysilicon-emitter (PE) bipolar junction transistors (BJTs). It includes noise results from unstressed as well as stressed devices. A discussion of the possible physical origins of low-frequency noise is presented. Details of noise measurement and a simple noise equivalent circuit, with related noise spectral-density expressions, are given. The first part of the paper deals with noise in virgin or unstressed devices. It starts with a detailed discussion of low-frequency noise results in virgin devices. This includes noise in ultra-small transistors and a discussion of the possible physical origin(s) of the noise. Low-frequency noise in npn and pnp transistors and a technique to estimate the collector 1/f noise are then presented. Since devices are scaled to achieve better high-frequency performance, a discussion of low-frequency noise as a function of emitter geometry for transistors from several different technologies is presented. The second part of the paper deals with degradation of the LFN performance due to electrical stress. It begins with a discussion of degradation mechanisms, followed by a phenomenological discussion of low-frequency noise after hot-carrier (HC) stress. A discussion of the modelling of low-frequency noise after hot-carrier degradation due to reverse current or reverse bias voltage stresses is presented. The effects of forward bias degradation on the low-frequency noise in BJTs are presented and discussed. Finally, it is demonstrated that the noise in PE-BJTs is more sensitive to degradation than the DC characteristics.