Noise associated with substrate current in fine-line NMOS field-effect transistors

The noise manifested by impact-ionization-generated substrate current in fine-line NMOS transistors is studied. It is found that this noise can be considerably above the shot noise level for high drain voltages. The magnitude of this noise is interpreted in terms of an avalanche gain produced by a multistep impact-ionization process involving both holes and electrons. The device structure imposes one positive and one negative feedback loop and exhibits a peak in the noise as a function of the drain voltage.