An explanation of [formula omitted] noise in LDD MOSFETs from the ohmic region to saturation

Abstract

1 f noise of the drain current S I vs drain-source voltage was studied in an LDD MOSFET. Analysis of the d.c. characteristics shows that the series resistance R Dd, introduced by the LDD structure on the drain side, increases greatly with the external drain-source voltage V DS increasing towards the value of the effective gate voltage V G ∗ . Consequently, the internal drain-source voltage V ds and the channel current I DS are reduced. This is used successfully to explain why the experimental results of S I as a function of V DS /V G ∗ in an LDD MOSFET are at variance with those in a conventional MOSFET. Here a model and a procedure are proposed from which the internal drain-source voltage V ds and series resistance are obtained from measuring results. After recalculation of the experimentally observed S I in terms of channel current fluctuations, we find that S I ch vs normalized internal drain-source voltage V ds /V G ∗ shows the same trend as in a conventional MOSFET. The role of R Dd becomes more important as the value of V DS /V G ∗ increases. Owing to the fact that the influence of R Dd becomes small in relation to the channel resistance itself, a long-channel device is affected less than one with a short channel.