AN EXPLANATION OF 1/F NOISE IN LDD MOSFETS FROM THE OHMIC REGION TO SATURATION

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.