Although there is still controversy about its origin, the designer requires accurate models to estimate 1/f noise of the MOS transistor in terms of its size, bias point and technology. Conventional models present limitations, they usually do not consistently represent the series-parallel association of transistors and they may not provide adequate results for all the operation regions, particularly moderate inversion. In this work we review current flicker noise models, paying particular attention to their behavior along the different operation regions and to their seriesparallel association properties. We present a physics based model for flicker noise following classical carrier fluctuation theory. With the aid of a compact, continuous model for the MOS transistor it has been possible to integrate the contribution to drain current noise of the whole channel area arriving at a consistent, continuous, and simple model for the 1/f noise.