The lateral distribution of the effective contact potential difference (ECPD or φMS) was determined experimentally for the first time over the gate area of metal-oxide-semiconductor (MOS) structures. The photoelectric method for measuring φMS in MOS devices was modified to characterize its two-dimensional lateral distribution. It is found that φMS values are different at the gate center, gate edges, and gate corners. In square MOS gates, it is found that φMS values are highest in the center area of the gate, lower along the gate edges, and lowest at the gate corners. These results are confirmed by several independent photoelectric and electrical measurement methods. This lateral distribution of φMS is attributed to the nonuniform two-dimensional distribution of mechanical stress under the gate, which is known to influence the barrier height and φMS of MOS structures. A simple physical model is introduced relating the two-dimensional (2D) distribution of φMS to the 2D mechanical-stress distribution. The model predictions are in agreement with φMS distributions obtained experimentally using photoelectric and electrical characterization methods in Al/SiO2/Si MOS structures. © 2004 Elsevier B.V. All rights reserved.