Nonuniform displacement of MOSFET channel pinchoff

The effects of fixed positive oxide charge under the gate on the characteristics of MOSFET devices are well known. The combined effects, however, of the oxide charge in the field and of the edge contour and impurity profile on the device characteristics have not been as extensively investigated in the past. In this paper we address this problem and show results which give a new insight in the performance of MOSFET devices. With the help of two- and three-dimensional numerical solutions of Poisson's equation, it was found that this oxide charge lowers the threshold potential resulting in an increase in conductivity towards the two edges of the channel along the width direction. As a consequence, the geometric channel pinchoff locus shifts towards the drain as the channel edge is approached. This is in contrast to the conventional assumption of the pinchoff locus being in parallel to the drain. The oxide charge and the net impurity profile under the field region adjacent to the channel causes the current density to increase gradually towards the edges of the channel in channel width direction. At high power densities, this may lead to drain-induced corner breakdown. Further, in the subthreshold region of device operation, the electric field at the corner of the drain junction is increased, leading possibly to corner breakdown.