Aspect ratio calculation in n-channel MOSFET with a gate-enclosed layout

Transistors with gate-enclosed layout can be used in radiation environments to prevent the onset of any leakage current through a radiation induced lateral path under the bird's beak or at the shallow trench corner. The gate-enclosed layout introduces a non-standard device geometry and its effect on the MOSFET characteristics has been investigated here. In particular, we have addressed the problem of evaluating the MOSFET aspect ratio for selected gate-enclosed shapes. To this purpose, we have developed different analytical models in order to evaluate analytically the drain current expression at a low drain bias. The simplest model has been proposed for the circular devices, derived from a solution of the corresponding problem featuring circular symmetry. Increasing model complexity was needed for square, broken square, and rectangular MOSFETs. In these devices, the transistor was broken into two or three separate parts, which give independent contributions to the total drain current and aspect ratio value. A fitting parameter (α) was needed to identify the borderline between edge and corner transistors. The model results have been successfully compared with the experimental measurements and the results obtained from numerical simulations.