An accurate and analytic threshold-voltage model for small-geometry MOSFETs with single-channel ion implantation in VLSI

This paper presents an accurate and analytic threshold-voltage model for n-channel enhancement MOSFETs single-channel boron implantation. In the developed model, the step-profile approximation is used to simulate the channel-implanted profile and an accurate inversion condition is used to calculate the surface potential. Moreover, the built-in voltage across the step junction is taken into consideration, which insures the continuities of surface potential and maximum depletion width changes with respect to the applied substrate bias as the edge of maximum depletion width passes through the step junction. A modified charge-sharing model is also developed to consider the charge-sharing effect due to source and drain diffusion islands and is incorporated into the developed threshold-voltage model by the depletion-charge-superposition method. In addition, the charge-sharing effect due to the bird's beak and field-encroachment implant has also been considered in the developed model. The experimental devices fabricated by a set of test keys have been characterized. The measured threshold voltages are compared to the developed model and excellent agreement between comparisons has been obtained for wide ranges of channel lengths, channel widths, and applied substrate biases. Moreover, a method of extracting the implant parameters for a step profile and the flatband voltage is also presented. (author)