The effect of focused ion-beam implantation on the threshold voltage of short-channel silicon metal–oxide–semiconductor field-effect transistors

Abstract

A theoretical modeling of the threshold voltage of short-channel silicon metal–oxide–semiconductor field-effect transistors (MOSFETs) taking into consideration the focused ion-beam technology for the direct implantation of dopants into semiconductor substrates has been performed. Based on a quasitwo-dimensional solution of Poisson’s equation, the surface potential distribution along the channel of a MOSFET has been derived. For this, implanted channel doping concentration is varied linearly along the channel and in a Gaussian-type fashion in a direction perpendicular to the channel. The threshold voltage has been determined from a knowledge of the minimum surface potential in the channel. The effects of finite source and drain junction depths have been included by modifying the depletion capacitance beneath the gate. Short-channel effects on the threshold voltage are thus taken into consideration. The model provides important insight of the physics controling the threshold voltage of a MOSFET. It is noted that a nonuniform doping with density lower in the drain end of the channel and higher in the source end of the channel, and a proper tailoring of the doses, straggles, and energy of implantation are keys to the improvement of the electrical characteristics of a MOSFET.