THE EFFECT OF FOCUSED ION-BEAM IMPLANTATION ON THE THRESHOLD VOLTAGE OF SHORT-CHANNEL SILICON METAL-OXIDE-SEMICONDUCTOR FIELD-EFFECT TRANSISTORS

A theoretical modeling of the threshold voltage of short-channel silic on metal-oxide-semiconductor field-effect transistors (MOSFETs) taking into consideration the focused ion-beam technology for the direct imp lantation of dopants into semiconductor substrates has been performed. Based on a quasitwo-dimensional solution of Poisson's equation, the s urface potential distribution along the channel of a MOSFET has been d erived. For this, implanted channel doping concentration is varied lin early along the channel and in a Gaussian-type fashion in a direction perpendicular to the channel. The threshold voltage has been determine d from a knowledge of the minimum surface potential in the channel. Th e effects of finite source and drain junction depths have been include d by modifying the depletion capacitance beneath the gate. Short-chann el effects on the threshold voltage are thus taken into consideration. The model provides important insight of the physics controling the th reshold 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 sourc e end of the channel, and a proper tailoring of the doses, straggles, and energy of implantation are keys to the improvement of the electric al characteristics of a MOSFET. (C) 1995 American Institute of Physics .