PHYSICS OF NANOTRANSISTORS: GATE VOLTAGE AND SURFACE POTENTIAL, MOBILE CHARGE IN BULK MOS AND IN THIN SOI

Abstract

In the third one from the line our new tutorial reviews, directed to serve students, university teachers and researchers, the physics of processes in the semiconductor substrate MOSFET was discussed. This physics is determined by the bending of the zones, which depends on the surface potential ψ S , which in its turn is determined by the gate voltage VG . A sufficient general equation connecting VG with ψ S is obtained. Under the conditions of the depletion regime, a simpler dependence of ψ S on VG is obtained. The behavior of the mobile electron charge Q ∝ψ S and Q V ∝ G is discussed: how does the electronic charge vary with the surface potential and with the gate voltage in the conditions before and after the threshold voltage. The correct results in the prethreshold range and in the strong inversion mode are obtained without resorting to a numerical solution of the Poisson – Boltzmann equation, but at the same time the numerical solution of this equation covers both the subthreshold region and the strong inversion region, and the transition region between them. The behavior Q ∝ψ S and Q V ∝ G is also considered for a completely different structure of MOS, a structure with an exceptionally thin silicon on insulator substrate ETSOI, typical for the current trend of miniaturization of transistors. Nevertheless, we demonstrate that the main features of the structure of ETSOI are similar to those of the massive structure of MOS. Dependences Q ∝ψ S and Q V ∝ G have also been obtained for the ETSOI structure both below and above the threshold. The results obtained suggest that 1D electrostatics is acceptable for both massive MOS structures and ETSOI structures. In the next articles we will show how 2D electrostatics explains why the ETSOI structure with a double gate is preferable for very short nanotransistors.