Quantum-mechanical effects on the threshold voltage of ultrathin-SOI nMOSFETs

A theoretical description is given of the dependence of the threshold voltage, V/sub TH/, of SOI MOSFETs on a wide range to top silicon layer thickness, t/sub s/, using both classical and quantum-mechanical methods. The quantum-mechanical effects become remarkable below the critical thickness and raise V/sub TH/ with decreasing t/sub s/. The classical method cannot be applied in such a thin t/sub s/ region, since classically obtained V/sub TH/ decreases monotonously with decreasing t/sub s/ even below the critical thickness. As a result, the V/sub TH/ curve as a function of t/sub s/ can be divided into two regions with a boundary at a critical t/sub s/, and the classical method can be applied above that critical thickness.<>