Semiclassical and wave-mechanical modeling of charge control and direct tunneling leakage in MOS and H-MOS devices with ultrathin oxides

Charge control and gate leakage in metal-oxide-semiconductor (MOS) structur es and heterojunction-MOS structures with ultrathin oxide (1 nm) are invest igated using both classical and wave-mechanical calculations. In the classi cal approach, direct tunneling gate current is determined using the formali sm of transmission probability whereas the notion of quasibound state lifet ime is applied in the wave-mechanical model, For conventional MOS structure , the threshold voltage VT significantly depends on the applied model but a n excellent agreement between both approaches is found about gate leakage p rovided that the correction of VT is taken into account. For buried-channel II-MOS structures the quantum-induced V-T-shift is smaller but the degrada tion of gate control efficiency dn(s)/dV(G) is increased, due to a large ch arge displacement from the oxide interface resulting from 2-D confinement i n the buried strained layer. Using semiclassical approach the error of inve rsion charge distribution yields an overestimation of gate leakage compared with the more rigorous wave-mechanical calculation. It is finally shown by properly solving self-consistently Poisson and Schrodinger equations that a heterojunction-channel architecture may reduce the gate leakage by at lea st two orders of magnitude compared with conventional MOS design. This impr ovement would be in addition to the expected increase of device performance due to the strain-induced enhancement of electron transport properties.