We study the Si(100) inversion layer quantisation, capacitance and tunnelling characteristics in the case of a gradual band gap transition at the Si/SiO
interface. A linear band gap transition of 0.5 nm at the SiO
side results in nearly 20% redistribution of carriers from the 2-fold to the 4-fold degenerate valley, due to the greater wave-function penetration and sub-band level lowering for the 4-fold valley. The gate capacitance is enhanced by up to 12% for a 1.0 nm nominal oxide thickness, and the direct tunnelling current density increases by an order of magnitude.