Carrier mobility reduces when the gate SiON becomes thinner than 2 nm or high-k layer is used. Agreement has not yet been reached on the level of reduction and on the underlying mechanism. Remote charge scattering has been proposed to be responsible for the mobility reduction and this work assesses its importance. By increasing charge density at 0.56-1 nm from the substrate interface to the order of 1020 cm-3, it is found that both electron and hole mobility changes little.