Electron mobility in n-channel depletion-type MOS transistors

It is found that surface electron mobility can exceed electron mobility in bulk. Field-effect mobilities of n-channel depletion-type MOS transistors were measured, comparing transistor
G_{m} - V_{g}
and diode
C - V_{g}
characteristics. Electron mobility in a highly doped n-type channel layer exceeds that in bulk at around the onset of surface accumulation. This can be explained by reduction in ionized impurity scattering under accumulation conditions. In accumulation conditions, excessive electrons screen the Coulombic field from ionized impurities. This is confirmed by theoretical calculation based on the Maxwell-Boltzmann distribution of electrons and electron-concentration-dependent electron mobility.