Origins of universal mobility violation in SOI MOSFETs

The relation between the effective mobility and the transversal field is systematically investigated for SOI-MOSFETs operated in single-gate and double-gate modes. We point out several practical situations where the Universal Mobility (UM) concept is not applicable. In particular, different carrier distributions can lead to the same value of effective field and to distinct mobility values, breaking the foundations of the UM curve. The presence of two interfaces and channels with different quality or the carrier redistribution within the transistor body cannot be accounted in the UM. In particular cases, very unusual mobility-field curves are obtained.