Failure of moments-based transport models in nanoscale devices near equilibrium

It is shown that the conductance in nanoscale devices near equilibrium strongly depends on the choice of the transport model. Errors larger than a factor of two can be encountered, if the drift-diffusion (DD) model is used instead of a model based on the full Boltzmann equation. This effect is due to a fundamental difference in carrier heating between bulk systems and devices. Although carrier heating is included in hydrodynamic models, this effect is captured only partially by these models due to the model inherent approximations. A direct consequence of the failure of the DD approximation is that the usual method for inversion layer mobility extraction from measurements in the linear regime becomes inaccurate for short gate lengths and the extracted mobilities might be too small. This error has also an impact on the modeling accuracy at strong nonequilibrium. In the case of the DD model, the overestimation of the conductivity in the linear regime can partly compensate the underestimation of the current at high bias, and the model accidentally appears to be more accurate than expected.