In this work we investigate the accuracy of the parabolic-band model for the simulation of cylindrical nanowire (CNW) FETs scaled down to a 1-nm diameter. In doing so, we rely on recently-published results based on a tight-binding computation of the band structure in square- and circular-section nanowires. The above results indicate that the FET characteristics are affected in two ways by the parabolic-band approximation: first, the conduction-band edge is shifted upwards in both nanowire types, leading to an overestimation of the FET threshold voltage at small wire areas; next, the transport effective masses are increased by the structural confinement of the electron charge, which is neglected in the parabolic-band model. Fitting functions of the tight-binding conduction-band edge and transport effective masses are worked out, thus providing the appropriate parameters for transport simulations. The output characteristics of the CNW-FET are then computed using the quantum-transmitting boundary method (QTBM) with and without the corrected conduction-band edge and transport effective masses, and the influence of the above corrections on threshold voltage and on-current is finally assessed.
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