Shape Effects on the Performance of Si and Ge Nanowire Field-Effect Transistors Based on Size Dependent Bandstructure

In this paper, we evaulated the shape and size effects of Si and Ge nanowire (NW) field-effect transistors (FETs) on device performance using sp(3)d(5)s* tight-binding (TB) model and semi-classical top-of-barrier ballistic transport model. Our simulation results show that smaller cross-sectional area is desirable for high frequency device applications and for larger ON-state currents, square cross-section may be desirable due to larger cross-sectional area and insulator capacitance. Furthermore, it is also observed that due to quantum effects, the gate capacitance to gate oxide capacitance (C-g/C-ox) ratio for the small size NW FETs could be much less than one, rendering the classical assumptions and calculations invalid for nano scale FETs. In this sub-nano region, therefore, a new set of assumptions and calculations in terms of effective mass, bandgap, and one-dimensional density-of-states should be implemented as quantum effects start to play an important role in device performance. (C) 2010 The Japan Society of Applied Physics