3D NEGF simulation of ‘ab initio’ scattering from discrete dopants in the source and drain of a nanowire transistor

In this paper we study the fluctuations in the I-V characteristics due to coherent scattering off different configurations of discrete dopants in the source/drain of an extremely thin Si nanowire transistor. The self-consistent screening potential at the impurities is calculated using a full 3D NEGF formalism coupled to the 3D solution of the Poisson equation [3]. The fluctuations in the I-V characteristics can be understood by examining the energy dependence of the transmission coefficients. We show that the effective potential presented to electrons by a positively charged ionised donor does not necessarily have the simple form of an attractive screened Coulomb potential. Instead, the self-consistent electrostatics leads to an effective potential that resembles an inverted sombrero: the core is Coulomb like, but the radial dependence rises through a maximum before flattening to an asymptotically zero value. This maximum introduces quasi-bound (resonant tunnelling) states. For the first time, we demonstrate the effects of the sombrero potential on scattering and ultimately device current transmission. The whole screening occurs due to electrons congregating in the quasi-bound states of the sombrero potential