Transport modeling for nanoscale semiconductor devices

Abstract

Due to extreme miniaturization of device dimensions the well established TCAD tools are pushed to the limits of their applicability. Since conventional MOSFETs are already operating in the sub-100 nm range, new physical effects and principles begin to determine the transport characteristics and the validity of conventional current transport models is in question. The classical drift-diffusion model of carrier transport in electronic devices has been widely employed in TCAD tools. However, it must be generalized to include hot-carrier effects. This motivated the development of higher-order moments transport models such as the hydrodynamic transport model, the energy transport model, and the six-moments model. With scaling continuing quantum mechanical effects begin to affect the transport properties. Parallel to the search for new technological solutions for MOSFET scaling, the development of conceptually new devices and architectures is becoming increasingly important. New nanoelectronic structures, such as carbon nanotubes, nanowires, and even molecules, are considered to be prominent candidates for the post-CMOS era. At this small device size the geometrical spread of the carrier wave packet in transport direction can no longer be ignored. When the device size becomes shorter than the coherence length, the complete information about the carrier dynamics inside the device including the phase of the wave function is needed and one has to resort to a full quantum mechanical description including scattering. Transport in advanced nanodevices is determined by the interplay between coherent propagation and scattering. Numerical methods for dissipative quantum transport based on the non-equilibrium Green's function formalism, the Liouville/von-Neumann equation for the density matrix, and the kinetic equation for the Wigner function are attaining relevance. In this work we review semi-classical and quantum mechanical modeling of carrier transport in nanoscale semiconductor devices.