  In this paper we review the theory of silicon nanowires. We focus on
nanowires with diameters below 10 nm, where quantum effects become important
and the properties diverge significantly from those of bulk silicon. These
wires can be efficiently treated within electronic structure simulation methods
and will be among the most important functional blocks of future nanoelectronic
devices. Firstly, we review the structural properties of silicon nanowires,
emphasizing the close connection between the growth orientation, the
cross-section and the bounding facets. Secondly, we discuss the electronic
structure of pristine and doped nanowires, which hold the ultimate key for
their applicability in novel electronic devices. Finally, we review transport
properties where some of the most important limitations in the performances of
nanowire-based devices can lay. Many of the unique properties of these systems
are at the same time defying challenges and opportunities for great
technological advances.
