  The structural and electronic properties of amorphous silicon ($a$-Si) are
investigated by first-principles calculations based on the density-functional
theory (DFT), focusing on the intrinsic structural defects. By simulated
melting and quenching of a crystalline silicon model through the Car-Parrinello
molecular dynamics (CPMD), we generate several different $a$-Si samples, in
which three-fold ($T_3$), five-fold ($T_5$), and anomalous four-fold ($T_{4a}$)
defects are contained. Using the samples, we clarify how the disordered
structure of $a$-Si affects the characters of its density of states (DOS). We
subsequently study the properties of defect complexes found in the obtained
samples, including one that comprises three $T_5$ defects, and we show the
conditions for the defect complexes to be energetically stable. Finally, we
investigate the hydrogen passivation process of the $T_5$ defects in $a$-Si and
show that the hydrogenation of $T_5$ is an exothermic reaction and that the
activation energy for a H$_2$ molecule to passivate two $T_5$ sites is
calculated to be 1.05 eV.
