A particular Silicon atom, with 4 electrons in its outer shell, can share its electrons with 4 of its nearby neighbours, giving a total of 8 electrons. This configuration, like that of the inert gas Argon, is more stable, and this energetically favoured.
Although the electrons in Silicon are bound to the atoms and also
bound in these valence bonds, when such a sample is subject to an
electrical potential difference, some of these electrons
will break free and contribute to a current. This
current is not as
easy to establish as in a good electrical conductor, but
it is definitely easier to do so than in an electrical insulator,
where the electrons are more tightly bound.