Semiconductors

Conductors are able to accomodate an electron flow through them because the electrons in the atoms are not tightly bound to thier atoms. When an electron leaves its atom, it leaves behind hole, which because of the absence of the electron, is positivly charged since the atom started out neutral. This hole can be modeled as a positive charge. As electrons move towards one end of the conductor, they will leave holes at the opposite end, thus creating a "flow" of holes.

The electrons in insulators are tightly bound to thier atoms, so they cannot flow.

Semiconductors allow us to build non-linear devices, such as computers.