"If circuits are like plumbing, then none of the "pipes" of a circuit are ever empty. This idea is extremely important, and without it we cannot understand semiconductors ...or even conductors! Metals contain a vast quantity of movable electrons which forms a sort of "electric fluid" within the metal. A simple hunk of copper is like a water tank! Physicists call this fluid by the name "electron sea of metals," or "the ocean of charge." Semiconductors are always full of this movable "charge-stuff." The movable charge is in there even when a transistor is sitting on the shelf and disconnected from everything"