Pure silicon and germanium are intrinsic semiconductors as they have no impurity whatsoever. Electrons in these elements are all tightly held in their crystalline structure, i.e., they are not free to move.
When energy is added to pure silicon in the form of heat, it can cause a few electrons to break free of their bonds, leaving behind a hole in each case. The absence of electrons is treated as positively charged particle having the same amount of positive charge as on an electron.
These electrons move randomly in the crystal. These electrons and holes are called free carriers, and move to create electrical current. However, there are so few of them in pure silicon that they are not very useful.
In an intrinsic semiconductor, electrons and holes are always generated in pairs and the negative charge of free electrons is exactly balanced by the positive charge of holes. However, a hole only shifts its position due to the motion of an electron from one place to another. So, when a free electron moves in a crystal because of thermal energy, its path deviates whenever it collides with a nucleus or other free electrons. This gives rise to a zig-zag or random motion, which is similar to that of a molecule in a gas.