When purified they have a crystalline construction like salt and sugar.
The atoms which make up the materials are rigidly locked together in a pattern (a LATTICE) in which the electrons, in the atoms, are unable to move.
This means that pure silicon and germanium are good insulators.

After purification, precise amounts of impurities are added (the materials are DOPED).
These impurities fit into the lattice but have associated electrons which are free to move about and produce a flow of electric current.
There is therefore a surplus of negative electrons and the material is called N-type semiconductor.

Other types of impurities can be added to pure silicon and germanium. These produce a shortage of electrons in the lattice.
Therefore there are HOLES in the lattice.
Electrons can jump into these holes, producing a flow of holes.
It's like sitting in a row of chairs in the doctor's waiting room.
When someone gets up and goes into the surgery there is an empty chair (a hole).
People (electrons) move along nearer to the surgery and a hole travels in the opposite direction.

Since there is a shortage of negative electrons there is an overall positive charge and the material is called  P-type semiconductor.

The resistance of semiconductors is about half way between conductors and insulators.
Hence the name, semiconductors.

Semiconductors are used in semiconductor devices such as diodes, transistors, integrated circuits etc.