What is the concept of a hole in semiconductors?

A semiconductor material forms a crystal structure where all the valence electrons "participate" in forming the lattice. There are neither "extra" nor "missing" electrons in the structure. If we dope the semiconductor with a "P-type" material, this sets the stage for the creation of a "hole" in that matrix. The P-type material will have one less valence electron than our semiconductor material. And when that P-type atom becomes part of the crystal matrix, it lacks that one electron to make the matrix "complete" or "uniform" as regards the electrons. That creates the hole in the matrix. When that P-type material is formed up against N-type material (which has an "extra" electron in its matrix), that extra electron will leave the N-type material and migrate to the P-type material to fill that hole. This sets up a condition where charges have shifted, and it creates a difference of potential (voltage) across the junction (owing to the shift of the electrons).