A very important feature of the semiconductor material is the electron-hole pair. To get a semiconductor to conduct a current, we must make an electron jump from an occupied to an unoccupied energy level. When it does this it leaves a hole (an empty state). This hole can be filled by another electron, which itself leaves a new hole. Therefore, we could say that both the hole and the electron contribute to the conductivity as they move around in the material. The hole is like a positive charge (lack of negative), the electron is negative.

It's a little bit like this simple puzzle game where you move pieces around to form an image. The moving pieces correspond to the electrons, of course.