The cause of electrical conduction in semiconductors is due to the movement of the holes in the valence band and the movement of the electrons in the conduction band.
When electric field is applied then as a result the electrons will start moving in the conduction band in the direction opposite to that of the electric field. The current produced at that time is known as hole current. In other words we can say that two types of currents are mainly working in the semi conductor. One is the hole current that is due to the movement of the holes in the valence band and other is the electronic current which is caused by the movement of the electrons in the conduction band. The total resultant current in the semi conductor is the sum of the both the currents.
