My textbook ("Electronic Principles" by Malvino and Bates) seems to suggest the free electrons in conduction band move left and reach the positive terminal of the battery. Does this mean electrons in valence band are not allowed to move left(hole moving right) and enter the positive terminal of the battery?
Similarly, it says the electrons from the negative terminal of the battery enter from the right directly into the valence band holes. Does this mean the electrons from the battery are not allowed to enter into the conduction band?
In summary: Electrons in an intrinsic semiconductor always leave from the conduction band and always enter from the valence band. This doesn't feel right. What am I missing?
EDIT: Saying electrons in the valence band are not free, so the battery cannot pull them doesn't make sense because of the following scenario: If a p-type semiconductor is connected across a battery, the battery's positive terminal has no problem attracting the electrons from valence band.
Best Answer
If it's intrinsic, there's as many electrons in the conduction band as holes in the valence band by definition, because each electron leaves a hole. The electrons in the conduction band are bound to their atoms by (relatively weak) electrostatic forces, and can move fairly freely.
Electrons in the valence band are bound more tightly, and largely move only from one atom to an adjacent one that contains a hole.
Bottom line is, all the electrons are moving to the left--some in long unfettered paths, and some from one atom to the next. Also, in the valence band, it's kind of tag-team...an individual electron moves to an adjacent atom leaving a hole, then another electron from the other side fills that hole. It's just as easy to model it as a hole propagating than a tag-team of electrons.