Holes in a P-type semiconductor under external force E Basically in almost every semiconductor texts, there will be all these concepts concerning electrons, holes, dopants, fermi-levels.
However, I have been always confused about the picture of hole transport in semiconductor device, say, a simple PN junction.
With a specific acceptor level and dopant concentration, we have some "holes" in the valence bands, which are in fact the absence of some electrons having gone to the acceptor levels, then all theses books seem to assume in the valence bands of the p-type region, there will be only holes that conduct current?
My questions:


*

*aren't there still many electrons in the valence band? though they have some negative effective masses, but still do contribute to the transport?

*under a certain external force(say,E), the electron and holes in the valence band are moving in the same directions since electrons have negative effective mass and holes have positive one, so the corresponding currents cancel with each other in a P-type semiconductor?
 A: There are two alternative ways to calculate the current from the valence band:
(A) Add up the current of all the electrons in the valence band. The unoccupied states are unoccupied; there's nothing there, they don't carry current, you should ignore them, obviously.
(B) Ignore the current from all the electrons in the valence band. At the same time, pretend that the unoccupied states are actually occupied by something called "holes", with positive effective mass and positive charge. Calculate the current due to the holes alone.
Of these (A) is obviously a correct way to do the calculation, but it's mathematically and intuitively a very difficult calculation to do. The surprising and important fact (explained here) is that the (B) calculation gives the exact same result as the (A) calculation. Since (A) gives the correct answer, so does (B). Since (B) is a simpler calculation than (A), and easier for visualization and intuition, everybody always calculates and describes valence-band current using the (B) approach.
Your proposal  for calculating the valence-band current is to track the current of the holes in the unoccupied states, and simultaneously track the current of all the electrons in the occupied states. That is neither (A) nor (B), it's doing both at once and adding them. So it will give you the wrong answer, off by a factor of 2.
A: As far as I understand, the electrons in the valence band do contribute to the transport, but only because they can move to holes, and this is equivalent to hole movement. There is no other place for these electrons to move to but to holes.
A: The lack of electrons in the structure of the substrate offer a resistance to electrons traveling its direction.And a doped substrate between two pn junctions can act as a control for electron flow.
