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I have some questions about holes in a semiconductor. When I checked the net I found holes are said to be equivalent positive charge and they say because the hole moves from one place to another when it is occupied by an electron, and electrons leave holes behind, etc...

But I couldn't find a clear answer for P-type semiconductor. It is written the holes in the valence band increase, but my problem is I see that lags and reduce the current not increase it because the holes somehow will attract electrons and get them from conduction band to valence band, and since the conduction band is the band that allows current to flow, won't that reduce the current instead of increasing it?

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It is not true that the valence band cannot contribute to conduction. That's just what happens in P-type semiconductors. The doping alters the band structure of the semiconductor so that there are "missing" electrons (holes) in the valence band. This allows other electrons to "move" from an atom to a nearby one without jumping into the conduction band: they fill a hole "near to them", leaving a hole "behind them". This mechanism is modeled by virtual charges (the holes) moving in the opposite direction. All this happens in the valence band, and this is (intuitively) the reason why the mobility of holes is less than that of electrons. Actual conduction is always due to moving electrons, but when conduction happens in the valence band all is more "difficult" (the energy of the moving electrons filling hole after hole is less than the energy they would have if they were in the conduction band). Bear in mind that this is only a qualitative explanation. This is something in the realm of quantum mechanics and solid-state physics, and the equations involved are rather nasty.

BTW, what you mention by "holes attract electrons from conduction band" is called recombination. In a P-type semiconductor there are very few electrons in conduction band, and they are due to thermal generation (the higher the temperature the higher the probability that a free electron-hole pair will be generated). So it is true that very few electrons in conduction band will contribute to current in P-type semiconductor (they are the thermally-generated minority carriers). But the bulk of the current is supported by holes "moving" in valence band, as I explained above.

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  • \$\begingroup\$ The problem is we take that as an introduction for first course electronics , as u said it is very qualitative and honestly i'm not convinced by most of what is written in the introductory chapters to electronics , i guess as you said the core of understanding lies within quantum mechanics and solid state. :( \$\endgroup\$ – Mohamed Osama Feb 28 '15 at 13:31
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    \$\begingroup\$ @Mohamed Osama: Fairly off-topic, but even widely used introductory EE book have "lies-to-children". Here's an example: chemistry.stackexchange.com/questions/9317/… \$\endgroup\$ – Fizz Feb 28 '15 at 13:34
  • \$\begingroup\$ IS the professional engineer must be aware of that ? i mean must he know quantum mechanics like physicists for research ? \$\endgroup\$ – Mohamed Osama Feb 28 '15 at 13:40
  • \$\begingroup\$ @MohamedOsama I don't know in which country you are attending college/university, and what's your course of study, so take the following with a grain of salt. I advice you not to try to understand the fine details of a device internal workings (you will probably study them in the future in a specific course, maybe microelectronics or similar). Just get the general feeling and concentrate on the "black box" operation of the devices, i.e. their mathematical models used in circuit analysis and design. ... \$\endgroup\$ – Lorenzo Donati Feb 28 '15 at 13:43
  • \$\begingroup\$ @MohamedOsama ... Unless you are going to become an electronic device designer, you could become a good system engineer even without understanding precisely how all that pesky charges "move" inside a crystal :-) Even the notion of movement is not so simple in quantum mechanics, and when you say an electron is "moving" and maybe imagine a little particle skirting around ions in a crystal, well, you are greatly oversimplifying things. So, don't waste too much time to get the finer details, but focus on device operation, characteristics and application circuits. \$\endgroup\$ – Lorenzo Donati Feb 28 '15 at 13:47
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I remember when I was following an introduction course about semiconductors, from which I can remember something that I never tought before. The very basic meaning of how holes are conducting and have less mobility, and before understand how a PN junction conducts, it's all about bubbles: an air bubble in the ocean does not move toward the surface, it is the water that slides down around the air due to gravity. It is intuitive, then, that moving everything around is more difficult, from which you have less mobility. This is the virtual hole which seems to move, like a bubble that is pushed up by the gravity (which pushes down, though).

How a PN junction conducts is a bit more technical, but if you understand the band diagrams of a junction, it is pretty easy.

Good luck. :)

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Since you asked an identical question on physics.SE, I'll give you an identical answer:

because the holes somehow will attract electrons and get them from conduction band to valence band

The reason that a p-type semiconductor is p-type is that it contains acceptor impurities. These are atoms that tend to capture electrons in localized states around their nucleus. For example, group III boron is a typical acceptor impurity in silicon.

Because the captured electrons are in localized states, they aren't free to contribute to conduction.

But, consider if we start with intrinsic material and start to increase the density of acceptor impurities. In intrinsic material, the conduction band has very low occupancy, so the electrons can't be captured from there. Instead, they're captured from the valence band, leaving holes behind.

And indeed, these holes do attract electrons from the conduction band, but to make p-type material you typically add many more (orders of magnitude more) impurities than the intrinsic carrier density, so there simply aren't enough conduction band electrons to fill the acceptor states or to fill the holes resulting from the acceptors attracting valence electrons.

Another way to look at this is to look at Fermi level. As the acceptors capture electrons and create holes, then we know the occupancy of the valence states has decreased. Since the valence state occupancy is reduced we realize the Fermi level must be closer to the valence band edge than in the intrinsic material. And since the Fermi level is closer to the valence band edge, it must be farther from the conduction band edge, resulting in even fewer conduction band electrons than in intrinsic material. In (quasi-)equilibrium, we find a Fermi level that gives a statistical balance between density of valence band holes, occupancy of acceptor states, and density of conduction band electrons. And this gives tells us in what ratio holes and electrons are available to carry current.

Short version: The holes can try to attract electrons from the conduction band all they want, there simply aren't enough electrons there in p-type material to fill all the holes.

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