# Doubt in working of p-type semiconductors

Correct me if I'm wrong, but as far as I know, in p-type semiconductors, first the excitation of an electron takes place from the valence band to the conduction band due to thermal energy, then this electron moves in the direction of the applied electric field until it encounters a hole, into which it goes to complete the missing bond.

However I fail to understand how this increases conductivity. According to me, these extra holes should lead to a decrease in conductivity as they cause a perfectly free electron moving in the conduction band conducting electricity to stop and enter them.

In a $p$ type semiconductor the electrons are thermally excited into localised gap states just above the top of the valence band, and they leave behind a hole in the valence band.

The electrons in the gap states are not mobile, so the only charge carriers that can flow are the positive holes in the valence band.

• Thank you for your answer. Just one more question; if there is a hole where there should have been a bond between two, say, Si atoms, what makes it favourable for an electron to jump from a bond to that hole, even if it is in the gap state, considering that the same hole is created at its original location? Is it the externally applied electric field that "persuades" the electron to move? – User Dec 2 '17 at 1:35

The presence of abundant holes attracts the neighboring electrons to sit in it. As long as the electron fills the holes in the silicon crystal there will be new holes behind the electron as it goes far from it. The newly created holes successfully attract the electrons, creating other new holes leads to the movement of holes, creating a standard electric current flow in the semiconductor. So the electron don't stop after merging with the hole instead it is attracted by the hole created due to the movement of electron preceding it.

first the excitation of an electron takes place from the valence band to the conduction band

You've forgotten about the acceptor impurity states.

The main thing about a p-type material is that there are positively charged impurity ions scattered around the material. These produce "acceptor states" with energy levels just above the top of the valence band.

Now, there's a bit of backwards logic to figure out why this matters: Since the energy levels are near the top of the valence band, they're (for moderate doping concentrations) below the Fermi level and therefore very likely to be occupied. Each acceptor site that is occupied means that a valence band state has become unoccupied, creating a hole.

As the other answers said, the holes are mobile and so they contribute to conductivity of the material, while the electrons in the acceptor states are localized so they do not.