# Current flow in semiconductors

1. Sep 11, 2015

### beyondlight

I have big trouble understanding how current flows in semiconductors.

As i have understood it it has to do with "valence" and "conduction" bands.

I see valence bands as an discrete energy level within an atom that are possessed by electrons. These electrons do not contribute to a net current if there is a electrif field applied over the semiconductor.

However I am confused about the conduction bands. Are they supposed to be viewed as an outer layer of the atom or as energy states, that if an electron can be in those energy states they are completely free from the atoms and just float around in the semiconductor??

The confusing thing is that is if the latter option is true, then it doesn't matter if the valence band is full or empty because they don't contribute to the condution. But if even holes float around in the conduction band then they cancel each other out and so there is not current?

And another confusing thing is that why is there needed to be holes in the valence band in order to enable the electrons to jump down into the holes and thus create a current flow? What is the need for electrons to exploit holes in the valence band if they are already in the conduction band?

How does holes in the valence band enable condution??? Isn't it a state where particles are trapped inside atoms? And so cant support conduction??

2. Sep 11, 2015

### Drakkith

Staff Emeritus
I'd think of them as energy states. Trying to think about which electrons are where is confusing, as any electron can be just about anywhere in the atom since the orbitals are probability densities, not set paths through space.

Holes don't exist in the conduction band, they exist in the valence band.

Electrons jumping back down from the conduction band to the valence band isn't what gives you electrical current. The movement of the electrons and the movement of the holes combined give you current flow. A hole on one side of a diode can move to the other side, acting as if it were a positive charge. Since the hole has moved, the location it was originally at is now more negatively charged, as it was replaced by an electron in the valence band (this is not recombination, its just electrons moving around in the valence band). So you have the movement of the original electron that was excited into the conduction band, plus the movement of electrons in the valence band (which is modeled as if the hole were a positive charge moving in the opposite direction).

See here: https://en.wikipedia.org/wiki/Electron_hole