# Doped semiconductor: "what if all donated electrons are gone?"

I have trouble understanding conductivity of a n-doped semiconductor in the band theory.

I know that donator atoms carry one excess electron that can enter the conduction band easily. If this happens, a fixed positive charge remains.

Assume that due to an applied voltage, the excess electrons in the conduction band are all removed from the semiconductor. How can the current continue to flow, how can new electrons enter the semiconductor or the conduction band?

Are the doping atoms "close enough" to each other so that excess electrons can move from doping atom to another?

I have less trouble understandung conductivity in a p-doped SC: an electron "fills up the missing bond", creating a fixed negative charge. Conduction occurs in the valence band by "passing the hole" from one Si atom to the next.

• There are two ways of looking at it. One: it's identical to the process you describe for SC. Why do you think it would be different? The other is that the extra electrons join the conduction band, and propagate (nearly) freely through the semiconductor in the same way current flow in a metal. Nov 14, 2015 at 14:55
• "it's identical to the process you describe for SC" do you mean p-doped SC? I don't think it's identical, because the Si-Atoms passing "holes" are "next to each other", as opposed to the donator atoms that are only ~1ppm. Nov 14, 2015 at 15:08
• I'm not clear on the distinction. Anyway, a "hole" that starts on a dopant atom can be filled by and "electron" from a neighboring Si atom. Now the dopant has four bonds and and Si has three. Nov 14, 2015 at 15:14
• OK, but the common representation puts the dopant atom's electrons close to the conduction band. "How do the Si electrons from the valence band get enough energy to reach the hole at the dopant?" Is there a difference between a hole due to a missing charge or due to an incomplete bond? Nov 14, 2015 at 15:43
• Electric forces are very large. It would take a huge voltage to remove an electron from 1 atom in a million. Nov 14, 2015 at 16:40