How is a semiconductor electrically neutral?

Question

I'm in the process of learning how transistors works, which starts with understanding how doping is used to create n-type and p-type semiconductor materials.

All the resources I read sort of explain this the same way, and I'm missing something. P-type semiconductors have extra holes and are predisposed to accept electrons, whereas n-type semiconductors have extra free electrons and are predisposed to donate them. This is the fundamental principle of how transistors work.

But every resource emphasizes that in spite of this both n-type and p-type semiconductors are electrically neutral, which is where I'm lost.

If one has extra electrons, and one is missing electrons, how are they electrically neutral and not charged? I seem to have a block about this or something, I just don't get it.

This whole diffusion process is just really confusing.

Answer 1

They don't have extra electrons or holes. They just have electrons or holes that do not fit in the crystal structure. So these are "free" to move.

• Think of a crystal of a material where the atoms have 4 valence electrons. Each atom has just as many electrons as protons and is neutral.

• Now replace one atom with a new atom of just one higher atomic number. This atom might have 5 valence electrons. It still has just as many electrons as protons and is neutral.

• But it doesn't fit. It can only fit into the existing structure by using 4 of the valence electrons. The last one will be pushed away and is now sort of "free" to move around.

When you know that there is an extra electron you are just forgetting that there is actually also and extra proton. Inside the dopant atom. The electric neutrality is kept.