I'm trying to understand how a diode works and for this I've used(among other resources) the book written by Albert Malvino, Electronic Principles.
Everywhere I read about this topic, it says that when the N-type and P-type semiconductors are joined together, the free electrons from the N-type diffuse to the P-type.
I don't understand why is the diffusion happening.
The book Electronic Principles contains an attempt to explain this and states that the electrons diffuse because they have the same charge and they repel each other, but in my understanding the P-type and the N-type semiconductors have a neutral charge, because the number of positive charges (protons in the nuclei) is equal to the number of negative charges (free electrons and covalent bonds electrons), so the electrons repeling each other can't actually be the cause of the diffusion.
Answer
The answer lies in a thermodynamic argument. The diffusion is a spontaneous process that occurs when particles with random motions are not uniformly distributed. The electrons in the conduction band in the n region are much more numerous that in the p junction. They will naturally tend to balance the concentration from the n junction to the p junction since the equilibrium tends to be the state with maximum disorder ie maximum entropy.
A good visual picture of how the diffusion process occurs is in this video at 4'50. https://www.youtube.com/watch?v=JBtEckh3L9Q
So the answer to what causes diffusion is thermal motion of quasi free electrons.
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