I've studied the Kronig–Penney model and learned about the effective mass theory and I know that at the top of the valence band the effective mass of electrons are negative, this is just an introduction about the information I know to ask the question properly.
My question is: In case of semiconductor an empty state is called hole and it has an positive charge and positive effective mass also, but what really is a hole and how did we introduce this concept and how to understand it? Because a lot of textbooks gives different interpretations to it and I can't understand it correctly.
