When two slabs of n-type and p-type semiconductors are put in contact, the relative affinities of the materials cause electrons to migrate out of the n-type material across the junction to the p-type material. What is the reason for this electron migration?
1) Difference in energy levels between the n-type and p-type materials
2) Difference in doping concentrations between the n-type and p-type materials
3) Difference in band gaps between the n-type and p-type materials
4) Difference in electron affinities between the n-type and p-type materials