1. The electron affinity to an isolated neutral Si atom is 1.39eV while to a neutral Si crystal it is 4.02eV. Why is it larger in Si crystal? Give a simple explanation based on electrostatic or Coulomb force.
2. Draw the electron energy level and transition energy diagrams of a second electron attached to a neutral atom, i.e., a neutral atom can bind two electrons in analogy to Figs (b) and (c). Use dark lines or curves for the potential energy well that binds the second electron in question. This situation actually occurs at some impurity and defect centers in semiconductors.
