Pure silicon is deliberately ‘impurified’ (doped) with tri- and pentavalent impurity atoms to make a p- or n-type semi-conductor. If we put a p-and n-type crystal together we get a junction (pn-junction, Fig. 3) whose electrical properties determine the performance of the solar cell. In equilibrium (with no external voltage) the Fermi characteristic energy level EF will be the same throughout. Because of the difference in the concentrations of electrons and holes in the p- and n-regions, electrons diffuse into the pregion and holes into the n-region. The immobile impurity atoms create a space charge-limited current region; the diffusion current and the field current offset one another in equilibrium. The diffusion potential UD in the pn-junction depends on the amount of doping and corresponds to the original difference between the Fermi energy levels of the separate p- and n-regions. The distance between the valence band and the conduction band in silicon at room temperature is