All magenta-colored horizontal bars (i.e., Ef and scroller) may be mouse-dragged. After becoming sufficiently familiar with the inter-relationship between the Fermi level (Ef) and the electron (n) and hole (p) concentrations, users may use the donor/acceptor impurity panels to learn how to use the doping of impurities to control the carrier concentrations (n & p) and thus the Fermi level.