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The red rectangle represents a p-type semiconductor and the blue rectangle an n-type semiconductor. At the moment, this applet has correct materials parameters for three semiconductors, Si, Ge and GaAs. All material parameters and energies in this applet are at room temperature only. Most material parameters, however, are temperature-dependent. For the temperature-dependence of the energy gap, see the ref.
You can change the doping level by typing in the value and clicking the 'setNewValue' button.
Equilibrium band diagrams appear below the semiconductor. The green horizontal line is the Fermi level. You may initiate the pn junction formation by clicking the 'FormJunction' button or using mouse drag and watch the physical system approach a new (electro-thermal) equilibrium which is characterized by a constant Fermi level throughout the material. The white region between the red (p-type) and blue (n-type) regions is the transition (or depletion) region where most of the free carriers are depleted. This white region is the space charge region with uncompensated ionized impurity charges which produce electric field in this region.
Repeat formation and separation of the PN junction several times trying out different doping values. Do until you become sufficietly familiar with the equilibrium band diagram and its formation principles.