The following links are to a variety of applets that should help you visualize what is happening inside pn junction diodes in equilibrium . This applet allows you to change the type of semiconductor and doping level on each side of the junction. This applet allows you to change the type of semiconductor and doping level on each side of the junction. This applet allows you to change the type of semiconductor and doping level on each side of the junction. A particularly neat thing about the lower diagram is that the shading in the conduction band and valence band schematically represent the density of electrons and holes as a function of energy.
Darker shade implies higher densities and lighter implies lower.
You can follow other links on these pages to find other applets you might find useful and interesting.