Click the "Create Ground State Band" button to the right of the energy level diagram. A valence band appears in the energy diagram that spans both the p side and n side of the LED. Then click the "Create Excited State Band" button. A conduction band appears in the diagram above the valence band. The valence band is shaded lighter than the conduction band to indicate it is populated with more electrons than the conduction band. Click and drag either band vertically to change the energy gap.
Click the "Create Built in Voltage" button to the right of the energy diagram. Now you can click and drag the right half of the energy band diagram independent of the left half. Physically, this action corresponds to doping the left side with accepters, to become P-type and doping the right side with donors, to become N-type. Drag the conduction or valence band on the right (i.e. N-side) downward, because the electron energies in the N-side are lower than the P-side. Observe the conduction band on the N-side now become dark to indicate population with electrons.
Drag the "Input Energy" slider to the right to increase the input energy supplied to the laser diode. Simultaneously, observe the increase movement of the energy bands on the right (N-side) until there is no energy difference between the energy bands on the P-side and N-side. The conduction band on the P-side now becomes dark to indicate that electrons from the N-side have flowed over to it. A thick transition arrow appears at the junction between the P-side and N-side. The trial spectrum below the energy level diagram indicates the energy of the light emitted by the laser. The laser begins to glow with light of the appropriate color. To make the trial spectrum match the Actual output spectrum with the trial spectrum click the "Clear All" button and repeat from STEP 1.