In the space below indicate the range of energy values (in eV) for each band and the resulting energy gap. Answer the following questions:
We are now able to apply energy diagrams to explain the spectra of LEDs. However, to be successful we needed to extend the concept of individual allowed energies to allowed energy bands. The spectra emitted by LEDs is the result of electrons making transitions from a number of energy levels in the conduction band to a number of energy levels in the valence band. The electron transitions that are allowed can range from the highest energy level of the conduction band to anywhere between the lowest and highest energy levels of the valence band (see the left side of the figure below). Other electronic transitions that are allowed can range from the lowest energy level of the conduction band to anywhere between the highest and lowest energy levels of the valence band (see B of the figure below). These electronic transitions result in the emission of a broad continuous spectrum that is concentrated on a particular color (and thus energy) of light. The size of the energy gap of the semiconductor solid determines the color of light emitted by the LED. Thus, if we know the spectra of light emitted by an LED we can predict its energy gap.