Final answer:
Energy is given off when an electron and a hole recombine in a semiconductor crystal. The process is equated to an electron dropping from a higher energy state to a lower energy state, emitting a photon. The released energy falls within the infrared or visible range, specific to the semiconductor's band gap.
Step-by-step explanation:
When an electron and a hole of a semiconductor crystal recombine, energy is given off. This process is analogous to an electron transitioning from a higher energy state to a lower one in an atom, where the energy difference is emitted in the form of a photon. In semiconductors, the electron transitions from the conduction band to the valence band, releasing energy that typically falls within the infrared or visible range of the electromagnetic spectrum, depending on the material's band gap.
There are two main approaches to determining the energy levels of an electron in a crystal: the band theory and the quantum mechanical model. The band theory explains the presence of energy bands and energy gaps in the energy structure of a crystal, where continuous ranges of allowed energy levels are separated by forbidden ranges or gaps. Materials with wide energy gaps tend to be good insulators, while those with very narrow or no gaps are good conductors. Semiconductors fall in between, with small enough band gaps that allow electrons to be thermally excited from the valence band to the conduction band at room temperature.
An insulator is characterized by a large energy gap typically greater than 1 eV, while a semiconductor has a smaller gap which makes electron excitation more feasible. The process of doping can introduce excess electrons or holes within a semiconductor, altering its electrical properties by changing the balance between electrons and holes, thus modifying the conductivity.