What happens when the energy of the incoming photon exceeds the band gap energy?

When the energy of the incoming photon exceeds the band gap energy of a semiconductor, the correct outcome is that it is absorbed and leads to electron excitation. This occurs because the band gap energy represents the minimum energy required to excite an electron from the valence band to the conduction band within the semiconductor material.

When a photon with energy greater than the band gap is absorbed, it provides enough energy to promote an electron to a higher energy state, creating a free electron in the conduction band and leaving behind a hole in the valence band. This process is fundamental to the operation of devices like photodetectors and solar cells, as it generates charge carriers that can be harnessed for electrical current.

In contrast, if the energy of the photon were less than the band gap energy, the photon would not have the necessary energy to excite an electron, leading to scenarios such as reflection or transmission without absorption. The other options, including thermal expansion or no interaction, do not directly relate to the specific response of a semiconductor to incoming photon energy that surpasses the band gap threshold. Thus, the absorption and resulting electron excitation is the expected and correct process that occurs.