One method to bring electrons from a valence band to a conduction band is by applying an external energy source, such as thermal energy or photon energy. This process is known as excitation.
In the case of thermal excitation, increasing the temperature of a material provides energy to the atoms or molecules, causing them to vibrate more vigorously. This increased thermal energy can enable some electrons in the valence band to gain enough energy to jump to the conduction band, creating electron-hole pairs.
In the case of photon excitation, electromagnetic radiation, typically in the form of photons, is absorbed by the material. The energy carried by the photons can be transferred to the electrons, elevating them from the valence band to the conduction band. The minimum energy required for this excitation is determined by the energy band gap of the material.
Once the electrons are in the conduction band, they are free to move and participate in electrical conduction, making the material more conductive.