Electron excitation is the transfer of a bound electron to a more energetic, but still bound state. This can be done by photoexcitation (PE), where the electron absorbs a photon and gains all its energy or by electrical excitation (EE), where the electron receives energy from another, energetic electron. Within a semiconductor crystal lattice, thermal excitation is a process where lattice vibrations provide enough energy to transfer electrons to a higher energy band. When an excited electron falls back to a state of lower energy, it undergoes electron relaxation. This is accompanied by the emission of a photon (radiative relaxation) or by a transfer of energy to another particle.