Electron Structure in Isolated Atoms vs. Solids

Electron Structure of Isolated Atoms vs. Solid Materials

• Electron structure of an isolated atom consists of discrete energy levels with electrons occupying specific orbitals defined by quantum numbers
• Electron structure of a solid material features continuous energy bands formed by the overlap of atomic orbitals due to close packing of atoms
• This interaction creates a valence band and a conduction band, separated by a band gap, determining the material's electrical properties (insulator, semiconductor, or conductor)

Electron Excitation Involving Donor Impurity Atom

• No hole is generated when an electron is excited from a donor energy level to the conduction band
• This is because the donor energy level lies just below the conduction band, and the excitation process releases a free electron into the conduction band without creating an electron vacancy in the valence band

Electron and Hole Motions in a p-n Junction

• Under forward bias, the potential barrier is reduced, allowing electrons from the n-side to move into the p-side and holes from the p-side to move into the n-side, resulting in significant current flow
• Under reverse bias, the potential barrier is increased, preventing majority carriers from crossing the junction, resulting in minimal current flow
• Minority carriers (electrons on the p-side and holes on the n-side) are pulled toward the junction, but their contribution to current is negligible

Electron Band Structures for Solid Materials

• Solid materials exhibit four possible electron band structures: metals, insulators, semiconductors, and semimetals
• Metals: conduction band and valence band overlap, allowing electrons to flow freely and making them good conductors of electricity
• Insulators: large band gap between valence band and conduction band, preventing electron flow at room temperature and thus inhibiting conductivity
• Semiconductors: smaller band gap, allowing some electron excitation at room temperature, enabling moderate conductivity that can be controlled by doping
• Semimetals: overlapping conduction and valence bands similar to metals but with fewer charge carriers, resulting in unique electrical properties that vary with temperature and doping

Electron Excitation Events

• Metals: electron excitation is unnecessary, as electrons can flow freely between conduction and valence bands
• Semiconductors (intrinsic): thermal energy can excite electrons from the valence band to the conduction band, creating free electrons and holes
• Semiconductors (extrinsic): doping with donor or acceptor impurities enables controlled excitation of electrons and creation of free electrons and holes
• Insulators: high-energy radiation or thermal energy can excite electrons from the valence band to the conduction band, creating free electrons and holes, but this is rare at room temperature

Learn about the differences in electron structure between isolated atoms and solid materials, including discrete energy levels and continuous energy bands.