Energy Bands

Why does pure silicon conduct a little, but not a lot? Why does copper conduct freely, and glass barely at all? The valence-electron picture from the last page gives an intuitive answer, but physicists have a more precise tool for it: the energy band model. It explains, with an actual number you can calculate, exactly how “hard to free” an electron is in any given material.

From single atoms to bands

In an isolated atom, electrons can only occupy specific, sharply defined energy levels — like rungs on a ladder. But in a solid crystal, atoms are packed closely together and their electron energy levels interact and blur into continuous ranges called energy bands, separated by ranges of energy that are completely off-limits, called band gaps.

Two bands matter most for conduction:

• The valence band — the range of energies where electrons are bound to atoms, still participating in chemical bonds. Electrons here cannot move freely through the crystal.
• The conduction band — a higher range of energies where electrons are no longer tied to any particular atom. Electrons here are free to roam through the crystal and carry electric current.

Between them sits the band gap: a forbidden zone of energies that no electron can occupy. To conduct, an electron in the valence band must gain enough energy to leap entirely across this gap into the conduction band. How wide that gap is determines how easy or hard that leap is — and that single number determines whether a material is a conductor, semiconductor, or insulator.

Comparing the three material types

The pattern across the three diagrams:

• Conductors have overlapping (or touching) valence and conduction bands — electrons sit in the conduction band with no jump required at all, so current flows with almost no resistance.
• Semiconductors have a small but nonzero gap. At room temperature, some electrons pick up just enough thermal energy to hop across, so there’s a modest, controllable population of free carriers.
• Insulators have a huge gap — so wide that essentially no electron at normal temperatures can ever pick up enough energy to cross it, so almost no current flows.

Silicon’s band gap: the number that makes everything work

Silicon’s band gap is about 1.12 electron-volts (eV) at room temperature. An electron-volt is just a convenient unit of energy for atomic-scale physics — one electron-volt is the energy a single electron gains crossing a 1-volt potential difference.

For comparison:

This 1.12 eV gap is the “sweet spot” that makes silicon so useful: it’s small enough that engineers can push electrons across it on purpose (with heat, light, or — most importantly — doping), but large enough that silicon doesn’t uncontrollably conduct all the time on its own.

Why this matters for doping

Crossing the full band gap by thermal energy alone is relatively rare at room temperature — that’s why pure silicon is a weak conductor. But if you introduce impurity atoms with a spare electron (or a spare hole), those atoms create new, much shallower energy levels sitting just inside the band gap, very close to the conduction band (or valence band). Electrons can hop that short remaining distance far more easily than crossing the full 1.12 eV gap. This is exactly the mechanism behind doping, covered next in Doping: N-Type and P-Type.

Key takeaways

• Electrons in a solid occupy energy bands, separated by forbidden band gaps.
• The valence band holds bound electrons; the conduction band holds free, current-carrying electrons.
• Band gap size determines material behavior: ~0 eV for conductors, a small gap (~1 eV) for semiconductors, and a large gap (~9 eV) for insulators.
• Silicon’s band gap is about 1.12 eV — small enough to control, large enough not to run away uncontrolled.
• Doping works by adding energy levels inside the gap, close to the existing bands, making it far easier for carriers to reach the conduction band or valence band. See Doping: N-Type and P-Type.
• The next practical device built on this idea is the PN Junction Diode.