Energy Bands and Intrinsic Semiconductors

Why silicon sits between metals and insulators.

In a solid the electron energy levels merge into bands. The valence band is filled with bound electrons and the conduction band holds free ones. A conductor has the bands overlapping, an insulator has a wide gap between them, and a semiconductor has a modest gap.

Silicon has a gap of about 1.1 electron-volts. Heat can lift an electron across, leaving a hole behind, so pure silicon conducts a little, and unlike a metal its conductivity rises as it warms up.

• Electrons and holes both carry current.
• Pure semiconductors are called intrinsic.
• Germanium has a smaller gap, about 0.67 eV.

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