Where conductivity comes from

Everything electrical about a material is decided by the electrons in its outermost shell.

• The Bohr model puts protons and neutrons in the nucleus and electrons in discrete shells around it. The atomic number is the proton count and identifies the element.
• The outermost shell is the valence shell, and the electrons in it are the valence electrons. They are the only ones loosely enough bound to take part in conduction.
• A conductor such as copper has one valence electron, easily freed. An insulator holds eight and gives up none. A semiconductor such as silicon has four, which is what makes it interesting.

Energy bands

In an isolated atom the energy levels are discrete. Pack atoms into a solid and those levels smear into bands.

• The valence band holds bound electrons; the conduction band holds free ones. Between them sits the band gap.
• An insulator has a wide gap that thermal energy cannot bridge. A conductor has bands that overlap, so free electrons exist at any temperature.
• A semiconductor has a small gap, around 1.1 eV for silicon. Heat alone frees some electrons, so conductivity rises with temperature, the opposite of a metal.

Intrinsic, and then doped

Pure silicon is a poor conductor. Its value is that its conductivity can be engineered.

• In a silicon crystal each atom shares its four valence electrons in covalent bonds with four neighbours.
• Break a bond and you create an electron-hole pair. The freed electron carries current one way, and the hole it leaves behind carries current the other way as neighbouring valence electrons shuffle across.
• Doping with a pentavalent donor such as phosphorus adds a spare electron and gives n-type material, where electrons are the majority carriers.
• Doping with a trivalent acceptor such as boron adds a hole and gives p-type material, where holes are the majority carriers. The other carrier type is present in both, as the minority carrier.