3_7 Semiconductors and Lead Pigments: Silicon, Doping, White Lead and Red Lead
Two very different technologies come from the bottom half of group 14. Silicon and germanium are semiconductors, the materials in every computer chip and solar cell. Lead forms brightly coloured oxides and carbonates that were used as paint pigments for centuries. In both cases the useful property comes from how electrons are held, and for lead it also depends on the inert pair effect from earlier in this chapter.
What Makes a Semiconductor
A semiconductor conducts electricity better than an insulator but much less well than a metal. In a silicon crystal, every atom forms four covalent bonds, just like the carbon atoms in diamond, so almost all the valence electrons are tied up in bonds. In band theory, the filled valence band is separated from the empty conduction band by a small band gap: about 1.1 eV for silicon and 0.67 eV for germanium, compared with about 5.5 eV for diamond. A little heat or light can lift some electrons across the gap. Each one leaves behind a hole, a missing electron that behaves like a positive charge, and both the electrons and the holes carry current.
This explains the key properties. Heating a semiconductor releases more charge carriers, so its resistance falls, while a metal's resistance rises with temperature. Light also releases carriers, so semiconductors are used in photocells, light sensors and solar cells. Pure silicon or germanium, where the carriers come only from the element itself, is called an intrinsic semiconductor. Other semiconductors include compounds such as gallium arsenide, cadmium sulphide, silicon carbide and lead sulphide.
Doping: n-Type and p-Type
Pure silicon conducts too poorly to be useful on its own. Adding a tiny, controlled amount of another element, roughly one atom per million, is called doping, and it produces an extrinsic semiconductor:
Doped silicon
Type | Dopant | Effect | Main charge carriers |
n-type | Group 15 (P, As) | Each dopant atom has one electron more than it needs for four bonds | Free electrons |
p-type | Group 13 (B, Ga) | Each dopant atom is one electron short, leaving a hole | Holes |
Where p-type and n-type silicon meet, they form a p–n junction, which lets current flow easily in one direction only. Junctions are the basis of diodes, LEDs, solar cells and, combined in pairs, transistors. Billions of transistors on a single silicon chip are what make modern computers, phones and calculators possible.
Lead Oxides as Pigments
Lead forms several coloured oxides, and their colours and oxidation states follow from the stable +2 state of lead and its oxidizing +4 state:
- Lead(II) oxide, PbO (litharge, massicot): yellow to reddish-yellow, depending on the crystal form. It is used in lead glass (flint glass), glazes and as a drier in varnishes.
- Red lead, $\mathrm{Pb_3O_4}$ (minium): a scarlet powder made by heating lead or PbO in air at about 340–450 °C. It contains both oxidation states,
\mathrm{2PbO \cdot PbO_2}
, and loses oxygen above about 470 °C to form PbO again. - Lead(IV) oxide, $\mathrm{PbO_2}$: a dark brown, strongly oxidizing solid and the positive plate material in lead–acid batteries.
\begin{aligned} \mathrm{6PbO + O_2} &\rightleftharpoons \mathrm{2Pb_3O_4} \\ \mathrm{Pb_3O_4 + 4HNO_3} &\rightarrow \mathrm{2Pb(NO_3)_2 + PbO_2 + 2H_2O} \end{aligned}
The reaction with nitric acid shows the mixed oxidation states clearly. The two Pb(II) units dissolve as lead nitrate, while the Pb(IV) unit is left behind as insoluble lead dioxide. Red lead was widely used in primer paints for iron and steel, because it slows rusting by keeping the metal surface in an oxidized, passive state.
White Lead and Chrome Yellow
White lead is basic lead carbonate,
\mathrm{2PbCO_3 \cdot Pb(OH)_2}
. It mixes well with linseed oil and covers surfaces very well, and it was the main white pigment for centuries. Its weakness is that hydrogen sulphide in polluted air turns it into black lead sulphide, so white paintwork darkens over time:[Pb+2].[O-]C([O-])=O.S>>[Pb+2].[S-2].O=C=O.OChrome yellow is lead chromate,
\mathrm{PbCrO_4}
, and boiling it with dilute alkali gives orange and red basic lead chromates. All of these pigments share one serious drawback: lead is a cumulative poison that damages the nervous system, especially in children who swallow flakes or dust from old paint. Lead pigments have therefore been banned or strictly limited in household paints in most places, and titanium dioxide has replaced white lead as the standard white pigment.Tips for remembering
- N for extra negative, P for missing electron (positive hole): phosphorus (group 15) adds an electron to silicon, and boron (group 13) leaves a hole.
- Heat helps semiconductors, hurts metals: warming frees more charge carriers in a semiconductor, but in a metal it only makes the atoms vibrate more and scatter the electrons.
- Red lead is two-faced lead: Pb₃O₄ is 2PbO·PbO₂, so it contains both Pb(II) and Pb(IV), and nitric acid separates them.