David W. Ball

The development of artificial light sources was a major step forward in technological development. No longer would humanity need to depend upon the sun or relatively dangerous open flames for light. The history of incandescent light bulbs — light sources that depend upon the electrical heating of a thin strip of wire called a filament so hot that it emits substantial amounts of light (and heat) — goes back to the early 1800s and the development of practical batteries. Although several patents were granted around the world for the development of incandescent lights through the 1800s, the first practical incandescent light bulb generally is credited to Thomas Edison, who developed his version in 1879 (Figure 1) (1).

Figure 1

There are other types of light sources. Fluorescent light bulbs take advantage of a different mechanism for generating light, based more upon quantum-mechanical rules rather than black-body radiation. There are also light sources that are based upon semiconductor constructions called diodes, and so they are more appropriately called light-emitting diodes (LEDs). This column is devoted to these types of lights.

Conductors, Nonconductors, and Semiconductors

The ability of a substance to move charged particles under the influence of an electric potential difference is called the substance's electrical conductivity. The quantity σ, called the conductivity, is the proportionality constant that relates the strength of the potential difference and the induced current density.

Substances that have a high value of σ are called conductors. Most metals, for example, have a high value of σ: for Ag, the most conductive metal, σ has a value of 6.3 × 10⁷ S/m (siemens per meter; a siemen is an inverse ohm, or Ω^–1 ). Substances that have a low value of σ are called nonconductors. The conductivity of glass ranges from 10^–10 –10^–14 S/m, whereas polytetrafluoroethylene (Teflon) has a σ of 10^–22 –10^–24 S/m. Another word for nonconductor is insulator.

Because all substances have values of conductivity, the difference between a conductor and a nonconductor is a matter of degree. On a table of conductivities, substances that are obvious conductors are separated by substances that are obvious nonconductors around σ ≈ 1, but that is a fluid boundary.

A semiconductor has electrical conductivity properties between those of conductors and nonconductors. Given what we said about the difference in the values of σ for conductors and nonconductors, being a semiconductor is also a question of degree. We can propose values of σ ranging from 10 to 1 × 10^–3 S/m as characteristic of semiconductors, but these values are not set in stone. Nonetheless, intermediate values of σ characterize semiconductors.