As long as inefficient metal wires were used, textbook writers were correct in asserting that thermoelectricity could never be used for power production. The secret of practical thermoelectricity is therefore the creation of better thermoelectric materials. (Creation is the right word since the best materials for the purpose do not exist in nature.) To perform this alchemy, we first have to understand the Seebeck effect.

Electrons and Holes

Let’s examine the latticework of atoms that make up any solid material. In electrical insulators all the atoms’ outer electrons[5] are held tightly by valence bonds to the neighboring atoms. In contrast, any metal has many relatively loose electrons which can wander freely through its latticework. This is what makes metals good conductors.

THERMOELECTRICITY

Figure 5 Thermoelectric couple made from p- and n-type semiconductors. The impurity atoms (I) are different in each leg and contribute an excess or deficiency of valence electrons. Heat drives both holes and electrons toward the cold junction.

Tc WASTE HEAT OUT ELECTRONS LOAD COLD JUNCTION HOLES ELECTRONS p SEMICONDUCTOR n SEMICONDUCTOR HOT JUNCTION Th HEAT IN Simplified Sketch of Atomic Lattice HOLE ELECTRON VALENCE BONDS SEMICONDUCTOR LATTICES I = Impurity atom

[Figure 5] suggests the latticework of a semiconductor. It is called a semiconductor because its conductivity falls far short of that of the metals. The few electrons available for carrying electricity are supplied by the deliberately introduced impurity atoms, which have more than enough electrons to satisfy the valence-bond requirements of the neighboring atoms. Without the impurities, we would have an insulator. With them, we have an n-type semiconductor. The n is for the extra negative electrons.

A p- or positive-type semiconductor is also included in [Figure 5]. Here the impurity atom does not have enough valence electrons to satisfy the valence-bond needs of the surrounding lattice atoms. The lattice has been short-changed and is, in effect, full of positive holes. Strangely enough, these holes can wander through the material just like positive charges.

The electron-hole model does not have the precision the physicist likes, but it helps us to visualize semiconductor behavior.