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.
The Seebeck effect is demonstrated when pieces of p- and n-type material are joined as shown in [Figure 5]. Heat at the hot junction drives the loose electrons and holes toward the cold junction. Think of the holes and electrons as gases being driven through the latticework by the temperature difference. A positive and a negative terminal are thus produced, giving us a source of power. The larger the temperature difference, the bigger the voltage difference. Note that just one thermocouple leg can produce a voltage across its length, but couples made from p and n legs are superior.