[Illustration: 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._]
T_c WASTE HEAT OUT
ELECTRONS
LOAD
COLD JUNCTION
HOLES
ELECTRONS
_p_ SEMICONDUCTOR
_n_ SEMICONDUCTOR
HOT JUNCTION
T_h 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.