ABSTRACT
The electrochemical potential (Fermi level) is the physical quantity that sets the essential transduction between electron density in a semiconductor and the voltage at the contacts. In this chapter, we examine the relationship among the density of states, the occupation of such states, and the voltage, and also several important cases for energy conversion and storage devices. Based on statistical concepts, we provide the interpretation of the variation of the Fermi level in terms of the change of entropy of an ensemble of electrons. We analyze various practical instances such as the occupation of band edges in doped semiconductors, the degenerate semiconductors used in highly conducting transparent oxides, and the properties of hot electrons. A device of fundamental importance in many applications is the diode that we describe in a simple model in terms of selective contacts and homogeneous Fermi levels. The recombination diode model serves to develop a detailed analysis of an electronic device as a thermal machine that uses the available energy to produce a quantity of work, conserving or decreasing the total entropy. In the final section of the chapter, we analyze in more depth the origin of voltage in the Li battery cell. To this end, we discuss the energy diagram containing a mixed ionic/electronic conductor that incorporates variations of chemical potential of both types of carrier. The model for the chemical potential of weakly interacting particles is applied to ions in the lattice gas model. Finally, to explain the voltage in more realistic situations, configurational phase transitions and the consequences of the changes of oxidation state of the intercalation material are discussed.
