Atom Energy Levels

In the isolated atomic structure model there are discrete (individual) energy levels associated with each orbiting electron, as shown in Fig. 1.3(a). Each material will, in fact, have its own set of permissible atom energy levels for the electrons in its atomic structure. The more distant the electron from the nucleus, the higher the energy state, and any electron that has left its parent atom has a higher energy state than any electron in the atomic structure.

Figure 1.3 Energy levels: (a) discrete levels in isolated atomic structures; (b) conduction and Valence bands of an insulator, semiconductor, and conductor. Between the discrete energy levels are gaps in which no electrons in the isolated atomic structure can appear. As the atoms of a material are brought closer together to form the crystal lattice structure, there is an interaction between atoms that will result in the electrons in a particular orbit of one atom having slightly different energy levels from electrons in the same orbit of an adjoining atom. The net result is an expansion of the discrete levels of possible energy states for the of valence electrons to that of bands as shown in Fig. 1.3(b)

Note that there are boundary levels and maximum energy states in which any electron in the atomic lattice can find itself, and there remains a forbidden region between the valence band and the ionization level. Recall that ionization is the mechanism whereby an electron can absorb sufficient energy to break away from the atomic structure and enter the conduction band. Note that the energy associated with each electron is measured in electron volts (eV). The unit of measure is appropriate, since

As derived from the defining equation for voltage V= W/Q. The charge Q is the charge associated with a single electron. Substituting the charge of an electron and a potential difference of 1 volt into Eq. (1) will result in an energy level referred to as one electron volt. Since energy is also measured in joules and the charge of one electron = 1.6*10^-19 coulomb,

At 0 K or absolute zero (-273.15°C), all the valence electrons of semiconductor materials find themselves locked in their outermost shell of the atom with energy levels associated with the valence band of Fig. 1.3b. However, at room temperature (300 K, 25°C) a large number of valence electrons have acquired sufficient energy to leave the valence band, cross the energy gap defined by Eg in Fig. 1.3b and enter the conduction band. For silicon Eg is 1.1 eV, for germanium 0.67 eV, and for gallium arsenide 1.41 eV. The obviously lower Eg for germanium accounts for the increased number of carriers in that material as compared to silicon at room temperature.

Note for the insulator that the energy gap is typically 5 eV or more, which severely limits the number of electrons that can enter the conduction band at room temperature. The conductor has electrons in the conduction band even at 0 K. Quite obviously, therefore, at room temperature there are more than enough free carriers to sustain a heavy flow of charge, or current.