PROF. MARY GRACE O. CATONG
ALAN, ARLAN H.
RAMIREZ, RONEL JAY S.
RUSIANA, RODOLFO O.
One of the most important electrical characteristics of a solid material is the ease with which it transmits an electric current. Ohm’s law relates the current I—or time rate of charge passage—to the applied voltage V as follows: V=IR.
Sometimes, electrical conductivity is used to specify the electrical character of a material. It is simply the reciprocal of the resistivity, or
ELECTRONIC AND IONIC CONDUCTION
An electric current results from the motion of electrically charged particles in response to forces that act on them from an externally applied electric field. Positively charged particles are accelerated in the field direction, negatively charged particles in the direction opposite.
ENERGY BAND STRUCTURES IN SOLIDS
In all conductors, semiconductors, and many insulating materials, only electronic conduction exists, and the magnitude of the electrical conductivity is strongly dependent on the number of electrons available to participate in the conduction process. For each individual atom there exist discrete energy levels that may be occupied by electrons, arranged into shells and subshells. Shells are designated by integers (1, 2, 3, etc.), and subshells by letters (s, p, d, and f ).
The electrical properties of a solid material are a consequence of its electron band structure—that is, the arrangement of the outermost electron bands and the way in which they are filled with electrons. Four different types of band structures are possible at 0 K. In the first (Figure18.4a), one outermost band is only partially filled with electrons. The energy corresponding to the highest filled state at 0 K is called the Fermi energy Ef.
CONDUCTION IN TERMS OF BAND AND ATOMIC BONDING MODELS
Only electrons with energies greater than the Fermi energy may be acted on and accelerated in the presence of an electric field. These are the electrons that participate in the conduction process, which are termed free electrons. In addition, the distinction between conductors and non-conductors (insulators and semiconductors) lies in the numbers of these free electron and hole charge carriers.
For an electron to become free, it must be excited or promoted into one of the empty and available energy states above Ef. For metals having either of the band structures shown in Figures 18.4a and 18.4b, there are vacant energy states adjacent to the highest filled state at Thus, very little energy is required to promote electrons into the low-lying empty states, as shown in Figure 18.5. Generally, the energy provided by an electric field is sufficient to excite large numbers of electrons into these conducting states.
Insulators and Semiconductors
For insulators and semiconductors, empty states adjacent to the top of the filled valence band are not available. To become free, therefore, electrons must be promoted across the energy band gap and into empty states at the bottom of the conduction band. The number of electrons excited thermally (by heat energy) into the conduction band depends on the energy band gap width as well as temperature. The larger the band gap, the lower is the electrical conductivity at a given temperature. Increasing the temperature of either a semiconductor or an insulator results in an increase in the thermal energy that is available for electron excitation.
When an electric field is applied, a force is brought to bear on the free electrons; as a consequence, they all experience acceleration in a direction opposite to that of the field, by virtue of their negative charge. These frictional forces result from the scattering of electrons by imperfections in the crystal lattice, including impurity atoms, vacancies, interstitial atoms, dislocations, and even the thermal...