Electrical Properties of Materials free essay sample

ELECTRICAL PROPERTIES OF MATERIALS SUBMITTED TO: PROF. MARY GRACE O. CATONG SUBMITTED BY: ALAN, ARLAN H. RAMIREZ, RONEL JAY S. RUSIANA, RODOLFO O. BSEE-3B OHM’S LAW 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. ELECTRICAL CONDUCTIVITY 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. Metals 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. ELECTRON MOBILITY 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 vibrations of the atoms themselves. ELECTRICAL RESISTIVITY OF METALS Metals have high conductivities because of the large numbers of free electrons that have been excited into empty states above the Fermi energy. Since crystalline defects serve as scattering centers for conduction electrons in metals, increasing their number raises the resistivity (or lowers the conductivity). The concentration of these imperfections depends on temperature, composition, and the degree of cold work of a metal specimen. known as Matthiessen’s Rule Influence of Temperature For the pure metal and all the copper–nickel alloys shown in Figure 18. 8, the resistivity rises linearly with temperature above about -200? C. Thus, This dependence of the thermal resistivity component on temperature is due to the increase with temperature in thermal vibrations and other lattice irregularities (e. g. , vacancies), which serve as electron-scattering centers. Influence of Impurities For additions of a single impurity that forms a solid solution, the impurity resistivity is related to the impurity concentration in terms of the atom fraction (at%/100) as follows: For a two-phase alloy consisting of and phases, a rule-of-mixtures expression may be utilized to approximate the resistivity as follows: Influence of Plastic Deformation Plastic deformation also raises the electrical resistivity as a result of increased numbers of electron-scattering dislocations. The effect of deformation on resistivity is also represented in Figure 18. 8. Furthermore, its influence is much weaker than that of increasing temperature or the presence of impurities. COMMERCIAL ALLOYS Electrical and other properties of copper render it the most widely used metallic conductor. Oxygen-free high-conductivity (OFHC) copper, having extremely low oxygen and other impurity contents, is produced for many electrical applications. Aluminum, having a conductivity only about one-half that of copper, is also frequently used as an electrical conductor. Silver has a higher conductivity than either copper or aluminum; however, its use is restricted on the basis of cost. SEMICONDUCTIVITY Two Types of Semiconductor * Intrinsic Semiconductor -are those the electrical behaviour is based on the electronic structure inherent to the pure material. Extrinsic Semiconductor -when the electrical characteristics are dictated by impurity atoms. Formula for Electrical Conduction for Intrinsic Conductivity * For intrinsic conductors, every electron promoted across the band gap leaves behind a hole in the valence band; thus, * Two Types of Change Carrier *free electrons *holes * Two Types of Extrinsic Semiconductor *n-type Extrinsic semiconductor *p-type Extrinsic semiconductor -The imp urity of the n-type is called donor. -The impurity of the p-type is called an acceptor. Doping- means adding impurities in various techniques. * The Fermi level of n-type semiconductor, is shifted upward in the band gap. * For p-type semiconductor, the Fermi level is positioned within the band gap and near to the acceptor level. * Factor that Affect Carrier mobility -the magnitude of electrons and hole mobilities are influenced by the presence of these of those some crystalline defects that are responsible for the scattering of electrons in metals. INFLUENCE OF TEMPERATURE * SEMICONDUCTOR DEVICES -Diode(rectifier diode) -Transistor Rectifier Diode- is an electronic device that allows the current to flow in one direction only. * Forward Bias -when a battery is used, the positive terminal may be connected to p-side and the negative terminal to the n-side. * Reverse Bias -opposite to forward bias that when minus to p and plus to n. TRANSISTORS Which extremely important semiconducting devices are in today’s microelectronic circuitry. Capable of two primary types of junction. * They can perform the same operation as their vacuum tube precursor, the triode;that is they can amplify an electrical signal. They serve as a switching device in computers for the processing and storage of information. TWO (2) MAJOR TYPES * JUNCTION (or BIMODAL) TRANSISTOR * MOSFET (METAL-OXIDE-SEMICONDUCTOR FIELD-EFFECT TRANSISTOR) JUNCTION TRANSISTOR The junction transistor is composed of two p-n junctions arranged back to back in either the n-p-n or the p-n-p configuration. A very thin n-type base region is sandwiched in between p-t ype emitter collector regions. (fig. 18. 22) The circuit that includes the emitter-base junction (junction 1) is forward biased. Whereas a reverse bias voltage is applied across the base-collector junction (junction 2). MOSFET One variety of MOSFET consists of two small islands of p-type semiconductor that are created within a substrate of n-type silicon. (Fig. 18. 24) The operation of a MOSFET is very similar to that described for the junction transistor but the primary difference is that the gate current is exceedingly small in comparison to the base current of a junction transistor. MOSFETs are, therefore, used where the signal sources to be amplified current. SEMICONDUCTORS IN COMPUTER In addition to their ability to amplify an imposed electrical signal, transistor and diodes may also act as switching devices, a feature utilized for arithmetic and logical operations and also for information storage in computers. Transistors and diodes within digital circuit operate as switches that also have two states -on and off, or conducting and non-conducting. MICROELECTRONIC CIRCUITRY During the past few years, the advent of microelectronic circuitry, where millions of electronic components and circuits are incorporated into a very small space, has revolutionized the field of electronic. This revolution was precipitated, in part, by aerospace technology, which necessitated computers and electronics devices that were small and had low power requirements. Also, the use of integrated circuits has become infused into many other facets of our lives- calculators, communications, watches, industrial production and control, and all phases of the electronics industry. ELECTRICAL CONDUCTION IN IONIC CERAMICS AND IN POLYMERS Most polymers and ionic ceramics are insulating materials at room temperature and, therefore, have electron energy band structures. Gives the room-temperature electrical conductivities of several of these materials. Of course many materials are utilized on the basis of their ability to insulate, and thus a high electrical resistivity is desirable. With rising temperature, insulating materials experience an increase in electrical conductivity, which may ultimately be greater than that for semiconductors. CONDUCTION IN IONIC MATERIALS Both cations and anions in ionic materials possess an electric charge and, as a consequence, are capable of migration or diffusion when an electric field is present. Thus an electric current will result from the net movement of these charged ions, which will be present in addition to current due to any electron motion. Of course, anion and cation migration will be opposite direction. The total conductivity of an ionic material ? total is thus equal to the sum of both electronic and ionic contribution, as follows: ELECTRICAL PROPERTIES OF POLYMERS Most polymeric materials are poor conductors of electricity because of the unavailability of large numbers of free electrons to participate in the conduction process. The mechanism of electrical conduction in these materials is not well understood, but it is felt that conduction in polymers of high purity is electronic. CONDUCTING POLYMERS Within the past several years, polymeric materials have been synthesized that have electrical conductivities on par with metallic conductors; they are appropriately termed conducting polymers. Conductivities as high as 1. 5107 (? -m)-1 have been achieved in these materials; on a volume basis, this value corresponds to one-fourth of the conductivity of copper or twice its conductivity on the basis of weight. These conducting polymers have the potential to be used in a host of applications in as much as they have low densities, are highly flexible and are easy to produce. OTHER ELECTRICAL CHARACTHERISTICS OF MATERIALS Two (2) other relatively important and novel electrical characteristics that are found in some materials deserve brief mention-namely: * Ferroelectricity * Piezoelectricity FERROELECTRICITY The group of dielectric materials called ferroelectrics exhibit spontaneous polarization- that is polarization in the absence of an electric field. They are the dielectric analogue of ferromagnetic materials, which may display permanent magnetic behaviour. There must exist in ferroelectric materials permanent electric dipoles, the origin of which is explained for barium titanate, one of the most common ferroelectric. PIEZOELECTRICITY An unusual property exhibit for a few ceramic materials is piezoelectricity, or, literally, pressure electricity: polarization is induced and an electric field is established across a specimen by the application of external forces reversing the sign of an external force ( i. e. from tension to compression) reverses the direction of the field. Piezoelectric materials are utilized in transducers, which are devices that convert electrical energy into mechanical strains, or vice versa. Some other familiar applications that employ piezoelectric include phonograph cartridges, microphones, speakers, audible alarms and ultrasonic imaging. In a phonograph cartridge, as the stylus traverses the grooves on a r ecord, a pressure variation is imposed on a piezoelectric material located in the cartridge, which is then transformed into an electric signal is amplied before going to the speaker.