Electrical World

At room temperature of 300°K, it requires an energy of E G = 1.12 eV to break covalent bonds in Silicon material and E G = 0.7 eV to break the covalent bonds in Germanium material and to produce some ‘electron–Hole pairs’. Even at room temperature, a few of the covalent bonds will be broken, leading to equal number of electrons and Holes in Conduction Band and Valence Band, respectively. Electrons in the Conduction Band and Holes in the Valence Band, in an intrinsic semiconductor, are shown in Fig. 2.12. Small dashes represent free or conduction electrons. Holes are represented by circles in valence band. Fig. 2.12 Energy-band diagram for an intrinsic semiconductor

The value of conductivity of a material gives us an estimate of the extent to which a material supports the flow of current through it. Electrical conductivity depends upon the number of electrons available in the conduction process. The concept of conductivity is useful in many engineering applications including medical electronics. J = nqμE Equation (2.17) derived in the previous section can also be written as is called as conductivity of the material. Thus, electrical conductivity of a material is defined as the ratio of current density J and electric field intensity E . Conductivity of semiconductor materials increases with temperature, as an increase in temperature causes increase in conduction current. This is due to increase in broken covalent bonds that result in more charge carriers for current flow. So more electrons from Valence Band jump to Conduction Band with increase in temperature. The conductivity of semiconductors varies comp...

Currents in metals are due to the movement of charge carriers ‘electrons’. where I is the current in Amperes and A is the cross-sectional area of conducting medium in metre 2 . Describing current density J as current per unit area has the advantage, since the dimensions of the conducting medium are not directly involved. Relation between current density and charge density ρ is described in the following: Current density: Current I (Amperes) through a conductor by definition is Charge (in Coulombs)/ Time (in seconds). Current is due to the movement of charges through a conducting medium in a given time. If, 1 C of charge moves through a conducting medium in 1 s, the resulting current is 1 A. electrons carry 1 Coulomb of charge. So the movement of 6.25 × 10 18 electrons for 1 s contributes to 1 A of current in a conductor. where q is the charge of an electron and N is the number of electrons in a given volume. If the charge passes through a distan...

Conduction in conductors and semiconductors Mobility μ : In good conductors like metals, free electrons exist in abundance. They are supposed to be accelerated under the influence of electric or magnetic field as per ballistic (dynamics) laws. But in practice it is found that the electrons move with a constant velocity proportional to the field. The reason for this is the random nature of the electron movement involved in repeated collisions. The loss of energy during collisions is supplemented due to acceleration caused by the applied field E . Thus it is observed that the random motion of electrons when resolved in the direction of the field, the electrons acquire a constant speed called the drift speed v that is proportional to the field E (V /m) and velocity v is in metres/second. where μ is the constant of proportionality. μ is called as mobility. It is measured as m 2 /V-s. Mobility of electrons and Holes due to the influence of electric field is give...

Purest semiconductor is known as intrinsic semiconductor . At 0°K, semiconductor behaves like an insulator, because energies of the order of E G cannot be acquired from an electric field. At room temperature, covalent bonds in the semiconductor may be broken into a few Hole–electron pairs, contributing to current flow through the material allowing the conductivity to increase. With respect to energy, if an electron is given additional energy, it breaks away from its covalent bond. When the free electron enters a Hole in a Valence Band, this excess energy is released as a quantum of heat or light. In turn this quantum of energy may be reabsorbed by another electron to break its covalent bond and create a new Hole–electron pair. Thus Holes and electrons appear to move. The moving charge carries form current. Ohm's law governs the conduction phenomena in conductors and resistors. Conduction by Holes is less when compared to that of electrons because of differences i...

When voltages are applied, materials offer different values of electrical resistances to the passage of currents through them. On the basis of electrical resistances, materials are classified as conductors, semiconductors and insulators . In solids, available energy states for the electrons form ‘bands of energy levels’ instead of discrete energy levels in atoms. Conductors: Materials with adjacent or over-lapped conduction and Valence Bands with zero forbidden band-gap energy ( E G = 0) are known as conductors . EBD for a conductor material is shown in Fig. 2.7. Fig. 2.7 Energy-band diagrams for conductors Initially, the energy levels in the Conduction Band are empty. But, electrons enter the Conduction Band due to increase in temperature or energy acquired from an applied electric field. Then the electrons move freely inside the Conduction Band as charge carriers with each electron carrying an electron charge q n = 1.6 × 10 –19 C. So in a conductor, elec...

Energy-band Concepts of Materials The electron energy levels for a single free atom in a gaseous medium are discrete, since the atoms are sufficiently far apart. So the energy levels of individual atoms are not perturbed. The proximity of neighbouring atoms in solid media such as crystals does not appreciably affect the energy levels of inner shell electrons. But, groups of energy levels of outer shell electrons are changed due to the influence of electrons in the neighbouring atoms. They allow sharing of electrons among them to form covalent bonds between neighbouring atoms in the process of getting on to stable ‘8-electron configuration’ in Silicon and Germanium semiconductors. Sharing of outer shell electrons to form covalent bonds is shown in Fig. 2.5. Valence band The coupling between the outer shell electrons of the atoms results in a group or a band of closely spaced energy levels or states instead of the widely spaced energy levels of the isolated atoms....

Fig. 2.3 Electron configuration of germanium atom Germanium semiconductor atom has ‘atomic number’ Z = 32. It has 32 positive charges in the nucleus and 32 electrons in various shells containing 2, 8, 18 and 4 electrons. Germanium atom is electrically neutral. Germanium semiconductor as a whole is electrically neutral. First, second and third orbits are completely filled. Fourth orbit (shell) is partially filled. Energy levels from fifth orbit onwards are empty energy levels. Germanium atom representation is shown in Fig. 2.4. It is a basis to know the formation of covalent bonds and so on. Germanium is also considered as a ‘tetravalent’ material, as it has 4 valence electrons in its outer incomplete shell. Thus, Silicon and Germanium materials are referred as tetravalent materials with similar electrical and chemical properties. Fig. 2.4 Representation of germanium atom with four valence electrons

REVIEW OF SEMICONDUCTOR PHYSICS The electronics subject begins from the concepts of behaviour of charge carriers in electron devices and Integrated Circuits (ICs) under influence of electric fields. A model of an atom is shown in Fig. 2.1. The aspect of electron motion is analogous to the planetary motion in which the planets rotate round the sun. On similar lines, electrons move in closed stationary orbits around the positive nucleus in an atom. Fig. 2.1 Electron configuration of silicon atom 2.1.1 Electron Configurations of Silicon and Germanium Atoms The shell structure and states occupied by electrons depend on the valence of material and its atomic number Z . Silicon and Germanium semiconductor materials are used for the manufacture of semiconductor devices. The distribution of electrons in the various orbits for Silicon and Germanium atoms is shown in Table 2.1 and in Figs. 2.1 and 2.3. Table 2.1 Element Atomic number ( Z ) Configuration ...