Electronic Properties of Materials

Electrons in a Crystal (Fundamentals of Electron Theory) Part 2

Consequences of the Band Model We mentioned in Section 6.4 that, because of the Pauli principle, each s-band of a crystal, consisting of N atoms, has space for 2N electrons, i.e., for two electrons per atom. If the highest filled s-band of a crystal is occupied by two electrons per atom, i.e., if the band […]

Electrical Conduction in Metals and Alloys (Electrical Properties of Materials) Part 1

Introduction The first observations involving electrical phenomena probably began with the study of static electricity. Thales of Miletus, a Greek philosopher, discovered around 600 BC that a piece of amber, having been rubbed with a piece of cloth, attracted feathers and other light particles. Very appropriately, the word electricity was later coined by incorporating the […]

Electrical Conduction in Metals and Alloys (Electrical Properties of Materials) Part 2

Experimental Results and Their Interpretation Pure Metals The resistivity of a metal, such as copper, decreases linearly with decreasing temperature until it reaches a finite value (Fig. 7.7) according to the empirical equation where a is the linear temperature coefficient of resistivity. We postulate that thermal energy causes lattice atoms to oscillate about their equilibrium […]

Electrical Conduction in Metals and Alloys (Electrical Properties of Materials) Part 3

Theory Attempts to explain superconductivity have been made since its discovery in 1911. One of these theories makes use of the two-fluid model, which postulates superelectrons that experience no scattering, have zero entropy (perfect order), and have long coherence lengths, i.e., an area 1000 nm wide over which the superelectrons are spread. The London theory […]

Electrical Conduction in Metals and Alloys (Electrical Properties of Materials) Part 4

Secondary Cells Rechargeable batteries are called secondary batteries. The price per KW-h is spread over several use cycles, for example $5 per KW-h for a Nickel-Cadmium battery (see below). In principle, the chemical reaction which is used for providing energy can be reversed in these devices. Important parameters for rechargeable batteries are the specific charge […]

Semiconductors (Electrical Properties of Materials) Part 1

Band Structure We have seen in topic 7 that metals are characterized by partially filled valence bands and that the electrons in these bands give rise to electrical conduction. On the other hand, the valence bands of insulators are completely filled with electrons. Semiconductors, finally, represent in some respect a position between metals and insulators. […]

Semiconductors (Electrical Properties of Materials) Part 2

Extrinsic Semiconductors Donors and Acceptors We learned in the previous section that in intrinsic semiconductors only a very small number of electrons (about 109 electrons per cubic centimeter) contribute to the conduction of the electric current. In most semiconductor devices, a considerably higher number of charge carriers are, however, present. They are introduced by doping, […]

Semiconductors (Electrical Properties of Materials) Part 3

Rectifying Contacts (Schottky Barrier Contacts) It is essential for further discussion to introduce the work function, f, which is the energy difference between the Fermi energy and the ionization energy. In other words, f is the energy which is necessary to transport an electron from EF to infinity. (Values for f are given in topic […]

Semiconductors (Electrical Properties of Materials) Part 4

Solar Cell (Photodiode) A photodiode consists of a p-n junction (Fig. 8.21). If light of sufficiently high energy falls on or near the depleted area, electrons are lifted from the valence band into the conduction band, leaving holes in the valence band. The electrons in the depleted area immediately "roll down" into the n-region, whereas […]

Semiconductors (Electrical Properties of Materials) Part 5

Transistors Bipolar Junction Transistor. An n-p-n transistor may be considered to be an n-p diode back-to-back with a p-n diode. A schematic band diagram for an unbiased n-p-n transistor is shown in Fig. 8.25. The three connections of the transistor are called emitter (E), base (B), and collector (C). Figure 8.25. Schematic band diagram of […]