A chlorine molecule forms a covalent bond

Modern electronics can trace its roots to the first electronic devices called vacuum tubes. Although, today, solid state devices have totally replaced the vacuum tube, the fundamental principle as to its usage remains relatively unchanged. For more than 40 years, until the late 1960s, the most important part in a consumer electronics product was the vacuum tube. It is with this historical perspective in mind that this section is presented so that readers will not lose sight of where it all started.

The vacuum tube got its start in 1883, when Edison was developing the incandescent lamp. To correct the premature burnout of the red-hot filament in light bulbs, Edison tried a number of experiments, one of which was to place a metal plate sealed inside a bulb and connect it to a battery and ammeter, as shown in Fig. 1.4. Edison observed that, when the filament was hot and the plate was positively (+) charged by the battery, the ammeter indicated a current flow through the vacuum, across the gap between the filament and the plate. When the charge on the plate was reversed to negative (–), the current flow stopped. As interesting as this phenomena was, it did not improve the life of Edison’s lamps and, as a result, he lost interest in this experiment and went on to other bulb modifications that proved more successful. For about 20 years, Edison’s vacuum tube experiment remained a scientific curiosity. In 1903, as radios were coming into use, J. A. Fleming, in England, found just.

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Bohr model of silicon atom

Electrons are being forced into the next higher shell. An atom is chemically stable if its outer shell is either completely filled with electrons, based on the 2n2 rule, or has eight electrons in it. The electrons in the outer shell are called valence electrons and, if their number is less than eight, the atom will have a tendency to interact with other atoms either by losing, acquiring, or merging its electrons with other atoms. In the periodic table (Fig. 1.1), elements with the same number of valence electrons have similar properties and are placed in the same group. For example, elements in Group I have atoms with one electron in their outer shell. Group II shows elements that have atoms with two electrons in their outer shell, and so on. Elements on the left side of the periodic table have a tendency to lose their valence electrons to other atoms, thus becoming electropositive. The elements on the right side of the periodic table show a tendency to acquire electrons from other atoms and become electronegative.

The type of interaction occurring between atoms, as they are brought together, depends largely on the properties of the atoms themselves. The interaction may form bonds that can be classified as ionic, covalent, molecular, hydrogen bonded, or metallic. Since this chapter is concerned with semiconductors, which tend to form covalent bonds with other elements and with themselves, the emphasis will be on covalent bonding. Covalent bonds occur when two or more atoms jointly share each other’s valence electrons. If the outer shell is partially filled with electrons, the atom will be attracted to other atoms also having a deficiency of electrons, so sharing each other’s valence electrons will result in a more stable condition. As an example, two chlorine atoms will attract and share each other’s single electron to orm a stable covalent bond with eight electrons in each shell (Fig. 1.3).

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