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In electrical engineering, materials are often classed as insulators (e.g. glass, rubber, mica, air) or conductors (e.g. copper, aluminum, platinum).Certain materials, however, fall into neither of these groups; their resistance is too high for them to be called conductors, too low for a classification as insulators. Among these semiconductors are silicon, germanium, selenium, and cuprous oxide, the last two being commonly used in the metallic rectifiers. Semiconductors may be made to have very useful properties in the conduction of electricity. For these applications, germanium and silicon are important materials; germanium will be selected for explanatory purposes.
A germanium crystal has an atomic structure containing four valence electrons; that is, each atom of germanium has four electrons in its outer orbit which are shared with neighboring atoms. If a slight impurity is added, the balance of electrons may be upset. The addition of a five-valence impurity, such as antimony, results in a crystal with an excess of electrons. Such a crystal is called n-type germanium (n for negative). The addition of a three-valence impurity, such as indium or gallium, results in a shortage of electrons. This deficiency gives rise to the term positive holes to describe the electron absences. The holes max become mobile as an electron absence is transferred from one germanium atom to another in the crystal structure. They may usually be thought of as the equivalent of positively charged particles capable of taking part in the conduction of electricity through the crystal, A p-type crystal is the result. For either n-type or p-type crystals, the amount of impurity is very small, about 1 part in 100 million. Extreme 2 must be exercised in the growth and manufacture of the crystals.
If pieces of n-type germanium are joined together, a pn- junction semiconductor diode is formed. In this diode there are nominally only p- type carriers to the left of the junction and n-type carriers to the right. Owing to the difference in the charge carrier concentrations the region in the immediate vicinity of the junction becomes depleted of mobile charges and referred to as the depletion region. When this condition exists free electrons in the n- section are lost across to the p- section.
Several pictorial methods can be used to illustrate the potential banner that exists across the junction.
One method of biasing a pn-diode is that in which an external voltage is applied in such a manner that it causes the electrons and holes to flow toward the junction and combine. This permits electrons to flow in the same direction in both sections, and high forward current results. The holes cross the junction from the n-section, and the electrons cross the junction from the n to p-section. Biasing of this type is called forward bias. Some current will, however, flow in the form of electrons and holes breaking their covalent bonds as the result of thermal agitation but the amount of current is very small. The current is referred to as reverse saturation current, the magnitude of which will increase with temperature and cause back resistance of a reverse-biased diode to decrease with an increase of temperature. The most important electrical characteristic of a pn-junction is that it can act as a diode by permitting easy flow of current in one direction and limiting current flow in the other. Because of this feature the pn-junction can be used for rectifying purposes.
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