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Semiconductors. General information

Semiconductors. General information

Before, how to start the topic, the question: why it'll of semiconductors? And that's because, what, as can be seen from the description of the development of electronics, in our time the greatest successes in this area involve the application of devices, performed on semiconductor devices.

Semiconductors (PE) belong to the class of substances, I have a solid crystalline structure and specific conductivity (104 - 10’10 characters per cm) occupy an intermediate position between conductors (104 - 106 SIM/cm) and dielectrics (1010 SIM/cm or less).

In the manufacture of PE devices often use silicon (Si - has an operating temperature up to 140°C), germanium (Ge - highest working temperature 75°C), gallium arsenide (GaAs - operates at temperatures up to 350-400°C).

NP also include selenium, tellurium, some oxides, carbides and sulfides. NP have such properties:

1)      negative temperature coefficient of resistance - with increasing temperature the resistance decreases (conductors - growing);

2)      the addition of impurities results in lower specific resistance (conductors - to increase);

3)      on the electrical conductivity of NP is influenced by radiation, electromagnetic radiation.

The processes of TMS and electrical conductivity of dielectrics such, but differ significantly from the conductivity of the conductors.Note, electrons, located on the outer orbit of the atom of matter, the name valence. They are weaker bound to the nucleus and determine the physical and chemical properties of matter.

In conductors electrons, located on the outer orbit of an atom, weakly bound to the nucleus and therefore easy enough to leave their atoms, then move erratically in the material - be free. If to the conductor to apply an external electric field, will arise the ordered movement of electrons - electric current.

In NP, all valence electrons are strongly coupled to crystalline radami thanks to the so-called covalent bond, You know from chemistry. As long as this linkage exists, electrons can carry electric charge in the material.

 

 

The mechanism of electrical conductivity of PE let's consider the example of crystal lattices Germany, that is an element of the IV group of the periodic system of Mendeleev. The lattice in the form plasse structure depicted in Fig Germany. 1. 1.

Germany atoms placed at the nodes of the lattices, their bond with other atoms is carried out through four valence electrons. Double lines between nodes indicate the covalent nature of the relationship, that is, each pair of valence electrons belongs simultaneously to two adjacent atoms. At a temperature of absolute zero and in the absence of exposure to NP no moving media and its electric resistance is large (endless).

Under normal conditions, as a result of the action of a substance thermal energy, some of the valence electrons leave the covalent bond and become conduction electrons - the process the generation of vapor media: of electrons and holes. The hole - vacancy in a covalent connection - has a positive charge, what we assign conditionally.

If now to place PE in an electric field, arise the directed movement of charges - electric current. Unlike in conductors the current in the NP is provided by carriers of the two charges - positive + (hole) and negative - (electrons).

The conductivity of the pure NP is the name of his own, he himself referred to and the NP-type. The formula is usually small. Significantly high conductivity have PE with impurities, besides its nature depends on the type of impurities.

Consider the example, when the melt is added to pure germanium impurity p echivalentul element (Group V of the periodic table), for example, Arsen (As), as shown in figure. 1.2.

When freezing up in some sites of the crystal lattices Germany its atoms are replaced by impurity atoms. Four of the valence electron of the impurity form the system of covalent bonds with the four valence electrons of Germany, and the fifth electron is superfluous impurities - free. Free electrons are left in the nodes of the crystal lattice of fixed positive ions, what they create in the crystal a positive volume charge.

The impurity, that gives up free electrons, called the donor. PE with a predominantly free electrons is called PE with electronic conductivity, or NP n-type.

Consider the introduction of germanium impurities with three valence electrons (Group III of the periodic table), for example, India (In), as shown in figure. 1.3.

For the formation of covalent bonds between Ge and In of one electron is not enough. Under the action of heat by the ambient electron from upper level of valence band moves to the level of impurity, creating a connection, which is not enough, through the lattice why is Germany in the valence band are formed moving with an acceptor impurity hole, as the impurity atoms become negative ions. This impurity is called acceptor, PE with an overwhelming amount of holes - PE with hole conductivity, or p-type.

полупроводники

Predominant in NP mobile the charge carriers have the name main, the rest - non-core.

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