Semiconductor Devices:
To understand how electronic devices operate, one has first to learn about the atomic structureof matter.
Structure of matter:
The matter consists of atoms, which contain electrons and a nucleus with protons and neutrons in a particularly intimate association. The electron has a negative charge. The proton has a positive charge equal to the negative charge carried by the electron. The neutron, as its name implies,has no charge; it is electrically neutral. Each element possesses a certain number of protons and an equal number of electrons to keep the atom electrically neutral. Each element is characterized by its number of electrons, or as it is called, its atomic number.The electrons are spread out in space around the nucleus in shells, which have been compared to the orbits of the planets round the sun. The electrons can be often stripped off the atom rather easily, leaving it positively charged, naturally, but it is much more difficult to break up the nucleus.
Current:
Electric current flows in a material being a result of the interaction of charged pieces called carriers
. A review of the mechanism for conducting electricity through various kinds of matter shows that in electrolytes and in gases, conduction occurs through the motion of ions.In metallic conductors, conduction takes place via the motion of electrons, and there is no conduction in insulators,but only a slight displacement of the charges within the atoms themselves. The number of free carriers in different materials varies in an extremely wide range. In metals, the density of free electrons is in order of 10^23 1/cm.In insulators, the free electron density is less than 10^3 1/cm . For this reason, the electrical conductivity of 33 various materials is very different, more than 10^6 S/cm for metals and less than 10^15 S/cm for insulators.
Energy levels:
The negatively charged electrons possess energy in discrete amounts, and therefore they
are placed only in certain energy levels without gaps between them. In the normal state, the electrons
tend to fill the lowest energy levels, leaving only the highest energy level unfilled. Electrons in this outer shell are loosely bound to the nucleus and can be freed or tied to neighboring atoms. In solids,atoms are situated very closely to each other. Neighboring atoms can derange their energy levels and combine to form energy bonds. Only the outer orbit is of interest to understanding the conductivity properties in a solid, also called the valence bond where electrons can move and participate in an electric current. Between the valence and other bonds, there is a forbidden gap, which the electrons can cross but where they cannot remain.
Conductivity:
The key to electrical conductivity of chemical elements is the number of electrons in the valence orbit. Insulators have up to eight valence electrons. Some of the atoms of the conductor have only one valence electron in their outer orbit. Since this single electron can be easily dislodged from its atom, it is called a free electron or a conduction-bond electron because it travels in a large orbit,equivalent to a high energy level. The slightest voltage causes free electrons to flow from one atom to another. The density of free carriers of metals and insulators is approximately constant and cannot be changed in a marked range. The electrical resistance of a metal changes slightly with temperature. The variation of resistance with temperature is accounted for as follows. In a metal only very few electrons are free to move upon application of a potential difference. The temperature of the conductor being lowered, the thermal vibration of its atoms’ lattice is decreased. As a result, the atoms interfere less with the motion of electrons, and consequently, the resistance is lowered. Such kind of resistance is known as an ohmic resistance or positive resistance. Only near the absolute zero does an abrupt change occur.
Summary:
Electric current is a flow and interaction of charged carriers. In conductors, conduction takes place via the motion of negatively charged electrons. The electrical conductivity depends on the number
of electrons in the valence orbit of chemical elements. Voltage causes free electrons to flow from one atom to another. The density of electrons in metal and therefore its resistance is approximately constant. Nevertheless, due to thermal vibration, the metal resistance slightly lowers when the temperature drops. Consequently, it is referred to as positive ohmic resistance of metals.
Current in Semiconductors:
Semiconductors are neither conductors nor insulators. The commonly used semiconductor elements are silicon, germanium, and gallium arsenide. Silicon is the most widely used semiconductor material. It has 14 protons and 14 electrons in orbits. An isolated silicon atom has four electrons in the valence bond.Germanium has 32 protons, 32 electrons, and 4 valence electrons like silicon.Crystal.Each atom that is normally bonded with the nearest neighbor atoms results in a special shape called a crystal.(Fig 1.1). A silicon atom that is a part of a crystal has eight electrons in the valence orbit and four neighbor atoms. Each of the four neighbors shares one electron. Since eachshared electron in Fig. 1.1 is being pulled in opposite directions, it is a kind of a bond between the opposite cores. This type of a bond is known as a covalent bond.The covalent bonds hold the tetravalent crystal together,ensuring its stability.
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