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15 - SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS Page 1 15.1 Introduction The word electronics is coined from the words electron mechanics . The subject of electronics deals with the study of devices in which specific current ( Ι ) versus voltage ( V ) relationship is obtained by controlling the production of electrons, theirs numbers and their conduction. Such relationships are different from the one obeyed by Ohm’s law in good conductors in which electric current is directly proportional to the electric potential difference. There are many substances found in nature in which the conduction of electricity is different from the one found in metals. Solid state devices are made having appropriate Ι - V relations by properly adding impurities in such a substance. Solid state devices are small in size and light in weight. They are very efficient and cheap. Semiconductor devices like the P – N junction diode, transistor LED ( Light emitting diode ), solar cell and logic circuits which is a basis for digital circuits shall be discussed in this chapter. 15.2 Conductors, Insulators and Intrinsic Semiconductors ( A Bond Picture ) The elements in the first three groups of the periodic table like alkali metals, noble metals, Aluminium, etc. are good conductors due to the presence of free electrons. Non-metals are bad conductors of electricity due to lack of free electrons. The elements in the fourth group of the periodic table like Si and Ge have greater resistance than good conductors but less than bad conductors. They are known as semiconductors. They behave as bad conductors at absolute zero temperature in their pure form. The resistivity of the good conductors increases with temperature, while the resistivity of the semiconductors decreases on increasing the temperature unto a certain limit. The conductivity of the semiconductors is changed by making radiation of suitable frequency incident on them. Two very important semiconductors Ge and Si are discussed here. Both have diamond crystal structure. If an atom of Si is considered at the centre of the tetrahedron, then its four nearest neighbours are at the vertices of a tetrahedron as shown in the figure. Diamond crystalline structure is obtained on extending this arrangement in a three dimensional space. The electronic arrangement of Si is 1s 2 2s 2 2p 6 3s 2 3p 2 . The electrons in 1s 2 2s 2 2p 6 completely occupy the K and L shells. 3s 2 3p 2 electrons are the valence electrons. These 2 s orbitals and 2 p orbitals combine to form 4 sp 3 complex orbitals. These orbitals combine with similar such orbitals of the neighbouring atoms and form covalent bonds. Thus, each of the four valence electrons of the silicon forms a covalent bond with its four neighbouring atoms as shown in the figure.
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15 - SEMICONDUCTOR ELECTRONICS: MATERIALS, DEVICES AND SIMPLE CIRCUITS Page 2 At absolute zero temperature, Si and Ge behave as insulators as the valence electrons are bound in covalent
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This note was uploaded on 11/28/2011 for the course PHYSICS 300 taught by Professor Smith during the Spring '06 term at ITT Tech Flint.

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