Unit I.1 (E.E.)

Electronics: -
     Electronics is the branch of engineering which deals with the flow of charged particles i.e. electrons or holes through the gases, liquids, or solids (mostly semiconductors) under different situations of externally applied fields.
 Classification of conductor, insulator and semiconductor based on energy band diagram: -
* Metals:- (conductors)                                                                                    (AKTU - 2005-06, 07-08)
    A metal such as copper or silver contains a large number of free electrons at room temperature. In fact, there is no forbidden energy gap between the valence and conduction bands. The two bands actually overlap each other.

* Insulators:-                                                                                                 (AKTU 2005-06, 07-08)
    A very poor conductor of electricity is called an insulator. It has a very wide forbidden energy gap (5 e V or more). The energy which can be supplied to an electron from an applied field is too small to carry the particle from the filled valence band into the vacant conduction band. At room tempe ature, an insulator does not conduct because there are no conduction electron in it. Shown in fig. (1)

* Semiconductors:-                                                                                      (AKTU 2005-06, 07-08)
    Semiconductors are a special class of elements having a conductivity between that of a good conductor and that of an insulator.
    Three semiconductors used most frequently in the construction of electronic devices are Ge, Si and Ga As. Forbidden energy gap is lower as compared to insulators. The energy provided by the heat at room temperature is sufficient to lift electrons from the valence band to the conduction band. Shown in fig. (2)

Note: -
(i) Valence Band : The energy band within an atom which contains the valence electrons is called valence band.
(ii) Conduction Band : The next higher energy band which contains the free electrons within an atom is called conduction band.
(iii) Forbidden Energy Gap : The energy gap between the valence band and conduction band is called forbidden energy gap. Electrons can’t stay in this band gap.
Semiconductor And Its Types: -
    Semiconductors are classified as follows :
* Intrinsic Semiconductors / Pure Semiconductors: -
    Intrinsic semiconductors or pure semiconductors are those which have very low level of impurities. In these types of semiconductors there is one part of impurity in 100 million parts of semiconductors. An intrinsic semiconductor (such as pure Ge or Si), has only four electrons in the outermost orbit of its atoms. Each of the four valence electrons take part in forming covalent bonds with the four neighbouring atoms.
    At absolute zero, all the valence electrons are tightly bound to the parent atoms.
The Semiconductor Therefore Behaves As a Perfect Insulator at absolute Zero Temperature: -
* Extrinsic Semiconductor: -
    A semiconductor material that has been subjected to the doping process is called an extrinsic material. The process of deliberately adding impurities to a semiconductor material is called doping. They have more conductivity than intrinsic semiconductor due to more number of charge carriers.
    They are of two types :

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