Semiconductor Physics - Talking Electronics

1 In Used Band Description of of Temperature on on n-type andp-type and of Voltage across pn-Junction or Biasing a pn- Flow in a ForwardBiased Characteristics of in the Operating Con-ditions of pn-JunctionINTRINTRINTRINTRINTRODUCTIONO DUCTIONODUCTIONODUCTIONODUCTIONC ertain substances like germanium, silicon, car-bon etc. are neither good conductors like cop-per nor insulators like glass. In other words,the resistivity of these materials lies inbetween conduc-tors and insulators. Such substances are classified assemiconductors. semiconductors have some usefulproperties and are being extensively used in electroniccircuits.

2 For instance, transistor a Semiconductor de-vice is fast replacing bulky vacuum tubes in almost allapplications. Transistors are only one of the family ofsemiconductor devices ; many other semiconductordevices are becoming increasingly popular. In this chap-ter, we shall focus our attention on the different aspectsof Principles of SemiconductorIt is not easy to define a Semiconductor if we want to take into account all its physical , generally, a Semiconductor is defined on the basis of electrical conductivity as under :A Semiconductor is a substance which has resistivity (10 4 to m) inbetween conductorsand insulators germanium, silicon, selenium, carbon reader may wonder, when a Semiconductor is neither a good conductor nor an insulator, thenwhy not to classify it as a resistance material ?

3 The answer shall be readily available if we study thefollowing table 10 8 m3 Glassinsulator9 1011 m4 Nichromeresistance material10 4 mComparing the resistivities of above materials, it is apparent that the resistivity of germanium( Semiconductor ) is quite high as compared to copper (conductor) but it is quite low when comparedwith glass (insulator). This shows that resistivity of a Semiconductor lies inbetween conductors andinsulators. However, it will be wrong to consider the Semiconductor as a resistance material. Forexample, nichrome, which is one of the highest resistance material, has resistivity much lower thangermanium. This shows that electrically germanium cannot be regarded as a conductor or insulatoror a resistance material.

4 This gave such substances like germanium the name of is interesting to note that it is not the resistivity alone that decides whether a substance issemiconductor or not. For example, it is just possible to prepare an alloy whose resistivity falls withinthe range of semiconductors but the alloy cannot be regarded as a Semiconductor . In fact, semicon-ductors have a number of peculiar properties which distinguish them from conductors, insulators andresistance of semiconductors (i)The resistivity of a Semiconductor is less than an insulator but more than a conductor.(ii) semiconductors have negative temperature co-efficient of resistance the resistanceof a Semiconductor decreases with the increase in temperature and vice-versa.

5 For example, germa-nium is actually an insulator at low temperatures but it becomes a good conductor at high tempera-tures.(iii)When a suitable metallic impurity ( arsenic, gallium etc.) is added to a Semiconductor , itscurrent conducting properties change appreciably. This property is most important and isdiscussed later in Bonds in SemiconductorsThe atoms of every element are held together by the bonding action of valence electrons. Thisbonding is due to the fact that it is the tendency of each atom to complete its last orbit by acquiring 8electrons in it. However, in most of the substances, the last orbit is incomplete the last orbit doesnot have 8 electrons.

6 This makes the atom active to enter into bargain with other atoms to acquire 8electrons in the last orbit. To do so, the atom may lose, gain or share valence electrons with otheratoms. In semiconductors , bonds are formed by sharing of valence electrons. Such bonds are calledco-valent bonds. In the formation of a co-valent bond, each atom contributes equal number of va-lence electrons and the contributed electrons are shared by the atoms engaged in the formation of Physics 57 Fig. shows the co-valent bonds among germanium atoms. A germanium atom has *4valence electrons. It is the tendency of each germanium atom to have 8 electrons in the last orbit. Todo so, each germanium atom positions itself between four other germanium atoms as shown in (i).

7 Each neighbouring atom shares one valence electron with the central atom. In this businessof sharing, the central atom completes its last orbit by having 8 electrons revolving around the this way, the central atom sets up co-valent bonds. Fig. (ii) shows the bonding following points may be noted regarding the co-valent bonds :Fig. (i)Co-valent bonds are formed by sharing of valence electrons.(ii)In the formation of co-valent bond, each valenceelectron of an atom forms direct bond with the valence electronof an adjacent atom. In other words, valence electrons areassociated with particular atoms. For this reason, valenceelectrons in a Semiconductor are not CrystalsA substance in which the atoms or molecules are arranged inan orderly pattern is known as a crystal.

8 All semi-conductorshave crystalline structure. For example, referring to Fig. ,it is clear that each atom is surrounded by neighbouring atomsin a repetitive manner. Therefore, a piece of germanium isgenerally called germanium Commonly Used SemiconductorsThere are many semiconductors available, but very few of themhave a practical application in Electronics . The two most fre-quently used materials are germanium (Ge) and silicon (Si). It is because the energy required tobreak their co-valent bonds ( energy required to release an electron from their valence bands) isvery small; being about eV for germanium and about eV for silicon. Therefore, we shalldiscuss these two semiconductors in detail.

9 *A germanium atom has 32 electrons. First orbit has 2 electrons, second 8 electrons, third 18 electrons andthe fourth orbit has 4 electrons. Bonds in Semiconductor58 Principles of Electronics (i) Germanium. Germanium has become the model substance among the semiconductors ; themain reason being that it can be purified relatively well and crystallised easily. Germanium is anearth element and was discovered in 1886. It is recovered from the ash of certain coals or from theflue dust of zinc smelters. Generally, recovered germanium is in the form of germanium dioxidepowder which is then reduced to pure atomic number of germanium is 32.

10 Therefore, it has 32 protons and 32 electrons. Twoelectrons are in the first orbit, eight electrons in the second, eighteen electrons in the third and fourelectrons in the outer or valence orbit [See Fig. (i)]. It is clear that germanium atom has fourvalence electrons , it is a tetravalent element. Fig. (ii) shows how the various germaniumatoms are held through co-valent bonds. As the atoms are arranged in an orderly pattern, therefore,germanium has crystalline structure.(ii) Silicon. Silicon is an element in most of the common rocks. Actually, sand is silicon diox-ide. The silicon compounds are chemically reduced to silicon which is 100% pure for use as atomic number of silicon is 14.