THE STRUCTURE OF ATOMS
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All matter is composed of atoms An atom is the smallest particle of an element that still retains the characteristics of that element Every element has a unique atomic structure, and atoms have a planetary type structure An atom consists of a central nucleus One or more electrons orbit the nucleus, like satellites around the earth, and are the basic particles of negative charge Nucleus contains one or more positively charged particles called protons Positive charge of proton is opposite to negative charge of electron The total (or net) charge of atom is thus zero An atom that has the same number of electrons in orbit as it has protons in its nucleus is electrically neutral Nucleus of every atom (except hydrogen) also contains one or more neutrons – they carry no electrical charge Each type of atom has a certain number of electrons and neutrons that distinguishes it from all other atoms of other elements
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THE PERIODIC TABLE
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The periodic table shows the known atoms with their respective atomic numbers, which is given by the number of protons in the atoms’ nuclei Hydrogen (H) is the simplest atom, and has one proton in its nucleus, and one electron in orbit around the nucleus Silver (Ag), gold (Au), carbon (C), copper (Cu), iron (Fe), oxygen (O), helium (He), mercury (Hg), lithium (Li), nitrogen (N), sodium (Na), neon (Ne), lead (Pb), silicon (Si), uranium (U)
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EXAMPLE STRUCTURE OF HELIUM ATOM
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Diagram shows the structure of the helium atom Nucleus is cluster of two protons (+) and two neutrons There are two electrons (-) in an orbit in the electron shell around the nucleus Atom is electrically neutral because the two protons (+) cancel out the two electrons (-) Important fact there is a force of attraction between oppositely charged particles and a force of repulsion between similarly charged particles Like charges repel and opposite charges attract 8. An Introduction to Semiconductors
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ELECTRON SHELLS (1)
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Other types of atoms contain additional electron shells (different layers where electrons orbit nucleus) that are farther removed from the nucleus Copper is the most commonly used metal in electrical applications Below shows the four electron shells that surround the copper atom, designated k, l, m, n If k regarded as 1, l as 2, and so on, then maximum number M of electrons contained in nth shell is given by M = 2n2 electrons (valid for only first 4 shells of atom) Innermost (k) shell contains 2 electrons, next has 8 electrons, next has 18 electrons, and m has 32
k Nucleus: 29 protons 34 neutrons
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ELECTRON SHELLS (2) • • • • • • •
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Nucleus of copper atom has 29 protons A neutral copper atom must therefore have 29 electrons distributed among its various shells Shells k, l, m of copper atom are filled to capacity with 28 electrons (2 + 8 + 18) Thus there is only one electron in the outer n shell Outermost shell (n for copper atom): valence shell Number of electrons in valence shell strongly influences electrical properties of elements Electrons in valence shell are farthest removed from the nucleus, they experience the least force attraction to the nucleus Electrons in a near empty shell can easily be dislodged from that shell The lone electron in the valence shell of copper can readily vacate that shell (free electron) An atom that produces a free electron acquires a net +ve charge because its total number of protons is then one greater than total number of electrons Such an atom is called a positive ion An electron must gain enough energy to overcome the force that binds it to its nucleus to become free For copper, room temperature provides enough energy to liberate valence electrons 8. An Introduction to Semiconductors
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CONDUCTORS, INSULATORS & SEMICONDUCTORS
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Presence of large number of free electrons in copper (and other metals) makes it a good conductor of electricity (silver, copper, gold – best conductors) Conduction is a process that is enhanced by the availability of large numbers of free electrons Materials whose valence electrons are tightly held to their parent atoms produce few free electrons Such materials are called insulators and are characterised by more than four valence electrons in their atomic structures Insulators are used for wire coatings Semiconductors are neither good conductors nor good insulators – characterised by atoms having 4 valence electrons in their structures Semiconductors have a structure where the electrons in the valence shell of every atom are simultaneously in the valence shell of all neighbouring atoms Consequently free electrons are produced much less readily than in conductors However a semiconductor can be manufactured to behave more like a conductor or an insulator by introducing impurities to control the total number of free electrons that result
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PURE SEMICONDUCTORS
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Semiconductors are materials whose resistivity falls between that of conductors and insulators Good conductors, semiconductors and insulators have resitivities of the order of 10-8Ωm, 10-2Ωm and 1010Ωm respectively Most important semiconductor is element called silicon (Si) Nucleus of silicon atom has 14 electrons arranged in three shells (2 inner, 8 middle, 4 outer) The 4 outer (valence) electrons determine conduction properties of Silicon In the body of a silicon formed crystal each atom shares its four valence electrons with four other atoms – making each atom effectively surrounded by 8 valence electrons
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ELECTRONS AND HOLES
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In a metal wire, electric current consists of a flow of electrons from –ve end to +ve end In semiconductors (e.g. silicon) current is carried by electrons and holes Holes are gaps left by electrons in valence shell which break away from their parent atom Holes behave more like positive charges Holes are low energy particles whereas free electrons, having gained enough energy to leave their parent atoms, are high energy particles Current consists of electrons moving from negative to positive, and a different quantity of positively charged holes moving from positive to negative In an intrinsic semiconductor, the number of holes must be equal to the number of electrons – i.e. there are no foreign atoms present in the crystalline structure The conductivity of a semiconductor can be increased by adding a small amount of controlled impurities (doping), thus making it an extrinsic semiconductor Impurities are called donor atoms
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N-TYPE AND P-TYPE SEMICONDUCTORS • • •
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2 kinds of extrinsic semiconductor: n-type and p-type N-type semiconductor made by doping silicon with e.g. phosphorous atoms which has 5 valence electrons When a phosphorous atom is introduced to a silicon crystal, 4 of its valence electrons from bonds with the 4 neighbouring silicon atoms 5th valence electron is thus spared and contributes to conduction as it is free So the impure silicon produced is called n-type since the majority charge carriers are electrons P-type semiconductor made by doping silicon with, say, boron which has 3 valence electrons On doping silicon with boron, the three boron valence electrons bond with three of the four neighbouring silicon atoms Thus one bond is incomplete, which is a hole as it is supposed to be filled by a missing electron, and thus behaves like a +ve charge Impurity atom (boron) also called acceptor Resulting impure silicon is called p-type since the majority charge carriers are positive holes
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