Energy Bands in Solids

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BAND THEORY OF SOLIDS 10/6/2010 1 weyes57

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Presented by V.SUBRAMANIAN, Post – Graduate Teacher(Physics), Government Higher Secondary School, MANALI, CHENNAI – 600 068, Tamil Nadu State, INDIA. 10/6/2010 2 weyes57

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Content 1.Introduction 2.Semiconductors 3.Energy band in solids 4. Conduction band, Valance band and Forbidden gap 5.Insulators, Conductors and Semiconductors 6.Electrons and holes in semi conductors 7.Intrinsic semiconductor 8.Doping of a semiconductor 9.Extrinsic semi conductors 10.N-type semi conductor 11.P-Type semiconductor 10/6/2010 3 weyes57

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INTRODUCTION In the year 1947, Transistor was invented by three scientists. William Shockley, John Bardeen and Walter Brattain in 1956 , shared Nobel Prize in physics for creating the transistor. 10/6/2010 4 weyes57

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Before World war II vacuum tubes were used in the process of Electrical signal generation, amplification, transmission and reception. After the invention of the transistor and other solid electronic devices, vacuum tubes were replaced. A solid state electronic device mainly consists of a semi conducting material. 10/6/2010 5 weyes57

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SEMICONDUCTORS Semiconductors have resistivity between good conductors and insulators. The resistivity of semiconductor lies approximately in between 10 -2 ohm m and 10 4 ohm m at room temperature. The resistance decreases with increase of temperature over a particular temperature range in the case of a semiconductor . 10/6/2010 6 weyes57

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This is contrary to that of metallic conductors, for which the resistance increases with increase of temperature. The elements that are classified as semiconductors are Si.Ge,In etc. Germanium and Silicon are most commonly used semiconductors. 10/6/2010 7 weyes57

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ENERGY BANDS IN SOLIDS There are discrete energy levels in the case of an isolated atom. 10/6/2010 8 weyes57

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Arrangement of electrons in an isolated Silicon atom 10/6/2010 9 weyes57

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Electron arrangement in a silicon atom 10/6/2010 10 weyes57

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The completely filled levels are called core levels and the electrons filling these levels are called core electrons. The electrons in the outermost level are called valence electrons. The partially filled outermost level is called valence level and the permitted levels which are vacant are known as conduction levels. 10/6/2010 11 weyes57

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In solids , the atoms are arranged in a systematic space lattice and each atom is influenced by neighbouring atoms. The closeness of atoms results in the intermixing of electrons of neighbourring atoms. 10/6/2010 12 weyes57

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Due to inter mixing of electrons, the number of permissible energy levels increases. Instead of a single energy level , there will be bands of energy levels. A set of such closely packed energy levels is called an energy band. 10/6/2010 13 weyes57

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ENERGY ATOMS ATOMS 28 Gram of silicon contains 6.023 x 1023 atoms 10/6/2010 14 weyes57

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A band which is occupied by the valence electrons or a band having highest energy is defined as valence band. The valence band may be partially or completely filled. This band can never be empty. 10/6/2010 15 weyes57

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In some materials valence electrons are loosely attached to the nucleus. Even at room temperature , some of the valence electrons leave the valence band. They are called free electrons. 10/6/2010 16 weyes57

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They are responsible for conduction and so they are called Conduction electrons. The band occupied by this electrons are called conduction Band. This band may be empty or partially filled. 10/6/2010 17 weyes57

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Energy bands in Silicon crystal 10/6/2010 18 weyes57 1s Completely Filled 2s Completely Filled 2p Completely Filled 3s Completely Filled 3p Partially Filled 3 d Completely Vacant Core Levels Valence Levels Conduction Levels 4s,4p,4d,4f Completely Vacant

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Energy bands in Silicon crystal Conduction Band ( Empty or Partially filled Band) Valence Band Fully or Partially filled Band) Completely filled Band 10/6/2010 19 weyes57

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Energy bands in Silicon crystal Conduction Band Valence Band Forbidden Energy Gap 1.11 eV 10/6/2010 20 weyes57

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The separation between valence band and conduction band is known as forbidden energy gap. If an electron is to be transferred from valence band to conduction band, external energy is required, which is equal to the forbidden energy gap. 10/6/2010 21 weyes57

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Insulators 10/6/2010 23 weyes57 Valence Band Conduction Band FORBIDDEN GAP Energy In an insulator, the forbidden gap is very large and in general is more than 3eV. No electron is available for conduction. Large amount of energy is needed to move electron from valance band to conduction band. Filled Band

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Insulators 10/6/2010 24 weyes57 Valence Band Conduction Band FORBIDDEN GAP Around 10eV (Glass) Energy In the case of materials like Glass at 0 K, valance band is completely filled and the forbidden gap energy is of the order of 10 eV. High electric field can not move electron from valance band to conduction band. When the electron is supplied with very high energy, then it can jump across the forbidden gap. Filled Band

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Insulators 10/6/2010 25 weyes57 FORBIDDEN GAP Energy When the temperature is increased, then, some electrons will move to go to conduction band. This is the reason why certain materials which are insulators at room temperature become conductors at high temperature. The resistivity of an insulator lies approximately between 10 11 and 10 16 Ωm Valence Band Conduction Band

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Semiconductors 10/6/2010 26 weyes57 Valence Band Conduction Band FORBIDDEN GAP Around 0.7eV (Ge) and 1.1 eV (Si) Energy In the case of semiconductors the forbidden gap is very small. At 0K the conduction band is empty and the valence band is completely filled. When a small amount of energy is supplied, the electrons can easily jump the forbidden gap. The conductivity of a semiconductor is of the order of 10 2mho m-1 Filled Band

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Conductors 10/6/2010 27 weyes57 Valence Band Conduction Band Energy In conductors there is no forbidden gap. The valence band and the conduction band overlap. The electrons from valence band freely enter into the conduction band due to overlapping of bands. Therefore very low potential difference can cause continuous flow of current. No forbidden gap

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10/6/2010 28 weyes57 Comparison of Insulator ,Semiconductor and Conductor

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10/6/2010 29 weyes57 Electrons and holes in semiconductors At absolute 0 temperature, in a pure semiconductor the valence band is completely filled and the conduction band is vacant. At 0 K Conduction Band Valence Band Electron Energy

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10/6/2010 31 weyes57 Electrons and holes in semiconductors At room temperature some of the electrons get energy to break the covalent bond and moves in to the conduction band. At Room temperature Conduction Band Valence Band Hole Electron Energy

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10/6/2010 33 weyes57 The electrons in a an intrinsic semiconductor, which moves in to the conduction band is called as intrinsic carriers. The vacancy created in the valence band is called as Hole. The importance of hole is that, it may serve as a carrier of electricity in the same manner as the free electron, but in opposite direction.

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Group 5 atoms have one more electron in their outer shell than group 4 elements such as silicon. Like all atoms, they have the same number of protons as electrons. This extra electron (which is negatively charged as are all electrons) is not tightly bound into the crystal lattice of the silicon atoms and can be easily separated from its parent atom with the addition of only a little energy. 10/6/2010 36 weyes57

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The thermal energy at room temperature is easily enough to do this, meaning that the electron is essentially free to move from the moment the phosphorus atom is introduced into the silicon crystal. 10/6/2010 37 weyes57

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FORMATION OF N - TYPE MATERIAL 10/6/2010 38 weyes57

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Group 3 atoms have one less electron than silicon so when they are introduced into the crystal, there is a ‘hole' where one extra electron should be. Electrons from neighbouring atoms can move into this hole, leaving a hole where they used to be which is in turn filled by another neighbouring electron. 10/6/2010 39 weyes57

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In this way, the hole can move through the crystal lattice. Although the hole carries no charge, the electrons which are moving into it represent a negative charge moving in the opposite direction to the hole. Therefore, we can think of the hole as a moving positive charge. 10/6/2010 40 weyes57

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FORMATION OF P - TYPE MATERIAL 10/6/2010 41 weyes57

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THE END 10/6/2010 42 weyes57 Thank you Visit www.freewebs.com/weyes57 and write your comments .

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