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Electrical Properties

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  • Rahul A

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Topic of this ppt is Properties and Effect of Temperature.

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    Electrical Properties
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    Electrical Conduction Ohm's law V = IR 1 Area, where lis current (Ampere), Vis voltage (Volts) and R is teeistance (Ohms or Q) of the conductor Resistivity Resistivity, p = RA/I Q-m), where A is the area and I is the (ength of the conductor. Electrical conductivity Conductivity, 1/ F I/RA ( Q
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    Band Theory ZEIectrons occupy energy states in atomic orbitals ZWhen several atoms are brought close to each other in a solid these energy states split in to a series of energy states (molecular orbitals). a The spacing between these states are so small that they overlap to form an energy band. Energy 2P orbital 2s orbital Interatomic spacing
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    Band Theory a The furthest band from the nucleus is filled with valence electrons and is called the valence band. a The empty band is called the conduction band. Z The energy of the highest filled state is called Fermi energy. a There is a certain energy gap, called band gap, between valence and conduction bands. Primarily four types of band structure exist in solids. Conduction band (5+eF1åp Valence band Empty band Empty states Filled states Metals Conduction band Valence band Semiconductors Conduction band Fenni energy Valence band Insulators
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    Band Theory uln metals the valence band is either partially filled (Cu) or the valence and conduction bands overlap (Mg). alnsulators and semiconductors have completely filled valence band and empty conduction band. alt is the magnitude of band gap which separates metals, semiconductors and insulators in terms of their electrical conductivity. a The band gap is relatively smaller in semiconductors while it is verylarge in insulators.
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    Conduction Mechanism ZAn electron has to be excited from the filled to the empty states above Fermi level (Ef) for it to become free and a charge carrier. aln metals large number of free valence electrons are available and they can be easily excited to the empty states due to their band structure. ZOn the other hand a large excitation energy is needed to excite electrons in Insulators and semiconductors due the large band gap. Empty states Ef Conduction in Metals Filled states
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    Intrinsic Semiconductors OSemiconductors like Si and Ge have relatively narrow band gap generally below 2 eV. a Therefore, it is possible to excite electrons from the valence to the conduction band. This is called intrinsic semi conductivity. ZEvery electron that is excited to the conduction band leaves behind a hole in the valence band. ZAn electron can move in to a hole under an electrical potential and thus holes are also charge carriers. Conduction band Band gap Hole Valence band
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    Intrinsic conductivity ZEIectricaI conductivity of a conductor primarily depends on two parameters — charge carrier concentration, n, and carrier Condüctivity, F n e mobility, is absolute charge (1.6 x 10-19 C). ulntrinsic semiconductors have two types charge carriers, namely electrons and holes where, n and p are concérfrati&n of electron and hole charge carriers respectively and e and Uh are their mobility. aSince each electron excited to conduction band leaves behind a hole in the valence band, n = p = njand = nje h) h)
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    Extrinsic Semiconductors u The conductivity is enhanced by adding impurity atoms (dopant) in extrinsic semi conductors . All semi conductors for practical purposes are extrinsic. QA higher valence dopant e.g. P (5+) in Si (4+) creates an extra electron (n-type) while a lower valence dopant like B (3+) creates a hole (p-type) as shown in the atomic bonding model below. Q This increases the charge carrier concentration and hence the enhancement in conductivity. 00 00 00 00 00 00 00 O Free oo electron 00 00 00 00 00 00 00 00 00 O 00 00 - 00-Hole 00 n-type 00 oo 00 00 00 p-type oo
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    Extrinsic Semiconductors a The band theory model of n-type and p-type extrinsic semiconductors are shown below. aln n-type, for each impurity atom one energy state (known as Donor state) is introduced in the band gap just below the conduction band. uln p-type, for each impurity atom one energy state (known as acceptor state) is introduced in the band gap just above the valence band. Donor state Conduction band Band gap Valence band n-type Acceptor state O p-type
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    Extrinsic conductivity al-arge number of electrons can be excited from the donor state by thermal energy in n-type extrinsic semiconductors. al-lence, number of electrons in the conduction band is far greater than number of holes in the valence band, i.e. n p and c = n lge aln p-type conductors, on the other hand, number of holes is much greater than electrons >> n) due to the presence of the acceptor states.
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    Effect of alncreasing temperature causes greater electron scattering due to increased thermal vibrations of atoms and hence, resistivity, , (reciprocal of conductivity) of metals increases (conductivity decreases) linnerlil 'Ali+h +ornnorn+l I ro cu- .16 at.O Ni 00 4 3 2 1 -250 -200 -150 -100 -50 Temperature (oc) O 50
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    Effect of a The resistivity of metals depends on two other factors namely, impurity level and plastic deformation as these generate scattering centers for electrons. alncrease in impurity level results in more scattering centers and decreases the conductivity. aSimilarIy plastic deformation introduces more dislocations which act as scattering centers and increase the resistivity. Ptotal= Pt+P i Pd
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    Effect of aln intrinsic semiconductors the carrier concentration increases with temperature as more and more electrons are excited due to the thermal energy. 1028 E c o O o 106 200 800 Temperature (K) 1800
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    Effect of Extrinsic Semiconductors a Temperature dependence of extrinsic semiconductors, on the other hand is totally different. OFor example, an n-type conductor exhibits three regions in the 400 500 -200 -100 o 100 200 300 1017 1016 temperature co 1015 re •on xtrin ic re on üååååZüü 1014 Fr eze-O t 1013 re on 1012 100 200 300 400 500 600 700 800
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    Effect of Extrinsic Semiconductors contd.. aln the low temperature region known as Freeze-out region, the charge carriers cannot be excited from the donor level to conduction band due to insufficient thermal energy. aln the intermediate temperature range ( 150 — 450 K) almost all the donor atoms are ionized and electron concentration is approximately equal to donor content. This region is known as Extrinsic region. aln the high temperature region sufficient thermal energy is available for electrons to get excited from the valence to the conduction band and hence it behaves like an intrinsic semi conductor.
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    Electrical properties of some metals at RT Metal Silver Copper Gold Aluminum Nickel Iron Platinum Conductivity 6.8 x 107 6.0 x 107 4.3 x 107 3.8 x 107 1.43 x 107 1.0 x 107 0.94 x 107 Resistivit 1.59 x 10-8 1.68 x 10-8 2.44 X 10-8 2.82 x 10-8 6.99 x 10-8 9.0 x 10-8 1.06 x 10-7
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    Electrical properties of some semi conductors Material Ge GaP InSb CdS ZnTe Band gap Conductivity (Q -1-m-l) (m2/V-s) (m2/V-s) 0.67 2.25 I .42 0.17 2.40 2.26 4 X 10-4 2.2 1 x 10-6 2 x 104 0.14 0.38 0.03 0.85 0.03 0.03 0.05 0.18 0.015 0.04 0.07 0.01
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    Dielectric Property dielectric material is an insulating material which can separate positive and negatively charged entities. >Dielectric materials are used in capacitors to store the electrical energy. Capacitance >Capacitance, C, is related to charge stored, Q, between two oppositely charged layers subjected to a voltage V. C = Q/V two parallel plates of area, A, are separated by a distance I in vacuum, then C = permittivity of vacuum = O' 8.85 x 10-12 F/m. adielectric material is present between the plates, C = AH, is tFe gerpittivity of the dielectric medium.
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    Capacitance and Polarization >The orientation of a dipole along the applied electric field is called polarization (P). At causes charge density to increase over that of a vacuum due to the presence of the dielectric material so !bat is the electric field. is surface charge density of a capacitor, also called dielectric displacement. acuum P Dielectric
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    Types of *our types of Ionic, Orientation, and Space charge (interfacial). >EIectronic polarization is due to displacement of the centre of the electron cloud around the nucleus under the applied field. >lonic polarization occurs in ionic material as the applied electric filed displaces the cations and anions in opposite directions resulting in a net dipole moment. >Orientation polarization can only occur in materials having permanent dipole moments. The rotation of the permanent moment in the direction of the applied field causes the polarization in this case. >Space charges polarization arises from accumulation of charge at interfaces in a heterogeneous material consisting of more than one phase having different resistivity.
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    Ferro-electricity *erro-electricity is defined as the spontaneous alignment of electric dipoles in the absence of an external field. > The spontaneous polarization results from relative displacement of cations and anions from their symmetrical positions. Therefore, ferroelectric materials must posses permanent dipoles. >ExampIes of ferroelectric materials: BaTi03, Rochelle salt (NaKC4H406.4H20), potassium dihydrogen phosphate (KH2P04), potassium niobate (KNb03), lead zirconate titanate [Pb (Zr03, Ti03)]. > These materials have extremely high dielectric constants at relatively low applied field frequencies. Hence, capacitors made from ferroelectric materials are
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    piezoelectricity >Piezo-eIectricity is defined as conversion of electrical energy into mechanical strain and vice versa. At arises due to polarization induced by an external force. Thus by reversing the direction of external force, direction of the established field can be reversed i.e. the application of an external electric field alters the net dipole length causing a dimensional change. >AppIication for these materials includes microphones, ultrasonic generators, sonar detectors, and mechanical strain gauges. >ExampIes: Barium titanate, lead titanate, lead zirconate (PbZr03),ammonium dihydrogen phosphate (NH4H2P04), and quartz.
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    Evaluation At the end of this chapter one should be able to understand > The source of electrical conductivity >Band theory, energy bands and band gap >Reasons for high conductivity of metals >Semi conductivity — Intrinsic and Extrinsic >Effect of temperature on conductivity >DieIectric behavior *erro and Piezo-electricity Key words: Electrical conductivity; Band theory; Band gap; Metallic conductors; Semi conductors; Dielectric; Ferroelectricity; Piezoelectricity.

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