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      Class 12 PHYSICS – JEE

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      • Class 12 PHYSICS – JEE
      CoursesClass 12PhysicsClass 12 PHYSICS – JEE
      • 1.Electrostatics (1)
        8
        • Lecture1.1
          Charge, Coulombs Law and Coulombs law in Vector form 41 min
        • Lecture1.2
          Electric Field; Electric Field Lines; Field lines due to multiple charges 42 min
        • Lecture1.3
          Charge Distribution; Finding Electric Field due to Different Object 01 hour
        • Lecture1.4
          Solid angle; Area Vector; Electric Flux; Flux of closed surface; Gauss Law 47 min
        • Lecture1.5
          Finding E Using Concept of Gauss law and Flux 01 hour
        • Lecture1.6
          Chapter Notes – Electrostatics (1)
        • Lecture1.7
          NCERT Solutions – Electrostatics
        • Lecture1.8
          Revision Notes Electrostatics
      • 2.Electrostatics (2)
        7
        • Lecture2.1
          Work done by Electrostatic Force; Work done by man in E-Field; Electrostatic Potential Energy 49 min
        • Lecture2.2
          Finding Electric Potential, Equipotential Surface and Motion in Electric Field 01 hour
        • Lecture2.3
          Electric Dipole and Dipole in Uniform and Non-uniform Electric field 01 hour
        • Lecture2.4
          Analysis of charge on conductors; Potential due to induced charge 58 min
        • Lecture2.5
          Conductors with cavity- Case 1: Empty cavity, Case 2: Charge Inside Cavity 41 min
        • Lecture2.6
          Connecting Two Conductors; Grounding of conductor; Electric field just outside conductor; Electrostatic pressure; Self potential Energy 54 min
        • Lecture2.7
          Chapter Notes – Electrostatics (2)
      • 3.Current Electricity (1)
        9
        • Lecture3.1
          Current, Motion of Electrons in Conductor; Temp. Dependence of Resistor 26 min
        • Lecture3.2
          Circuit Theory and Kirchoffs Laws 31 min
        • Lecture3.3
          Some Special Circuits- Series & Parallel Circuits, Open Circuit, Short Circuit 26 min
        • Lecture3.4
          Wheatstone Bridge, Current Antisymmetric 21 min
        • Lecture3.5
          Equivalent Resistance- Series and parallel, Equipotential Points, Wheatstone Bridge 25 min
        • Lecture3.6
          Current Antisymmetric, Infinite Ladder, Circuit Solving, 3D circuits 20 min
        • Lecture3.7
          Chapter Notes – Current Electricity
        • Lecture3.8
          NCERT Solutions – Current Electricity
        • Lecture3.9
          Revision Notes Current Electricity
      • 4.Current Electricity (2)
        4
        • Lecture4.1
          Heating Effect of Current; Rating of Bulb; Fuse 19 min
        • Lecture4.2
          Battery, Maximum power theorem; Ohmic and Non Ohmic Resistance; Superconductor 31 min
        • Lecture4.3
          Galvanometer; Ammeter & Voltmeter and Their Making 44 min
        • Lecture4.4
          Potentiometer and its applications ; Meter Bridge; Post Office Box; Colour Code of Resistors 32 min
      • 5.Capacitor
        6
        • Lecture5.1
          Capacitor and Capacitance; Energy in Capacitor 38 min
        • Lecture5.2
          Capacitive Circuits- Kirchoff’s Laws; Heat Production 01 hour
        • Lecture5.3
          Equivalent Capacitance; Charge on both sides of cap. Plate 52 min
        • Lecture5.4
          Dielectric Strength; Polar and Non-Polar Dielectric; Equivalent Cap. with Dielectric 01 hour
        • Lecture5.5
          Inserting and Removing Dielectric- Work (Fringing Effect), Force; Force between plates of capacitor 38 min
        • Lecture5.6
          Revision Notes Capacitor
      • 6.RC Circuits
        3
        • Lecture6.1
          Maths Needed for RC Circuits, RC circuits-Charging Circuit 19 min
        • Lecture6.2
          RC circuits-Discharging Circuit, Initial & Steady State, Final (Steady) State, Internal Resistance of Capacitor 44 min
        • Lecture6.3
          Revision Notes RC Circuits
      • 7.Magnetism and Moving Charge
        16
        • Lecture7.1
          Introduction, Vector Product, Force Applied by Magnetic Field, Lorentz Force, Velocity Selector 40 min
        • Lecture7.2
          Motion of Charged Particles in Uniform Magnetic Field 40 min
        • Lecture7.3
          Cases of Motion of Charged Particles in Uniform Magnetic Field 56 min
        • Lecture7.4
          Force on a Current Carrying Wire on Uniform B and its Cases, Questions and Solutions 59 min
        • Lecture7.5
          Magnetic Field on Axis of Circular Loop, Magnetic field due to Moving Charge, Magnetic Field due to Current 52 min
        • Lecture7.6
          Magnetic Field due to Straight Wire, Different Methods 40 min
        • Lecture7.7
          Magnetic Field due to Rotating Ring and Spiral 41 min
        • Lecture7.8
          Force between Two Current Carrying Wires 36 min
        • Lecture7.9
          Force between Two Current Carrying Wires 58 min
        • Lecture7.10
          Miscellaneous Questions 55 min
        • Lecture7.11
          Solenoid, Toroid, Magnetic Dipole, Magnetic Dipole Momentum, Magnetic Field of Dipole 54 min
        • Lecture7.12
          Magnetic Dipole in Uniform Magnetic Field, Moving Coil Galvanometer, Torsional Pendulum 01 hour
        • Lecture7.13
          Advanced Questions, Magnetic Dipole and Angular Momentum 56 min
        • Lecture7.14
          Chapter Notes – Magnetism and Moving Charge
        • Lecture7.15
          NCERT Solutions – Magnetism and Moving Charge
        • Lecture7.16
          Revision Notes Magnetism and Moving Charge
      • 8.Magnetism and Matter
        10
        • Lecture8.1
          Magnetic Dipole, Magnetic Properties of Matter, Diamagnetism; Domain Theory of Ferro 47 min
        • Lecture8.2
          Magnetic Properties of Matter in Detail 39 min
        • Lecture8.3
          Magnetization and Magnetic Intensity, Meissner Effect, Variation of Magnetization with Temperature 55 min
        • Lecture8.4
          Hysteresis, Permanent Magnet, Properties of Ferro for Permanent Magnet, Electromagnet 31 min
        • Lecture8.5
          Magnetic Compass, Earth’s Magnetic Field 20 min
        • Lecture8.6
          Bar Magnet, Bar Magnet in Uniform Field 49 min
        • Lecture8.7
          Magnetic Poles, Magnetic Field Lines, Magnetism and Gauss’s Law 32 min
        • Lecture8.8
          Chapter Notes – Magnetism and Matter
        • Lecture8.9
          NCERT Solutions – Magnetism and Matter
        • Lecture8.10
          Revision Notes Magnetism and Matter
      • 9.Electromagnetic Induction
        14
        • Lecture9.1
          Introduction, Magnetic Flux, Motional EMF 01 min
        • Lecture9.2
          Induced Electric Field, Faraday’s Law, Comparison between Electrostatic Electric Field and Induced Electric Field 43 min
        • Lecture9.3
          Induced Current; Faraday’s Law ; Lenz’s Law 56 min
        • Lecture9.4
          Faraday’s Law and its Cases 50 min
        • Lecture9.5
          Advanced Questions on Faraday’s Law 37 min
        • Lecture9.6
          Cases of Current Electricity 59 min
        • Lecture9.7
          Lenz’s Law and Conservation of Energy, Eddy Current, AC Generator, Motor 01 hour
        • Lecture9.8
          Mutual Induction 53 min
        • Lecture9.9
          Self Inductance, Energy in an Inductor 34 min
        • Lecture9.10
          LR Circuit, Decay Circuit 01 hour
        • Lecture9.11
          Initial and Final Analysis of LR Circuit 38 min
        • Lecture9.12
          Chapter Notes – Electromagnetic Induction
        • Lecture9.13
          NCERT Solutions – Electromagnetic Induction
        • Lecture9.14
          Revision Notes Electromagnetic Induction
      • 10.Alternating Current Circuit
        8
        • Lecture10.1
          Introduction, AC/DC Sources, Basic AC Circuits, Average & RMS Value 46 min
        • Lecture10.2
          Phasor Method, Rotating Vector, Adding Phasors, RC Circuit 35 min
        • Lecture10.3
          Examples and Solutions 21 min
        • Lecture10.4
          Power in AC Circuit, Resonance Frequency, Bandwidth and Quality Factor, Transformer 51 min
        • Lecture10.5
          LC Oscillator, Question and Solutions of LC Oscillator, Damped LC Oscillator 53 min
        • Lecture10.6
          Chapter Notes – Alternating Current Circuit
        • Lecture10.7
          NCERT Solutions – Alternating Current Circuit
        • Lecture10.8
          Revision Notes Alternating Current Circuit
      • 11.Electromagnetic Waves
        4
        • Lecture11.1
          Displacement Current; Ampere Maxwell Law 14 min
        • Lecture11.2
          EM Waves; EM Spectrum; Green House Effect; Ozone Layer 36 min
        • Lecture11.3
          Chapter Notes – Electromagnetic Waves
        • Lecture11.4
          Revision Notes Electromagnetic Waves
      • 12.Photoelectric Effect
        5
        • Lecture12.1
          Recalling Basics; Photoelectric Effect 50 min
        • Lecture12.2
          Photo-electric Cell 35 min
        • Lecture12.3
          Photon Flux; Photon Density; Momentum of Photon; Radiation Pressure- Full Absorption, Full Reflection; Dual nature 52 min
        • Lecture12.4
          Chapter Notes – Photoelectric Effect
        • Lecture12.5
          Revision Notes Photoelectric Effect
      • 13.Ray Optics (Part 1)
        12
        • Lecture13.1
          Rays and Beam of Light, Reflection of Light, Angle of Deviation, Image Formation by Plane Mirror 01 hour
        • Lecture13.2
          Field of View, Numerical on Field of Line, Size of Mirror 42 min
        • Lecture13.3
          Curved Mirrors, Terms Related to Curved Mirror, Reflection of Light by Curved Mirror 40 min
        • Lecture13.4
          Image Formation by Concave Mirror, Magnification or Lateral or Transverse Magnification 01 hour
        • Lecture13.5
          Ray Diagrams for Concave Mirror 45 min
        • Lecture13.6
          Image Formation by Convex Mirror; Derivations of Various Formulae used in Concave Mirror and Convex Mirror 01 hour
        • Lecture13.7
          Advanced Optical Systems, Formation of Images with more than one Mirror 24 min
        • Lecture13.8
          Concept of Virtual Object, Formation of Image when Incident ray are Converging, Image Characteristics for Virtual Object, 55 min
        • Lecture13.9
          Newton’s Formula, Longitudinal Magnification 23 min
        • Lecture13.10
          Formation of Image when Two Plane Mirrors kept at an angle and parallel; Formation of Image by two Parallel Mirrors. 43 min
        • Lecture13.11
          Chapter Notes – Ray Optics
        • Lecture13.12
          NCERT Solutions – Ray Optics
      • 14.Ray Optics (Part 2)
        13
        • Lecture14.1
          Refractive Index, Opaque, Transparent, Speed of Light, Relative Refractive Index, Refraction and Snell’s Law, Refraction in Denser and Rarer Medium 42 min
        • Lecture14.2
          Image Formation due to Refraction; Derivation; Refraction and Image formation in Glass Slab 57 min
        • Lecture14.3
          Total Internal Reflection, Critical Angle, Principle of Reversibility 01 hour
        • Lecture14.4
          Application of Total Internal Reflection 45 min
        • Lecture14.5
          Refraction at Curved Surface, Image Formation by Curved Surface, Derivation 56 min
        • Lecture14.6
          Image Formation by Curved Surface, Snell’s Law in Vector Form 01 hour
        • Lecture14.7
          Lens, Various types of Lens, Differentiating between various Lenses; Optical Centre, Derivation of Lens Maker Formula 01 hour
        • Lecture14.8
          Lens Formula, Questions and Answers 39 min
        • Lecture14.9
          Property of Image by Convex and Concave Lens; Lens Location, Minimum Distance Between Real Image and Object 01 hour
        • Lecture14.10
          Power of Lens, Combination of Lens, Autocollimation 35 min
        • Lecture14.11
          Silvering of Lens 44 min
        • Lecture14.12
          Cutting of Lens and Mirror, Vertical Cutting, Horizontal Cutting 49 min
        • Lecture14.13
          Newton’s Law for Lens and Virtual Object 01 hour
      • 15.Ray Optics (Part 3)
        6
        • Lecture15.1
          Prism, Angle of Prism, Reversibility in Prism 51 min
        • Lecture15.2
          Deviation in Prism, Minimum and Maximum Deviation, Asymmetric, Thin Prism, Proof for formula of Thin Prism 59 min
        • Lecture15.3
          Dispersion of Light, Refractive Index, Composition of Light, Dispersion through Prism 01 hour
        • Lecture15.4
          Rainbow Formation, Scattering of Light, Tyndall Effect, Defects of Image, Spherical Defect, Chromatic Defect, Achromatism. 57 min
        • Lecture15.5
          Optical Instruments, The Human Eye, Defects of Eye and its Corrections 01 hour
        • Lecture15.6
          Microscope & Telescope 02 hour
      • 16.Wave Optics
        21
        • Lecture16.1
          Introduction to Wave Optics 11 min
        • Lecture16.2
          Huygens Wave Theory 14 min
        • Lecture16.3
          Huygens Theory of Secondary Wavelets 10 min
        • Lecture16.4
          Law of Reflection by Huygens Theory 10 min
        • Lecture16.5
          Deriving Laws of Refraction by Huygens Wave Theory 10 min
        • Lecture16.6
          Multiple Answer type question on Huygens Theory 41 min
        • Lecture16.7
          Conditions of Constructive and Destructive Interference 22 min
        • Lecture16.8
          Conditions of Constructive and Destructive Interference 06 min
        • Lecture16.9
          Conditions of Constructive and Destructive Interference 23 min
        • Lecture16.10
          Incoherent Sources of Light 38 min
        • Lecture16.11
          Youngs Double Slit Experiment 12 min
        • Lecture16.12
          Fringe Width Positions of Bright and Dark Fringes 15 min
        • Lecture16.13
          Numerical problems on Youngs Double Slit Experiment 11 min
        • Lecture16.14
          Numerical problems on Youngs Double Slit Experiment 19 min
        • Lecture16.15
          Displacement of Interference Pattern 19 min
        • Lecture16.16
          Numerical problems on Displacement of Interference Pattern 28 min
        • Lecture16.17
          Shapes of Fringes 37 min
        • Lecture16.18
          Colour of Thin Films 59 min
        • Lecture16.19
          Interference with White Light 32 min
        • Lecture16.20
          Chapter Notes – Wave Optics
        • Lecture16.21
          NCERT Solutions – Wave Optics
      • 17.Atomic Structure
        6
        • Lecture17.1
          Thomson and Rutherford Model of Atom; Trajectory of Alpha particle; Bohr’s Model ; Hydrogen Like Atom; Energy Levels 58 min
        • Lecture17.2
          Emission Spectra, Absorption Spectra; De Broglie Explanation of Bohr’s 2nd Postulate; Limitations of Bohr’s Model 37 min
        • Lecture17.3
          Momentum Conservation in Photon Emission, Motion of Nucleus, Atomic Collision 58 min
        • Lecture17.4
          Chapter Notes – Atomic Structure
        • Lecture17.5
          NCERT Solutions – Atomic Structure
        • Lecture17.6
          Revision Notes Atomic Structure
      • 18.Nucleus
        6
        • Lecture18.1
          Basics- Size of Nucleus, Nuclear Force, Binding Energy, Mass Defect; Radioactive Decay 01 hour
        • Lecture18.2
          Laws of Radioactive Decay 36 min
        • Lecture18.3
          Nuclear Fission; Nuclear Reactor; Nuclear Fusion- Reaction Inside Sun 30 min
        • Lecture18.4
          Chapter Notes – Nucleus
        • Lecture18.5
          NCERT Solutions – Nucleus
        • Lecture18.6
          Revision Notes Nucleus
      • 19.X-Ray
        4
        • Lecture19.1
          Electromagnetic Spectrum, Thermionic Emission; Coolidge Tube – Process 1 22 min
        • Lecture19.2
          Coolidge Tube – Process 2; Moseley’s Law; Absorption of X-rays in Heavy Metal 39 min
        • Lecture19.3
          Chapter Notes – X-Ray
        • Lecture19.4
          Revision Notes X-Ray
      • 20.Error and Measurement
        2
        • Lecture20.1
          Least Count of Instruments; Mathematical Operation on Data with Random Error 18 min
        • Lecture20.2
          Significant Digits; Significant Digits and Mathematical Operations 30 min
      • 21.Semiconductors
        9
        • Lecture21.1
          Conductor, Semiconductors and Insulators Basics Difference, Energy Band Theory, Si element 21 min
        • Lecture21.2
          Doping and PN Junction 01 hour
        • Lecture21.3
          Diode and Diode as Rectifier 01 hour
        • Lecture21.4
          Voltage Regulator and Zener Diode and Optoelectronic Jn. Devices 01 hour
        • Lecture21.5
          Transistor, pnp, npn, Modes of operation, Input and Output Characteristics, , Current Amplification Factor 01 hour
        • Lecture21.6
          Transistor as Amplifier, Transistor as Switch, Transistor as Oscillator, Digital Gates 01 hour
        • Lecture21.7
          Chapter Notes – Semiconductors
        • Lecture21.8
          NCERT Solutions – Semiconductors
        • Lecture21.9
          Revision Notes Semiconductors
      • 22.Communication Systems
        5
        • Lecture22.1
          Basic working and terms; Antenna; Modulation and Types of Modulation 32 min
        • Lecture22.2
          Amplification Modulation, Transmitter, Receiver, Modulation index 40 min
        • Lecture22.3
          Chapter Notes – Communication Systems
        • Lecture22.4
          NCERT Solutions – Communication Systems
        • Lecture22.5
          Revision Notes Communication Systems

        Chapter Notes – Semiconductors

        On the basis of electrical conductivity, the materials can be divided into three categories :
        (1)  Conductors (e.g., Cu, Al, Fe, etc.),
        (2)  Insulators (e.g., wood, diamond, mica, etc.), and
        (3)  Semiconductors (e.g., Ge, Si, GaAs, … etc.)

        At room temperature (≈300 K), the conductivity of conductors is in the range 106 – 108 S/m, and that of insulators, 10–8 to 104 S/m.  The conductivity of semiconductors lies in between that of conductors and insulators.
        Electronic devices (such as diodes, transistors, ICs) are made from silicon (Si) and germanium (Ge).  In the early days (around 1950), mostly germanium was used, because it was comparatively easier to purify germanium.  However, it was found that the devices made from silicon are more stable and their functioning is less dependent on temperature variations.  Now, we have much improved metallurgical processes to purify silicon.  So, now-a-days mostly the devices are of Si.  An integrated circuit (IC) is made from a silicon chip or wafer.

        Energy Bands in Solids

        The electrons in an isolated atom have discrete energy levels.  However, in a crystal an atom is surrounded by a large number of other atoms.  Due to interatomic interactions, the energy levels are modified.  This modification is more prominent for the electrons in the outermost shell.  (The electrons in the inner shells are shielded by the electrons in outer shells and are not much affected by the electric fields of the neighbouring atoms.)  Due to this modification, each energy level splits into a very large number of levels (~ 1023) lying close to one another.  We can regard a bunch of these energy levels as a continuous energy distribution, and call it energy band.
        The energy bands, which are completely filled at 0 K are called valence bands (VB).  The bands with higher energies are called conduction bands (CB).  We are generally concerned with the highest valance band and the lowest conduction band.

        Note that a conduction band is either completely empty or partially filled.
        The difference between the highest energy in a valence band and the lower energy in the next higher conduction band is called forbidden energy gap (Eg).
        As an example, consider a specimen of sodium (Na) containing N atoms.  Its atomic number is 11 (1s2, 2s2, 2p6, 3s1).  The details of its energy bands are shown in the table.

        As temperature is raised, the electrons may collide with each other and with ions to exchange energy.  The order of energy exchanged is kT.  At room temperature (300 K), kT is 0.026 eV.  Ordinarily, the energy gaps are much larger than kT.  An electron in a completely filled band (i.e., a valence band) does not find an empty state with a slightly higher or lower energy.  Hence, it cannot accept or donate any energy of the order of kT.  However, the outermost electrons, which are in the highest occupied energy band, may take up this energy ≈kT provided some empty states are available in the same band.
        Similar thing happens when a piece of sodium is connected to a battery.  The electric field can supply only a small amount of energy to the electrons.  Only the electrons in the highest occupied band can accept this energy and then move according to the field.  This gives rise to an electric current.  The electrons in the inner (valence) bands cannot accept this small amount of energy and hence cannot take part in electric conduction.

        Why Materials Have Different Conductivities
        There can be four broad types of energy band structures, as shown.
        (A) The highest occupied energy band is only partially filled at 0 K.  (Such is the case with sodium, copper, etc.)  When electric field is applied, the electrons in the partially filled band can accept energy from the field and can drift accordingly.  Hence, such materials are good conductors of electricity.

        (B) The highest occupied energy band (VB) is completely filled at 0 K and next higher band is completely empty (CB).  But the two are overlapping.  (Zinc has such energy band structure.)  Therefore, there are empty energy stats close to the occupied states.  Hence, such solids are also good conductors.

        (C) The highest occupied energy band (VB) is completely filled and the next higher band is completely empty (CB).  There is a large gap (Eg > 5 eV) between these two bands.  (Diamond is of this type.)  The electrons in VB refuse to accept any energy from electric field, because there is no empty state nearby.  Only when the energy supplied (either by applied field or by raising the temperature) is more that Eg (~ 5 eV), an electron from VB can jump to CB, after which it can take part in electrical conduction.  Therefore, at ordinary temperatures, these materials behave as insulators.
        (D) The VB and CB are separated by small gap at 0 K (Eg ~ 1 eV).  [For Ge, Eg = 0.72 eV ; and for Si,  Eg = 1.12 eV.]  At 0 K, an ordinary battery cannot supply even this much energy.  Hence electrical conduction cannot take place.  That is, at 0 K, these materials behave as perfect insulators.  However, at room temperature, thermal energy pushes some of these electrons in VB to CB.  Thus, small conduction becomes possible.  Such solids are therefore called semiconductors.

        INTRINSIC SEMICONDUCTORS

        To make a diode or a transistor, the first step is to obtain a sample of semiconductor in its purest form.  This is called intrinsic semiconductor.  The impurity content is less than one part impurity in 100 million parts of semiconductor.

        Crystal Structure of Semiconductors

        Each atom of an intrinsic semiconductor (Ge or Si) has four valence electrons.  These four electrons of each atom form covalent bonds with the four neighbouring atoms.  Thus, the semiconductor has tetrahedral lattice structure.  A simplified two-dimensional representation of this crystalline structure is shown in figure.  The core represents the nucleus and all the orbiting electrons except the four valence electrons.  Therefore the core has +4 charge.  A covalent bond consists of two electrons, one from each adjacent atom.  At 0 K, all the valence electrons are tightly bound to the parent atoms.  No free electrons are available for electrical conduction.  Hence, the semiconductor behaves as a perfect insulator at 0 K.

        Charge Carriers in Intrinsic Semiconductors

        At room temperature, thermal energy is sufficient to make a valence electron jump to the conduction band.  It starts orbiting the nucleus at a larger radius, and frequently it jumps from one nucleus to the other.  It has become  free electron.When an electron breaks a covalent bond and moves away, a vacancy is created in the bond.  A positive charge is associated with this vacancy.  This vacancy is called a hole.  Free electrons and holes are always generated in pairs.  At any time, the concentration of free electrons is same as the concentration of holes in an intrinsic semiconductor,

        ni=pi

        Just as free electrons move randomly in the crystal, so do the holes.  An electron from the neighbouring bond can make a jump to fill the vacancy, thereby shifting the vacancy (or the holes) to new location.  Much energy is not needed to induce such a transfer, as all the electrons in the valence band have roughly the same energy.  A free electron carries negative charge (–1.6 x 10–19 C) with it.  A hole carries a positive charge (+1.6 x 10–19 C) with it.
        A metal (such as Cu, Al, etc.) has only one type of charge carriers, namely, free electrons.  But a semiconductor has two types of charge carriers ¾ free electrons and holes.
        When an electric field is applied, the free electrons (in the conduction band) drift opposite to the field and the holes drift along the field.  Thus, both types of carriers contribute to electric conduction.

        In a semiconductor, not only the thermal generation of electron-hole pairs takes place, but also there is simultaneous pair recombination.  When a free electron encounters a hole, during their random motion, the electron occupies the vacancy, re-establishing the covalent bond.  The individual identity of both is lost.  In this recombination process, same amount of energy is given out as was taken to generate electron-hole pair.  At equilibrium the rate of pair recombination is equal to the rate of pair generation.  If temperature increases, more bonds are broken, that is, the rate of generation increases.  This increases the concentration of free electrons and holes, which in turn increases the rate of recombination.  Equilibrium is again established.
        Note that Eg is more in silicon (Eg = 1.12 eV) than in germanium (Eg = 0.72 eV).  Therefore, at a given temperature, less number of electron-hole pairs will be generated in silicon than in germanium.  Hence, the conductivity of silicon is less than that of germanium. 

        Conduction in Intrinsic Semiconductor

        When a battery is connected across a semiconductor, the free electrons drift towards +ve terminal and holes drift towards –ve terminal.  The total current I is summation of the current due to electron flow In and the current due to hole flow.  The current in the connecting wire is due to electron flow.

        Effect of Temperature on Conductivity of a Semiconductor

        When temperature is raised, more electron-hole pairs are generated.  The higher the temperature, the higher is the concentration of charge carriers.  Because of this, the conductivity increases with temperature.  In other words, the resistivity (ρ=1/σ) decreases with rise in temperature.  That is, the semiconductors have negative temperature coefficient of resistance.

        EXTRINSIC SEMICONDUCTORS     

        Intrinsic (pure) semiconductors are of little use.  For making a semiconductor device, we deliberately add a tiny and controlled amount of desired impurity to the highly purified semiconductor.  This process is called doping.  A doped semiconductor is called extrinsic semiconductor.  The proportion of impurity added is about 1 in 106.

        N-type, or Donor Type

        Pentavalent impurity (from group V, such as phosphorous, arsenic, or antimony) is added so that an impurity atom substitutes for a silicon atom in the crystalline structure.  Four of its valence electrons make four covalent bonds with neighbouring atoms.  The fifth electron remains unpaired, and is quite loosely bound to the nucleus.  It needs very little energy (0.01 eV in Ge, 0.05 eV in Si) to free itself from the attractive force of the nucleus.  At room temperature, the thermal energy is enough to do this job for all the impurity atoms added.  Since each impurity atom donates one electron to the conduction band, this type of impurity is called donor type.

        After donating an electron, the impurity atom becomes +ve ion.  However, this +ve charge is immobile as the ion is held in its place by covalent bonds.  In addition to the free electrons donated by impurity atoms, there are some more due to breaking of covalent bonds.  Thus, a few holes are also produced.  If ND is the concentration of donor (impurity) atoms, n that of free electrons and p that of hole, we have

        ND           +           p           =              n

        (+ve ions)            (holes)               (electrons)

        As a whole, the N-type semiconductor is neutral.  It has electrons in majority and holes in minority (n>>p).

        P– type, or Acceptor Type

        Here, trivalent impurity (from group III, such as boron, aluminium, gallium and indium) is added.  The three valence electrons make only three covalent bonds with neighbours.  The fourth bond remains incomplete.  There exists a vacancy of an electron in this bond.

        Note that this vacancy is not a hole, as no charge is associated with it.  However, the single electron in the incomplete bond has a great tendency to snatch an electron from neighbouring  bonds.  Only a little (about 0.01 eV) additional energy is needed by the electron in adjacent bond to jump and occupy the vacancy around the impurity atom.  When this happens, a hole is now formed in the adjacent bond.  This hole goes on moving around randomly in the crystal carrying +ve charge with it.
        When impurity atom accepts an electron to complete fourth bond, it becomes –ve ion (immobile).  At room temperature, all impurity atoms (concentration NA) convert into –ve ions.  In addition to the holes created due acceptor impurity atoms, there will be some covalent bonds broken due to thermal energy.

        NA           +             n              =            p

        (–ve ions)            (electrons)               (holes)

        The holes are in majority (p>>n).

        Doping Level

        If the doping level (i.e., the impurity concentration) is increased in an extrinsic semiconductor, the concentration of majority carriers increases.  As a result, the chances of their recombining with minority carriers increase; and hence the concentration of minority carriers decreases.  In fact, in an extrinsic semiconductor, we have

        np=n2i
        where ni is the intrinsic concentration of free electrons (or of holes).

        Effect of Temperature on Extrinsic Semiconductor

        The number of charge carriers in an extrinsic semiconductor is much larger than that in intrinsic semiconductor.  Hence, its conductivity is also many times that of an intrinsic semiconductor.
        Consider an N-type semiconductor.  All the donors have already donated the electrons to the crystal at room temperature.  If the temperature is raised further, more covalent bonds are broken.  As a result, the concentration of minority carriers increases.  Eventually a temperature is reached when the concentration of minority carriers becomes almost same as that of majority carriers.  It will then behave like an intrinsic semiconductors (with higher conductivity).  Any device made of P- and N-type semiconductor will fail at such a temperature.  This critical temperature is about 85 °C for Ge and 200 °C for Si.

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