UNIT-5•Semiconductors•Superconductivity                     1
APPLIED PHYSICS   CODE : 07A1BS05       I B.TECH  CSE, IT, ECE & EEE     UNIT-5 : CHAPTER:1  NO. OF SLIDES :20            ...
UNIT INDEXS.No.      Module      Lectur PPT Slide                       e      No.                       No.   1    Introd...
Lecture-1• Solids are classified as metals,  semiconductors and insulators.• Solids with either overlapping  valence band ...
• Germanium and silicon are  important semiconductors which are  widely used in the manufacturing of  diodes and transisto...
• Germanium and silicon are pure  semiconductors with no impurities.• At room temperature the thermal enrgy  is sufficient...
Lecture-2• In an intrinsic semiconductor, the  Fermi energy level is at the middle of  valence and conduction bands.• If E...
• The density of electrons is  given by n= 2(2пme*kT/h2)3/2 exp[ (EF-Ec)/kT]• The density of holes is given by  p = 2(2пmh...
Lecture-3Extrinsic semiconductors• A semiconducting material in which  the charge carriers originate from  impurity atoms ...
N-type semiconductor• There are two types of impurities  possible namely pentavalent and  trivalent.• If a pentavalent ato...
• Antimony, phosphorous, arsenic etc.,  are examples of pentavalent  elements. When they are added to Si  or Ge as impurit...
• At 0kEF =(Ed+Ec)/2• i.e. at 0k Fermi level lies exactly at  the middle of the donor level Ed and  the bottom of the cond...
P-type semiconductor• If a trivalent atom is doped into the  trivalent host crystal, its three  valence electrons fill onl...
L                e                c•   When they t are added to Si or Ge as    impurities, uthey are called acceptors     ...
• At 0k EF =(Ev+Ea)/2 i.e. Fermi level  lies exactly at the middle of the  acceptor level and the top of  the valence band...
• For a semiconducting material  the electrical conductivity σ is  given by     σ = (neμe + peμh)Since n=p=ni σ = (μe + μh...
Lecture-4 EINSTEIN EQUATION• The relation between diffusion  coefficient and mobility of a  charge carrier is termed  Eins...
HALL EFFECT• When a piece of semiconductor  carrying a current is placed in a  transverse magnetic field, an  electric fie...
Lecture-5• The Hall coefficient    RH = -1/ne (for n-type  semiconductors)         = 1/pe (for p-type  semiconductors)    ...
• Mean life time is the time taken for  the injected concentration to fall to  1/e of its initial value.• Minority carrier...
UNIT INDEXS.No.                  Module            Lectur   PPT                                         e        Slide No....
APPLIED PHYSICS            CODE : 07A1BS05                I B.TECH           CSE, IT, ECE & EEE              UNIT-5: CHAPT...
Lecture-7SUPERCONDUCTIVITY.• Superconductivity is a  phenomenon occurring  in certain materials at  extremely low  tempera...
• Superconductivity  occurs in a wide variety  of materials, including  simple elements like tin  and aluminium, various  ...
Pu                    re                               e                          p ur                     Im   Resistiv  ...
Critical temperature• The temperature at which  the transition from normal  state to superconducting  state takes place on...
• A magnet levitating   above a high-   temperature   superconductor,   cooled with liquid   nitrogen. Persistent   electr...
Lecture-8Persistent current• The electrical current in a  superconducter,in  superconducting state  remains for a long tim...
Effect of magnetic field.• By applying magnetic field of  sufficient strength,  superconductivity of material  can be dest...
Meissner effect.  NORMAL CONDUCTER.           SUPERCONDUCTER                                 B                  B         ...
Levitation ExperimentsMagnets in repulsive mode forlevitation Meissner EffectHigh Tc Superconductor and High Energy    04/...
L           Lecture-9Types of             e             c             tSuperconductors.             u             r       ...
TYPE-ISUPERCONDUCTORS• Superconductors exhibiting  complete Meissner effect  (perfect diamagnetism) are  called Type-I  Su...
• Transition between  normal and  superconducting states  is sharp and well  defined.• There is only one value  of critica...
• Critical temperatures are  low. Hence these are not  commercially useful but  are useful to understand  the exciting phe...
TYPE-I SUPERCONDUCTERS               SUPER CONDUCTING               STATE       M                                  NORMAL ...
TYPE-IISUPERCONDUCTERS• They are developed from  alloys, compounds,  ceramics, transition metals  etc.• For any Type2 mate...
• The material behaves as  a perfect superconductor  in the range 0<H<Hc1.• When H>Hc2 the material  returns to normal sta...
TYPE-II SUPERCONDUCTERS.         SUPERCONDUCTIONG         STATE. M                     MIXED                       STATE  ...
Super electrons• According to London  brothers, a  superconductor is  composed of two distinct  type of electrons, i.e.,  ...
Lecture-10Penetration depth• According to London  equations, the magnetic flux  does not drop to zero  suddenly at the sur...
BCS theory• According to BCS theory,  superelectrons are  responsible for the  superconductivity. They  exist as Cooper pa...
L    Quantum Tunneling                e                c                tMetal                u                r   Metal  ...
Quantum TunnelingSuperconducter            Metal                     Insulater      I            Vc   V
L    Available States.   QUANTUM TUNNELING     e     c     t     uE     r     e     - Ef                                Ef...
04/10/13   46
Cooper Pairs04/10/13   47
Flux quantization•The magnetic flux enclosed by a ring is quantized. This concept is known as flux quantization.04/10/13  ...
Lecture-11Josephson effect• When a thin insulating  layer is sandwiched  between a metal and a  superconductor or two  sup...
d.c. Josephson effect•A d.c. current flows across the junction of two superconductors separated by a thin insulating layer...
a.c.Josephson effect•When d.c. voltage applied across the junction of the two superconductors separated by a thin insulati...
Applications ofJosephson effect                    Lecture-12• Josephson effect is  used to generate  microwaves with  fre...
Applications ofJosephson effect• A.C. Josephson effect is  used to measure very low  temperatures based on the  variation ...
Applications of                                             LecturSuperconductors                              e-131.It is...
from conventional power lines in the form of  useless radiation and heat..3.In japan, Superconducting magnets have been   ...
4.High efficiency ore-separating machines are built  using Super-conducting magnets, which are also  used to separate tumo...
6.Using Superconducting elements one  can build up an extremely fast and  large-scale computer in a compact size.  The pow...
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Unit 5

  1. 1. UNIT-5•Semiconductors•Superconductivity 1
  2. 2. APPLIED PHYSICS CODE : 07A1BS05 I B.TECH CSE, IT, ECE & EEE UNIT-5 : CHAPTER:1 NO. OF SLIDES :20 2
  3. 3. UNIT INDEXS.No. Module Lectur PPT Slide e No. No. 1 Introduction L1-2 4-8 2 Extrinsic L3 9-16 semiconducto rs 3. EINSTEIN L4-5 17-20 EQUATION 3
  4. 4. Lecture-1• Solids are classified as metals, semiconductors and insulators.• Solids with either overlapping valence band and conduction band or partially filled valence bands are metals.• Solids with finite forbidden gap in the range 1-3ev are semi conductors.• Insulators have much larger band gap. 4
  5. 5. • Germanium and silicon are important semiconductors which are widely used in the manufacturing of diodes and transistors.• Germanium and silicon are tetravalent atoms i.e they have four valence electrons. Since all the four valence electrons are covalently bound to the four neighboring atoms the crystal acts as a perfect insulator at 0k. 5
  6. 6. • Germanium and silicon are pure semiconductors with no impurities.• At room temperature the thermal enrgy is sufficient to break covalent bonds. When a covalent bond is broken a free electron-hole pair is generated.• Conductivity increases with temperature as more and more electrons cross over the small energy gap. 6
  7. 7. Lecture-2• In an intrinsic semiconductor, the Fermi energy level is at the middle of valence and conduction bands.• If Ev and Ec are the energy levels respectively at the top of the valance band and bottom of conduction band, the enerrgygap Eg is given by Eg =Ec-Ev• And EF=(Ec+Ev)/2 7
  8. 8. • The density of electrons is given by n= 2(2пme*kT/h2)3/2 exp[ (EF-Ec)/kT]• The density of holes is given by p = 2(2пmh*kT/h2)3/2 exp[ (Ev-EF)/kT] 8
  9. 9. Lecture-3Extrinsic semiconductors• A semiconducting material in which the charge carriers originate from impurity atoms added to the material is called impurity semiconductor or extrinsic semiconductor.• The addition of impurity increases the carrier concentration and hence the conductivity of the conductor. 9
  10. 10. N-type semiconductor• There are two types of impurities possible namely pentavalent and trivalent.• If a pentavalent atom is doped to the tetravalent host crystal, four of the five valence electrons of the impurity atom form covalent bonds with four neighboring host atoms and one electron is left unpaired. 10
  11. 11. • Antimony, phosphorous, arsenic etc., are examples of pentavalent elements. When they are added to Si or Ge as impurities, they are called donors as they donate free electrons.• The semiconductor prepared in this way will have more electrons than holes.• Since the excess free charge is negative, these are named as N-type 11 semiconductors.
  12. 12. • At 0kEF =(Ed+Ec)/2• i.e. at 0k Fermi level lies exactly at the middle of the donor level Ed and the bottom of the conduction band Ec.• The density of electrons in the conduction band is given by n = 2(2пme*kT/h2)3/4 exp[ (Ed-Ec)/kT] 12
  13. 13. P-type semiconductor• If a trivalent atom is doped into the trivalent host crystal, its three valence electrons fill only three of the four covalent bonds of the host atoms and one vacancy exists in the fourth bond.• Thus in this case one extra hole per doped atoms is formed.• The examples of trivalent atoms are boron, gallium, indium etc. 13
  14. 14. L e c• When they t are added to Si or Ge as impurities, uthey are called acceptors r as they readily accept electrons due e to the presence of the hole. - 3• Since the holes behave like positive charges, the acceptors are called P- type impurities and these impure semiconductors are called P-type semiconductors. 14
  15. 15. • At 0k EF =(Ev+Ea)/2 i.e. Fermi level lies exactly at the middle of the acceptor level and the top of the valence band.• Density of holes in valence band is given by p = 2(2пmh*kT/h2)3/4 exp[ (Ev-Ea)/kT] 15
  16. 16. • For a semiconducting material the electrical conductivity σ is given by σ = (neμe + peμh)Since n=p=ni σ = (μe + μh) 2e (2пkT/h2)3/2 (me*mh*)3/4 exp(-Eg/2kT) 16
  17. 17. Lecture-4 EINSTEIN EQUATION• The relation between diffusion coefficient and mobility of a charge carrier is termed Einstein equation.• D n = μekT/e (For electrons)• Dp = μfkT/e (For holes) 17
  18. 18. HALL EFFECT• When a piece of semiconductor carrying a current is placed in a transverse magnetic field, an electric field is produced inside the conductor in a direction normal to both the current and magnetic field.• This phenomenon is known as the Hall effect and the generated voltage is known as Hall voltage. 18
  19. 19. Lecture-5• The Hall coefficient RH = -1/ne (for n-type semiconductors) = 1/pe (for p-type semiconductors) 19
  20. 20. • Mean life time is the time taken for the injected concentration to fall to 1/e of its initial value.• Minority carrier life time can be defined as the time taken for the excess charge carriers to reduce to 1/e times its initial value, once the source generating these excess charge carriers is cut off. 20
  21. 21. UNIT INDEXS.No. Module Lectur PPT e Slide No. No. 1 properties of superconductors. L7-8 3-11 2 Types of superconductors L9-10 12-28 3. DC & AC Josephson effect L11-12 29-33 4. Applications L13 34-37 04/10/13 21
  22. 22. APPLIED PHYSICS CODE : 07A1BS05 I B.TECH CSE, IT, ECE & EEE UNIT-5: CHAPTER-2 NO. OF SLIDES :3704/10/13 22
  23. 23. Lecture-7SUPERCONDUCTIVITY.• Superconductivity is a phenomenon occurring in certain materials at extremely low temperatures, characterized by almost zero electrical resistance04/10/13 23
  24. 24. • Superconductivity occurs in a wide variety of materials, including simple elements like tin and aluminium, various metallic alloys and some heavily-doped semiconductors.04/10/13 24
  25. 25. Pu re e p ur Im Resistiv ity O T TEMP(K) ctance of superconducter suddenly drops t
  26. 26. Critical temperature• The temperature at which the transition from normal state to superconducting state takes place on cooling in the absence of magnetic field is called the critical temperature or the transition temperature04/10/13 26
  27. 27. • A magnet levitating above a high- temperature superconductor, cooled with liquid nitrogen. Persistent electric current flows on the surface of the superconductor, acting to exclude the magnetic field of the magnet (the Meissner effect). This current04/10/13 27 effectively forms an
  28. 28. Lecture-8Persistent current• The electrical current in a superconducter,in superconducting state remains for a long time .• This current remains for very long period without attenuation.• The time taken by the super04/10/13 28
  29. 29. Effect of magnetic field.• By applying magnetic field of sufficient strength, superconductivity of material can be destroyed.• The minimum magnetic field strength required to destroy superconductivity of substance,below Tc is called critical magnetic field (Hc) at04/10/13 29
  30. 30. Meissner effect. NORMAL CONDUCTER. SUPERCONDUCTER B B T<Tc T > TcSUPERCONDUCTER EXPELS MAGNETIC LINES OF FORCE. 04/10/13 30
  31. 31. Levitation ExperimentsMagnets in repulsive mode forlevitation Meissner EffectHigh Tc Superconductor and High Energy 04/10/13 31Permanent Magnet
  32. 32. L Lecture-9Types of e c tSuperconductors. u r e• Depending on the way of - 1 transition from superconducting state to normal state by the application of magnetic field, superconductors are classified into04/10/13 32
  33. 33. TYPE-ISUPERCONDUCTORS• Superconductors exhibiting complete Meissner effect (perfect diamagnetism) are called Type-I Superconductors.• They are also known as soft Superconductors.04/10/13 33
  34. 34. • Transition between normal and superconducting states is sharp and well defined.• There is only one value of critical magnetic04/10/13 34
  35. 35. • Critical temperatures are low. Hence these are not commercially useful but are useful to understand the exciting phenomenon of superconductivity.• Type-I Superconductors are mostly of pure04/10/13 35
  36. 36. TYPE-I SUPERCONDUCTERS SUPER CONDUCTING STATE M NORMAL STATE. O HcRELATION BETWEEN MAGNETIZATION ANDAPPLIED MAGNETIC FIELD FOR TYPE-I SUPERCONDUCTERS.
  37. 37. TYPE-IISUPERCONDUCTERS• They are developed from alloys, compounds, ceramics, transition metals etc.• For any Type2 material, two critical values of applied magnetic field Hc1 and Hc2 can be identified. In04/10/13 37
  38. 38. • The material behaves as a perfect superconductor in the range 0<H<Hc1.• When H>Hc2 the material returns to normal state.• Nb and Zr are some examples of this type.04/10/13 38
  39. 39. TYPE-II SUPERCONDUCTERS. SUPERCONDUCTIONG STATE. M MIXED STATE NORMAL (OR) STATE. VORTEX STATE O Hc1 Hc H 2Variation of Magnetization with applied magneticfield for Type –II superconducters.
  40. 40. Super electrons• According to London brothers, a superconductor is composed of two distinct type of electrons, i.e., normal electrons and super electrons. super electrons experience no04/10/13 40
  41. 41. Lecture-10Penetration depth• According to London equations, the magnetic flux does not drop to zero suddenly at the surface of Type-I superconductors, but decreases exponentially. The depth from the surface at which the magnetic flux04/10/13 41
  42. 42. BCS theory• According to BCS theory, superelectrons are responsible for the superconductivity. They exist as Cooper pairs. They form a bound single system. Their motions are correlated.04/10/13 42
  43. 43. L Quantum Tunneling e c tMetal u r Metal e - 1 Insulater I V
  44. 44. Quantum TunnelingSuperconducter Metal Insulater I Vc V
  45. 45. L Available States. QUANTUM TUNNELING e c t uE r e - Ef Ef 1 E1Super Conducter Insulater METAL
  46. 46. 04/10/13 46
  47. 47. Cooper Pairs04/10/13 47
  48. 48. Flux quantization•The magnetic flux enclosed by a ring is quantized. This concept is known as flux quantization.04/10/13 48
  49. 49. Lecture-11Josephson effect• When a thin insulating layer is sandwiched between a metal and a superconductor or two superconductors, electrons can tunnel through the junction. Their wave functions on04/10/13 49
  50. 50. d.c. Josephson effect•A d.c. current flows across the junction of two superconductors separated by a thin insulating layer in04/10/13 50
  51. 51. a.c.Josephson effect•When d.c. voltage applied across the junction of the two superconductors separated by a thin insulating layer then04/10/13 51
  52. 52. Applications ofJosephson effect Lecture-12• Josephson effect is used to generate microwaves with frequency W = 2eVo/ħ• A.C. Josephson effect is used to define standard volt04/10/13 52
  53. 53. Applications ofJosephson effect• A.C. Josephson effect is used to measure very low temperatures based on the variation of frequency of the emitted radiation with temperature• A Josephson junction is used for switching of signals04/10/13 53
  54. 54. Applications of LecturSuperconductors e-131.It is a basis of new generation of energy saving power system. Superconducting generators are smaller in size and less in weight compare with conventional generators. These generators consume very low energy, hence more energy will be saved.2.All electric power companies are looking forward to the superconducting transmission system that would save most of the energy now being last04/10/13 54
  55. 55. from conventional power lines in the form of useless radiation and heat..3.In japan, Superconducting magnets have been used to levitate an experimental train above its track and can drive it at a great speed of 500 Km/h with minimum expenditure of energy. A similar magnetic propulsion system may be used to launch satellites into orbits directly from the earth without the use of rockets.04/10/13 55
  56. 56. 4.High efficiency ore-separating machines are built using Super-conducting magnets, which are also used to separate tumour cells from healthy cell by High Gradient Magnetic separation method.5.Superconducting materials can be used as a memory or storage device in computers, since the current in it can flow without any change in its value with time.04/10/13 56
  57. 57. 6.Using Superconducting elements one can build up an extremely fast and large-scale computer in a compact size. The power consumed by this computer will be less than 0.5 watt. 7. The Josephon devices are used to produce microwaves, which are made up of superconductors.04/10/13 57

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