Electronics: Semiconductors and diodes s

Semiconductors and diodes

1. Semiconductors and Diodes Chapter 1

2. Outlines

3. Outlines

4. Bohr Atom The Bohr model of the atom is that electrons can circle the nucleus only in specific orbits, which correspond to discrete energy levels called shells . The atomic number is the number of protons in the nucleus. The outermost occupied shell is called the valence shell and electrons that occupy this shell are called valence electrons . Atom The Maximum Number of Electrons in Each Shell is:

5. Conductors Materials can be classified by their ability to conduct electricity. This ability is related to the valence electrons. Copper is an example of an excellent conductor . It has only one electron in its valence band, which can easily escape to the conduction band, leaving behind a positive ion (the core). Like all metals, copper has many free electrons which are loosely held by the attraction of the positive metal ions. Core (+1) Materials Used in Electronic Devices

6. Conductors Materials Used in Electronic Devices

7. Insulators have tightly bound electrons with few electrons available for conduction. Nonmetals, such as glass, air, paper, and rubber are excellent insulators and widely used in electronics. Even these materials can break down and conduct electricity if the voltage is high enough. Insulators Materials Used in Electronic Devices

8. Semiconductors Silicon is an example of a single element semiconductor . It has four electrons in its valence band. Unlike metals, silicon forms strong covalent bonds (shared electrons) with its neighbors. Intrinsic silicon is a poor conductor because most of the electrons are bound in the crystal and take part in forming the bonds between atoms. Semiconductors are between conductors and insulators in their ability to conduct electricity. Core (+4) Materials Used in Electronic Devices

9. The Silicon, Si, Atom These shared electrons – bonds – are shown as horizontal and vertical lines between the atoms. This picture shows the shared electrons Silicon has a valency of 4 i.e. 4 electrons in its outer shell Each silicon atom shares its 4 outer electrons with 4 neighbouring atoms

10. Silicon – the crystal lattice If we extend this arrangement throughout a piece of silicon… We have the crystal lattice of silicon This is how silicon looks when it is cold It has no free electrons – it cannot conduct electricity – therefore it behaves like an insulator

11. Electron Movement in Silicon However, if we apply a little heat to the silicon…. An electron may gain enough energy to break free of its bond… It is then available for conduction and is free to travel throughout the material

12. Hole Movement in Silicon Let’s take a closer look at what the electron has left behind There is a gap in the bond – what we call a hole

13. Hole Movement in Silicon This hole can also move… An electron – in a nearby bond – may jump into this hole… Effectively causing the hole to move… Like this…

14. Electrons Holes Within the crystalline structure, there are two types of charge movement (current): The bound (valence) electrons move between atoms, effectively moving holes from one atom to another as illustrated. Holes act like positive charges, with their own mobility. Current in Semiconductors The conduction band electrons are free to move under the influence of an electric field.

15. Doping Relying on heat or light for conduction does not make for reliable electronics To make the semiconductor conduct electricity, other atoms called impurities must be added. “Impurities” are different elements. This process is called doping.

16. N-Type Semiconductors Certain impurities will change the conductivity of silicon. An impurity such as Antimony has an electron that is not part of the bonding electrons so is free. This creates an n -material. N-Type and P-Type Semiconductors

17. The Phosphorus Atom Phosphorus is number 15 in the periodic table It has 15 protons and 15 electrons – 5 of these electrons are in its outer shell

18. Doping – Making n-type Silicon Suppose we remove a silicon atom from the crystal lattice… and replace it with a phosphorus atom We now have an electron that is not bonded – it is thus free for conduction

19. Doping – Making n-type Silicon Let’s remove another silicon atom… and replace it with a phosphorus atom As more electrons are available for conduction we have increased the conductivity of the material If we now apply a potential difference across the silicon… Phosphorus is called the dopant

20. Extrinsic Conduction – n-type Silicon A current will flow Note: The negative electrons move towards the positive terminal

21. From now on n-type will be shown like this. N-type Silicon This type of silicon is called n-type This is because the majority charge carriers are negative electrons A small number of minority charge carriers – holes – will exist due to electrons-hole pairs being created in the silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons

22. An impurity such as boron leaves a vacancy in the valence band, creating a p -material. Both p- and n- materials have energy levels that are different than intrinsic silicon. Hole from B atom P-Type Semiconductors N-Type and P-Type Semiconductors

23. Doping – Making p-type Silicon As before, we remove a silicon atom from the crystal lattice… This time we replace it with a boron atom Notice we have a hole in a bond – this hole is thus free for conduction

24. Doping – Making p-type Silicon Let’s remove another silicon atom… and replace it with another boron atom As more holes are available for conduction we have increased the conductivity of the material If we now apply a potential difference across the silicon… Boron is the dopant in this case

25. P-type Silicon This type of silicon is called p-type This is because the majority charge carriers are positive holes A small number of minority charge carriers – electrons – will exist due to electrons-hole pairs being created in the silicon atoms due to heat The silicon is still electrically neutral as the number of protons is equal to the number of electrons From now on p-type will be shown like this.

26. The pn junction A p- and an n -material together form a pn junction. A potential is built up (called the barrier potential ) that prevents further charge migration. When the junction is formed, conduction electrons move to the p- region, and fall into holes. Filling a hole makes a negative ion and leaves behind a positive ion in the n -region. This creates a thin region that is depleted of free charges at the boundary.

27. The pn Junction When initially joined electrons from the n-type migrate into the p-type – less electron density there When an electron fills a hole – both the electron and hole disappear as the gap in the bond is filled This leaves a region with no free charge carriers – the depletion layer – this layer acts as an insulator

28. As the p-type has gained electrons – it is left with an overall negative charge… As the n-type has lost electrons – it is left with an overall positive charge… Therefore there is a voltage across the junction – the junction voltage – for silicon this is approximately 0.6 V 0.6 V The pn Junction

29. The Reverse Biased pn Junction Take a p-n junction Apply a voltage across it with the p-type negative n-type positive Close the switch The voltage sets up an electric field throughout the junction The junction is said to be reverse – biased

30. Negative electrons in the n-type feel an attractive force which pulls them away from the depletion layer Positive holes in the p-type also experience an attractive force which pulls them away from the depletion layer Thus, the depletion layer ( INSULATOR ) is widened and no current flows through the p-n junction The Reverse Biased pn Junction

31. Take a p-n junction Apply a voltage across it with the p-type positive and n-type negative Close the switch The voltage sets up an electric field throughout the junction The junction is said to be forward – biased The Forward Biased pn Junction

32. Negative electrons in the n-type feel a repulsive force which pushes them into the depletion layer Positive holes in the p-type also experience a repulsive force which pushes them into the depletion layer Therefore, the depletion layer is eliminated and a current flows through the p-n junction The Forward Biased pn Junction

33. At the junction electrons fill holes They are replenished by the external cell and current flows Both disappear as they are no longer free for conduction This continues as long as the external voltage is greater than the junction voltage i.e. 0.6 V The Forward Biased pn Junction

34. If we apply a higher voltage… The electrons feel a greater force and move faster The current will be greater and will look like this… The pn junction is called a DIODE and is represented by the symbol… The arrow shows the direction in which it conducts current The Forward Biased pn Junction

35. Diodes A diode is a semiconductor device with a single pn junction and metal connections to leads. It has the ability to pass current in only one direction.

36. Forward bias Forward bias is the condition which allows current in the diode. The bias voltage must be greater than the barrier potential. Diodes

37. Reverse bias Reverse bias is the condition in which current is blocked. Diodes

38. The complete V-I characteristic curve Diodes

39. Three diode approximations are: The Ideal Diode Model The Practical Diode Model The Complete Diode Model Diode Approximations

40. The Ideal Diode Model Diode Approximations

41. The Ideal Diode Model Diode Approximations Forward Voltage: Forward Current: Reverse Current: Reverse Voltage: Characteristic curve

42. The Practical Diode Model Diode Approximations

43. The Practical Diode Model Diode Approximations Characteristic curve Forward Voltage: Forward Current: Reverse Current: Reverse Voltage:

44. Diode Approximations The Complete Diode Model

45. The Complete Diode Model Diode Approximations Characteristic curve Forward Voltage: Forward Current: Reverse Current: Reverse Voltage:

46. Example: Diode Approximations

47. Solution: Diode Approximations

48. Solution: Diode Approximations

49. Half-Wave Rectifier The diode conducts during the positive half cycle because it is forward biased. During the positive half cycle, the output voltage looks like the positive half of the input voltage. The current path is through ground back to the source. Diode Applications

50. Half-Wave Rectifier The diode does not conduct during the negative half cycle because it is reverse biased. During the negative half cycle, the current is 0, so the output voltage is also 0. Diode Applications

51. Half-Wave Rectifier Half-wave output voltage for three input cycles Diode Applications Average Value of the Half-Wave Output Voltage: The average value of the half-wave rectified output voltage is the value you would measure on a dc voltmeter

52. Half-Wave Rectifier Diode Applications

53. Half-Wave Rectifier Diode Applications Effect of the Barrier Potential on the Half-Wave Rectifier Output The effect of the barrier potential on the half-wave rectified output voltage is to reduce the peak value of the input by about 0.7 V.

54. Half-Wave Rectifier Diode Applications

55. Half-Wave Rectifier Diode Applications Peak Inverse Voltage (PIV): The peak inverse voltage (PIV) is equal to the peak input voltage and is the maximum voltage across the diode when it is not conducting.

56. Half-Wave Rectifier Diode Applications Transformer Coupling Transformer coupling provides two advantages. First, it allows the source voltage to be stepped up or down as needed. Second, the ac source is electrically isolated from the rectifier, thus avoiding a shock hazard in the secondary circuit for lower voltages. Half-wave rectifier with transformer coupled input voltage

57. Half-Wave Rectifier Diode Applications Transformer Coupling Determine the peak value of the output voltage for the figure if the turns ratio is 0.5.

58. Full-Wave Rectifier A full-wave rectifier allows unidirectional (one-way) current through the load during the entire 360° of the input cycle, whereas a half-wave rectifier allows current through the load only during one-half of the cycle. The result of full-wave rectification is an output voltage with a frequency twice the input frequency and that pulsates every half-cycle of the input, as shown in the figure below. Diode Applications

59. Full-Wave Rectifier Full-wave output voltage for two input cycles Diode Applications Average Value of the Full-Wave Output Voltage: The average value, which is the value measured on a dc voltmeter, for a full-wave rectified sinusoidal voltage is twice that of the half-wave, as shown in the following formula: V AVG is approximately 63.7% of V p for a full-wave rectified voltage

60. Full-Wave Rectifier Diode Applications

61. Full-Wave Rectifier Diode Applications Center-Tapped Full-Wave Rectifier Operation A center-tapped rectifier is a type of full-wave rectifier that uses two diodes connected to the secondary of a center-tapped transformer.

62. Full-Wave Rectifier Diode Applications Center-Tapped Full-Wave Rectifier Operation (a) During positive half-cycles, D 1 is forward-biased and D 2 is reverse-biased. (b) During negative half-cycles, D 2 is forward-biased and D 1 is reverse-biased.

63. Full-Wave Rectifier Diode Applications Effect of the Turns Ratio on the Output Voltage

64. Full-Wave Rectifier Diode Applications Peak Inverse Voltage

65. Full-Wave Rectifier Diode Applications

66. Full-Wave Rectifier Diode Applications

67. Full-Wave Rectifier Diode Applications Bridge Full-Wave Rectifier Operation (a) During the positive half-cycle of the input, D 1 and D 2 are forward-biased and conduct current. D 3 and D 4 are reverse-biased.

68. Full-Wave Rectifier Diode Applications Bridge Full-Wave Rectifier Operation b) During the negative half-cycle of the input, D 3 and D 4 are forward-biased and conduct current. D 1 and D 2 are reverse-biased.

69. Full-Wave Rectifier Diode Applications Bridge Full-Wave Rectifier Operation

70. Full-Wave Rectifier Diode Applications Bridge Full-Wave Rectifier Operation

71. Full-Wave Rectifier Diode Applications

72. Full-Wave Rectifier Diode Applications

73. References Electronic Devices Book Slides.