Chapter 17

2. Chapter 17 Introduction to Semiconductors and Power Supplies

3. Objectives • Define electronics. • Explain the doping process. Explain how N-type and P-type materials are made. • Discuss how N-type and P-type materials conduct electrical energy. • Explain forward and reverse biasing. Permission granted to reproduce for educational use only.

4. Objectives • Discuss various types of semiconductor diodes. • Explain the operation of a half-wave and full-wave rectifier. • Explain power supply filtering. Permission granted to reproduce for educational use only.

5. Objectives • Explain power supply load characteristics. • Outline various methods for regulating voltage. • Discuss methods for raising voltages. • Construct simple power supplies. Permission granted to reproduce for educational use only.

6. Development of integrated circuits, transistors, and vacuum tubes J.A. Fleming, rectification Bell laboratories, transistor Introduction to Electronics Permission granted to reproduce for educational use only.

7. Conductors • Copper and several other metals make good conductors • Certain materials (glass and rubber) are insulators • Semiconductors are made from materials that are not good conductors or insulators Permission granted to reproduce for educational use only.

8. Semiconductors • Atomic characteristics • Conduction of electricity • Doping Permission granted to reproduce for educational use only.

9. Atomic Characteristics • Atomic number • Silicon’s number is 14 • 14 protons in nucleus • 14 electrons outside nucleus • Germanium’s number is 32 • 32 protons in nucleus • 32 electrons outside nucleus Permission granted to reproduce for educational use only.

10. Atomic Characteristics (Cont.) • Valence electrons • Covalent bonds • Lattice crystalline structure Permission granted to reproduce for educational use only.

11. Conduction of Electricity • Current is flow of electrons • Results in energy • Chain reaction of electrons being added • Chain reaction of electrons moving and leaving holes Permission granted to reproduce for educational use only.

12. Doping • Intrinsic versus extrinsic semiconductors • Impure materials • Trivalent impurities • Pentavalent impurities Permission granted to reproduce for educational use only.

13. Pentavalent Impurities • Also called donor impurities • Result in N-type crystals Permission granted to reproduce for educational use only.

14. Pentavalent Impurities (Cont.) • Conduction through an N-type crystal Permission granted to reproduce for educational use only.

15. Trivalent Impurities • Also called acceptor impurities • Result in P-type crystals Permission granted to reproduce for educational use only.

16. Trivalent Impurities (Cont.) • Conduction through a P-type crystal • Hole moves in opposition to electron flow Permission granted to reproduce for educational use only.

17. Review What is rectification? Changing an alternating current to a direct current Permission granted to reproduce for educational use only.

18. Review What structure does a covalent bond create? Lattice crystalline structure Permission granted to reproduce for educational use only.

19. Review Describe the action of the energy resulting from electron flow. An electron is added to one end of the conductor. Another electron leaves the opposite end. Permission granted to reproduce for educational use only.

20. Review What is the result of doping? An extrinsic semiconductor Permission granted to reproduce for educational use only.

21. Review What kind of crystals result from trivalent impurities? P-type crystals Permission granted to reproduce for educational use only.

22. Review What is another name for pentavalent impurities? Donor impurities Permission granted to reproduce for educational use only.

23. Semiconductor Diodes • Diodes • Cathodes • Anodes • Semiconductor diodes • Potential hills, orpotential barriers Permission granted to reproduce for educational use only.

24. Semiconductor Diodes (Cont.) • Forward and reverse biasing • Types of semiconductor diodes • Diode characteristics and ratings • Series and parallel rectifier arrangements • Testing diodes Permission granted to reproduce for educational use only.

25. Forward Biased Diodes • Voltage connected across diode • Positive terminal is connected to the P crystal • Negative terminal is connected to the N crystal Permission granted to reproduce for educational use only.

26. Forward Biasing • Negative electrons in N crystal move to the barrier • Positive holes in P crystal move to the barrier • Opposition of voltage reduces barrier effect • Electrons and holes are joined at the barrier Permission granted to reproduce for educational use only.

27. Reverse Biased Diodes • Positive source is connected to the N crystal • Negative source is connected to the P crystal Permission granted to reproduce for educational use only.

28. Reverse Biasing • Electrons in N crystal flow to positive terminal • Barrier effect is increased because of source voltage • Little current flows in a reverse-biased circuit Permission granted to reproduce for educational use only.

29. Types of Semiconductor Diodes • Point contact diodes • Silicon rectifiers • Zener diodes • Light emitting diodes (LEDs) Permission granted to reproduce for educational use only.

30. Point Contact Diodes • Used for detection and rectification • Contain a small N-type germanium crystal • Made of a fine phosphor bronze wire • P-type region forms around contact point in germanium crystal • Has both P- and N-type crystals Permission granted to reproduce for educational use only.

31. Silicon Rectifiers • High forward-to-reverse current ratios • Can achieve rectification efficiencies of more than 99% • Can be made resistant to shock • Do not decay with age Permission granted to reproduce for educational use only.

32. Zener Diodes • When forward biased, act similar to a closed switch • Go into reverse bias at various voltages • Zener breakdown point Permission granted to reproduce for educational use only.

33. LEDs • Emit light in the forward bias direction • Made from semiconductor compounds • Energy used creates photons Permission granted to reproduce for educational use only.

34. Diode Characteristics and Ratings • Commonly based on current and voltage capabilities • Also based on peak inverse voltage (PIV) Permission granted to reproduce for educational use only.

35. Series and Parallel Rectifier Arrangements • Connecting diodes or rectifiers in series increases voltage ratings beyond the value of a single rectifier • Connecting in parallel improves current handling ability beyond that of a single diode • Electrical characteristics must match, or current or voltage drop will not be equal across both sides Permission granted to reproduce for educational use only.

36. Testing Diodes • Forward-biased diodes have low resistance • Reverse-biased diodes have high resistance Permission granted to reproduce for educational use only.

37. Review What is a diode? A device that permits electron flow in one direction, while blocking flow from the other direction Permission granted to reproduce for educational use only.

38. Review What are the two electrodes in a diode called? Anode and cathode Permission granted to reproduce for educational use only.

39. Review What type of biasing reduces the barrier effect? Forward biasing Permission granted to reproduce for educational use only.

40. Review What type of diode has both the P- and the N-type crystals? Point contact diodes Permission granted to reproduce for educational use only.

41. Review What happens at the zener breakdown point? The diode is able to maintain a fairly constant voltage as the current varies over a certain range Permission granted to reproduce for educational use only.

42. Review What kind of diodes are made from semiconductor compounds? Light emitting diodes (LEDs) Permission granted to reproduce for educational use only.

43. Power Supplies • Power supply functions • Power transformers • Half-wave and full-wave rectification • Filters Permission granted to reproduce for educational use only.

44. Power Supply Functions • Stepping voltages up or down • Dividing voltage • Changing ac to dc voltage • Changing pulsating dc to pure dc • Regulating power supply output Permission granted to reproduce for educational use only.

45. Power Transformers • Step ac voltage up or down to necessary values • Isolation transformers • Connections • Hot • Grounded, or neutral Permission granted to reproduce for educational use only.

46. Power Transformers (Cont.) • Center taps • Voltages are 180 degrees out of phase with respect to center tap • Windings can provide different voltages Permission granted to reproduce for educational use only.

47. Half-Wave Rectifiers • Output is pulses of current flowing in one direction at same frequency as input voltage Permission granted to reproduce for educational use only.

48. Negative Rectifiers • Inverted diodes • Negative supply voltage Permission granted to reproduce for educational use only.

49. Full-Wave Rectifiers • Use two diodes • Both half cycles of input wave can be used • Center tap made on secondary winding • Both positive and negative half cycles of input voltage cause current through R in the same direction Permission granted to reproduce for educational use only.

50. Bridge Rectifiers • For full-wave rectification • Can be used in circuits without transformers • Line-operated bridge circuits Permission granted to reproduce for educational use only.