Electronics Principles & Applications Seventh Edition

1. Electronics Principles & Applications Seventh Edition Charles A. Schuler Chapter 3 Diodes (student version) McGraw-Hill © 2008 The McGraw-Hill Companies Inc. All rights reserved.

2. INTRODUCTION • The PN Junction • Characteristic Curves of Diodes • Diode Lead Identification • Diode Types and Applications

3. Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

4. Concept Preview • Diodes have both P-regions and N-regions. The boundaries are called junctions. • The P-region is the anode side. The N-region is the cathode side. • There are three possible bias conditions: zero, forward, and reverse. • Zero bias is accompanied by a depletion region. • Forward bias can collapse the depletion region. • Reverse bias enhances the depletion region. • Bias determines if diodes will be off or on.

5. The P-side of a junction diode is doped with acceptor atoms. The N-side of a junction diode is doped with donor atoms. junction P N A silicon crystal Cathode Anode Schematic Symbol

6. Depletion region Zero Bias The electrons near the junction cross over and fill the holes near the junction. Having no carriers, the depletion region is an insulator.

7. Forward Bias The carriers move toward the junction and collapse the depletion region. The diode is on.

8. Reverse Bias The carriers move away from the junction. The depletion region is reestablished and the diode is off.

9. Concept Review • Diodes have both P-regions and N-regions. The boundaries are called junctions. • The P-region is the anode side. The N-region is the cathode side. • There are three possible bias conditions: zero, forward, and reverse. • Zero bias is accompanied by a depletion region. • Forward bias can collapse the depletion region. • Reverse bias enhances the depletion region. • Bias determines if diodes will be off or on. Repeat Segment

10. Concept Preview • The volt-ampere graph for a resistor is a straight line (a resistor is a linear device). • The volt-ampere graph for a diode is non-linear. • The knee voltage for a silicon diode is approximately 0.7 volts. This is the voltage required to collapse the depletion region. • Excess reverse bias will result in diode breakdown. • The cathode end might be marked with a band. • Diodes can be tested with an ohmmeter.

11. 5 ohms 200 175 150 10 ohms 125 100 mA 75 20 ohms 50 25 0 0 0.5 1.0 1.5 Volts Resistor volt-ampere characteristic curves

12. knee 200 175 150 125 100 Forward current in mA 75 50 25 0 0 0.5 1.0 1.5 Forward bias in volts Silicon diode volt-ampere characteristic curve

13. Linearity • The volt-ampere characteristic curve for a resistor is a straight line (linear). • A diode has a non-linear characteristic curve. • The barrier potential produces a knee in the diode curve. • The knee voltage is approximately 0.6 to 0.7 volts for silicon diodes.

14. 200 175 100 oC 150 125 25 oC 100 Forward current in mA 75 -50 oC 50 25 0 0 0.5 1.0 1.5 Forward bias in volts The effect of temperature

15. breakdown Reverse bias in Volts 400 600 200 0 20 40 Reverse current in mA 60 80 100 120 140 Silicon diode reverse bias characteristic curve

16. Cathode lead Anode lead

17. The diode is forward biased by the ohmmeter.  0 Cathode lead V mA Anode lead

18.  0 Cathode lead V mA Anode lead The diode is reverse biased by the ohmmeter.

19. Silicon diode ohmmeter testing • Low resistance in both directions: the diode is shorted. • High resistance in both directions: the diode is open. • Relatively low resistance in the reverse direction: the diode is leaky. • The ratio of reverse resistance to forward resistance is > 1000: the diode is good.

20. Diode quiz At zero bias, diodes show a depletion region which acts as an________. insulator The depletion region is collapsed by applying _________ bias. forward The depletion region is made wider by applying ________ bias. reverse A forward-biased diode has its anode________ with respect to its cathode. positive Diode forward voltage drop decreases as temperature ________. increases

21. Concept Review • The volt-ampere graph for a resistor is a straight line (a resistor is a linear device). • The volt-ampere graph for a diode is non-linear. • The knee voltage for a silicon diode is approximately 0.7 volts. This is the voltage required to collapse the depletion region. • Excess reverse bias will result in diode breakdown. • The cathode end might be marked with a band. • Diodes can be tested with an ohmmeter. Repeat Segment

22. Concept Preview • Zener diodes have a relatively low breakdown voltage and are often used as voltage regulators. • Diode clippers limit signal amplitude. • Diode clamps produce a dc signal component. • Diodes can be used to suppress inductive kick. • Optocouplers combine light emitting diodes and photodiodes. • Fiber optic cables carry light signals. • Varicap diodes are used to tune electronic circuits.

23. Reverse bias in Volts 4 6 2 0 20 40 Reverse current in mA 60 80 100 120 140 A zener diode is designed to break down and conduct backwards at lower voltages.

24. V I V Reverse bias in Volts 4 6 2 0 20 40 Reverse current in mA 60 80 100 120 140 The voltage across a conducting zener is relatively constant.

25. Load Unregulated Supply Using a zener diode as a voltage regulator The load is in parallel with the zener and will see a relatively constant voltage as long as the zener is conducting.

26. VOUT is not clipped This circuit is called a clipper or limiter. When VIN < 1.2 VPP VIN VOUT The diodes do not conduct.

27. + 0.6 V VOUT is clipped The diodes conduct. - 0.6 V When VIN > 1.2 VPP VIN VOUT

28. C is charged. dc VPP - 0.6 V VOUT(dc) = 2 0.6 V VOUT is clamped. This circuit is called a clamp or dc restorer. When VIN > 1.2 VPP C VIN VOUT

29. CEMF The inductive kick can cause damage.

30. CEMF Transient suppression diodes can be used with inductive loads. The coil discharges through the diode and there is no arc.

31. LED As the electrons cross the junction, they lose energy in the form of photons.

32. VD IS = VS - VD RS LED circuit RS LED Power supply VS The typical voltage drop for most LEDs is from 1.5 to 2.5 V.

33. RS Power supply VS Photodiode Photodiodes are reverse biased and conduct in the presence of light.

34. Input Output Output Input Optocoupler

35. Step-index multimode fiber Input pulse Output pulse The combined shorter and longer path lengths act to stretch the output pulse.

36. Step-index multimode fiber Due to pulse stretching, high speed data transmission is not possible. Input Output

37. Single mode fiber A single path means no pulse stretching and high speed data transmission is possible. Input Output

38. Tuning diode L C ZERO BIAS C is maximum and fR is minimum.

39. Tuning diode L C REVERSE BIAS C is less and fR increases.

40. Tuning diode L C MAX. REVERSE BIAS C is minimum and fR is maximum.

41. Diode applications quiz A circuit used to control the amplitude of a signal is the ________. clipper A circuit used to add a dc component to a signal is the ________. clamp A device containing an LED and a photo- diode is the ________. optocoupler A tuning diode shows less capacitance as reverse bias ________. increases The device that is often used to regulate voltage is the ________ diode. zener

42. Concept Review • Zener diodes have a relatively low breakdown voltage and are often used as voltage regulators. • Diode clippers limit signal amplitude. • Diode clamps produce a dc signal component. • Diodes can be used to suppress inductive kick. • Optocouplers combine light emitting diodes and photodiodes. • Fiber optic cables carry light signals. • Varicap diodes are used to tune electronic circuits. Repeat Segment

43. REVIEW • The PN Junction • Characteristic Curves of Diodes • Diode Lead Identification • Diode Types and Applications