1. EENG 3510 Chapter 3
Diodes

2. Chapter 3 Homework
3.2 (c & d), 3.3 , 3.9, 3.19, 3.23

3. 3.1.1 Current-Voltage Characteristic
diode circuit symbol
i–v characteristic
equivalent circuit in the reverse direction
equivalent circuit in the forward direction

4. 3.1.1 Current-Voltage Characteristic
an external circuit to limit the forward current
the reverse voltage

5. 3.1.2 A Simple Application: The Rectifier

6. 3.1.3 Another Application: Diode Logic Gates (In a positive-logic system)
AND gate (in a positive-logic system)
OR gate
Y = A + B + C
Y = ABC

7. Example 3.2 Find values of I and V

8. Example 3.2a Find values of I and V
1. Both diodes are conducting.
Voltage at B is zero.
ID2 = 10 V -0 V / 10 k = 1 mA
I + ID2 = (0 – (-10) V ) / 5 k = 2 mA
I + 1 = 2mA
I = 1 mA
5. V = 0

9. Example 3.2b Find values of I and V
Assume both diodes are conducting.
VB = 0
ID2 = (10 V – 0 V) / 5k = 2 mA
I + 2 = (0 – (-10)) V / 10 k
I = - 1 A This is not correct.
Assume D1 is off and D2 is on.
ID2 = (10 V – (-10 V) )/ 15k = 1.33 mA
V = VB = -10 V + (10 k X 1.33 mA)
V = -10 V V = 3.3 V

10. Exercise 3.4a - Find: I & V
I = 5 V / 2.5 K = 2 mA V
V = 0, Why ?

11. Exercise 3.4b - Find: I & V
I = 0 A, Why? V
V = 5 V, Why ?

12. Exercise 3.4c - Find: I & V V
I = 0 A, Why?
V = 5 V, Why ?

13. Exercise 3.4d - Find: I & V V
I = 5 V / 2.5 K = 2 mA
V = 0, Why ?

14. Exercise 3.4e - Find: I & V
I = 3 V / 1 K = 3 mA
V = 3 V, Why ?

15. Exercise 3.4f - Find: I & V
I = 4 V / 1 K = 4mA
V = 1 V, Why ?

16. 3.2 Terminal Characteristics of Junction Diodes

17. 3.2 Terminal Characteristics of Junction Diodes

18. 3.2.1 The Forward-Bias Region
Is: Saturation current, in the order of 10-15A, doubles in value for every 5°C rise in temperature
n = 1, 2: material and physical structure
V = forward voltage

19. 3.2.1 The Forward-Bias Region (cont.)
Silicon diodes conduct when the forward voltage = 0.7 volts
Germanium diodes conduct when the forward voltage = 0.3volts

20. Example
Given: A forward biased diode, forward voltage drop is 0.7 V at 2 mA, n = 1 at 0.6 V Find : the current i2

21. 3.2.2 The Reverse-Bias Region
If |v|>> |VT|(25mV)
i≅-Is (Saturation current)

22. 3.2.3 The Breakdown Region
If the power dissipated in the diode is limited to a “safe” level, the breakdown is normally not destructive
VZK: Z →Zener, K →Knee

23. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics The Exponential Model
Graphical Analysis

24. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics Iterative Analysis Using theExponential Model

25. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics The Piecewise-Linear Model

26. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics The Piecewise-Linear Model (cont.)
Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation

27. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics The Constant-Voltage-Drop Model
Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (B) is used to approximate the fast-rising exponential. Observe that this simple model predicts VD to within 0.1 V over the current range of 0.1 mA to 10 mA.

28. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics The Constant-Voltage-Drop Model
The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation.

29. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics Use of the Diode Forward Drop in Voltage Regulation
A voltage regulator is a circuit whose purpose is to provide a constant dc voltage between its output terminals
The output voltage is required to remain as constant as possible in spite of
Changes in the load current drawn from the regulator output terminal
Changes in the dc power-supply voltage that feeds the regulator circuit

30. 3. 3 Modeling the Diode Forward Characteristics 3. 3
3.3 Modeling the Diode Forward Characteristics Use of the Diode Forward Drop in Voltage Regulation

31. 3. 4 Operation in the Reverse Breakdown Region –Zener Diodes 3. 4
3.4 Operation in the Reverse Breakdown Region –Zener Diodes Specifying and Modeling the Zener Diode
Circuit symbol for a zener diode.

32. 3. 4 Operation in the Reverse Breakdown Region –Zener Diodes 3. 4
3.4 Operation in the Reverse Breakdown Region –Zener Diodes A Final Remark
In recent years, zener diodes are replaced in voltage-regulator design by specially designed integrated circuits (ICs) that perform the voltage regulation function much more effectively and with greater flexibility than zener diodes.

33. 3.5 Rectifier Circuits 120(N2/N1) V Coils wound around an iron core
Remove pulsation
Remove ripple

34. 3.5.1 The Half-Wave Rectifier
Transfer characteristic of the rectifier circuit
Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model.
Input and output waveforms, assuming that rD ! R.

35. 3.5.1 The Half-Wave Rectifier (cont.)
Two important parameters:
1) Current-handling capability: the largest current the diode
is expected to conduct
2) Peak inverse voltage (PIV): the diode must be able to
withstand without break

36. 3.5.2 The Full-Wave Rectifier
Full-wave rectifier utilizing a transformer with a center-tapped secondary winding
transfer characteristic assuming a constant-voltage-drop model for the diodes;
PIV = 2Vs-VD
input and output waveforms

37. 3.5.3 The Bridge Rectifier
Most Popular Rectifier Circuit Configuration
The bridge rectifier
input and output waveforms

38. 3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier
A simple circuit used to illustrate the effect of a filter capacitor.
Note that the circuit provides a dc voltage equal to the peak of the input sine wave. The circuit is therefore known as a peak rectifier or a peak detector.
Input and output waveforms assuming an ideal diode.

39. 3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier

40. 3.5.4 The Rectifier with a Filter Capacitor The Peak Rectifier
Waveforms in the full-wave peak rectifier

41. 3.7.1 Basic Semiconductor Concepts
Simplified physical structure of the junction diode.
(Actual geometries are given in Appendix A.)

42. 3.7.1 Basic Semiconductor Concepts (cont.)
Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

43. 3.7.1 Basic Semiconductor Concepts (cont.)
At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

44. 3.7.1 Basic Semiconductor Concepts (cont.)
The concentration of free electrons n, and the concentration of holes p

45. 3.7.1 Basic Semiconductor Concepts (cont.)
Diffusion: electrons (holes) will diffuse from the region of high concentration to the region of low concentration
Drift:when an electric field is applied across a piece of silicon, free electrons and holes are accelerated by the electric field E. The positively charged holeswill drift in the direction of E, while the negatively charged electronswill drift in a direction oppositeto that of electric field.

46. 3.7.1 Basic Semiconductor Concepts (cont.)
Doping of a silicon crystal to turn it into n type or p type is achieved by introducing a small number of impurity atoms.
Ex: phosphorus
A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

47. 3.7.1 Basic Semiconductor Concepts (cont.)
Doping of a silicon crystal to turn it into n type or p type is achieved by introducing a small number of impurity atoms.
Ex: boron
A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

48. 3.7.2 The pn Junction Under Open-Circuit Conditions
Equilibium: Is= ID,
Maintained by the barrier voltage V0
(a) The pn junction with no applied voltage (open-circuited terminals). (b) The potential distribution along an axis perpendicular to the junction.

49. 3.7.2 The pn Junction Under Open-Circuit Conditions
Barrier Voltage:

50. 3.7.2 The pn Junction Under Open-Circuit Conditions
Width of the Depletion Region

51. 3.7.3 The pn Junction Under Reverse-Bias Conditions
Anode
Cathode
Depletion width: increases
Barrier voltage v0: increase
I = IS-ID
The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than IS. Note that the depletion layer widens and the barrier voltage increases by VR volts, which appears between the terminals as a reverse voltage.

52. 3.7.3 The pn Junction Under Reverse-Bias Conditions
Anode
Cathode
Depletion width: decrease
Barrier voltage v0: decrease
I = ID -IS
The pn junction excited by a constant-current source supplying a current I in the forward direction. The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.