1. EE2301: Basic Electronic Circuit Let’s start with diode EE2301: Block C Unit 11

2. EE2301: Basic Electronic Circuit Examples of Diode EE2301: Block C Unit 12

3. EE2301: Basic Electronic Circuit The Basic Property of a Diode Let’s have a demo EE2301: Block C Unit 13

4. EE2301: Basic Electronic Circuit How does it work? EE2301: Block C Unit 14

5. EE2301: Basic Electronic Circuit EE2301: Block C Unit 15 Block C Unit 1 Outline  Semiconductor materials (eg. silicon) > Intrinsic and extrinsic semiconductors  How a p-n junction works (basis of diodes)  Large signal models > Ideal diode model > Offset diode model  Finding the operating point  Application of diodes in rectification

6. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes6 Electrical Materials InsulatorsConductors Semi- Conductors

7. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes7 Semiconductor Applications Integrated Circuit

8. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes8 Semiconductor Applications TFT (Thin Film Transistor)

9. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes9 Intrinsic Semiconductor Si Covalent Bonds

10. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes10 Silicon Crystal Lattice In 3-D, this looks like: Number atoms per m 3 : ~ 10 28

11. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes11 Growing Silicon We can grow very pure silicon

12. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes12 Conduction

13. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes13 Currents in Semiconductor Source: http://hyperphysics.phy-astr.gsu.edu/HBASE/solids/intrin.html

14. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes14 Carrier Concentration The number of free electrons available for a given material is called the intrinsic concentration n i. For example, at room temperature, silicon has: n i = 1.5 x 10 16 electrons/m 3 1 free electron in about every 10 12 atoms

15. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes15 Doping: n-type 1 Si atom substituted by 1 P atom P has 5 valence electrons (1 electron more) 1 free electron created Si P - Electrically neutral

16. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes16 Doping: p-type Si B 1 Si atom substituted by 1 B atom + B has 3 valence electrons (1 electron short) 1 hole created Electrically neutral

17. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes17 p-n Junction

18. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes18 Diode Physics + + + + + + + + + - - - -- - -- - + + - - - - - + + + + + + + + + + + + - - - -- - -- - + + - -- + + - - - + +

19. EE2301: Basic Electronic Circuit Semiconductor Electronics - Unit 1: Diodes19 Diode Physics + + + + + + + + + - - - -- - -- - + + - - - - - + + + + + + + + + + + + - - - -- - -- - - + - - - - - + + + + + Website: http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/diode.html

20. EE2301: Basic Electronic Circuit EE2301: Block C Unit 120 Biasing and Conventions v D : Voltage of P (anode) relative to N (cathode) i D : Current flowing from anode to cathode

21. EE2301: Basic Electronic Circuit EE2301: Block C Unit 121 Diode Diode begins to conduct a significant amount of current: Voltage V γ is typically around 0.7V Diode equation: I D = I 0 [exp(eV D /kT) - 1]

22. EE2301: Basic Electronic Circuit EE2301: Block C Unit 122 Diode Symbol and Operation Forward-biased Current (Large) Reverse-biased Current (~Zero) + - Forward Biased: Diode conducts - + Reverse Biased: Little or no current iDiD

23. EE2301: Basic Electronic Circuit EE2301: Block C Unit 123 Real diode circuits +-+- +VL-+VL- + V D - IDID VTVT RTRT To find V L where V T and R T are known, First apply KVL around the loop: V T = V D + R T I D Then use the diode equation: I D = I 0 [exp(eV D /kT) - 1] At T = 300K, kT/e = 25mV We then need to solve these two simultaneous equations, which is not trivial. One alternative is to use the graphical method to find the value of I D and V D.

24. EE2301: Basic Electronic Circuit EE2301: Block C Unit 124 Graphical method Operating point is where the load line & I-V curve of the diode intersect Equation from KVL

25. EE2301: Basic Electronic Circuit EE2301: Block C Unit 125 Diode circuit models  Simplify analysis of diode circuits which can be otherwise difficult  Large-signal models: describe device behavior in the presence of relatively large voltages & currents > Ideal diode model > Off-set diode mode

26. EE2301: Basic Electronic Circuit EE2301: Block C Unit 126 Ideal diode model In other words, diode is treated like a switch here v D > 0: Short circuit v D < 0: Open circuit

27. EE2301: Basic Electronic Circuit EE2301: Block C Unit 127 Ideal diode model Circuit containing ideal diode Circuit assuming that the ideal diode conducts Circuit assuming that the ideal diode does not conduct

28. EE2301: Basic Electronic Circuit EE2301: Block C Unit 128 Ideal diode example 1 Problems 9.7 and 9.8 Determine whether the diode is conducting or not. Assume diode is ideal Repeat for V i = 12V and V B = 15V

29. EE2301: Basic Electronic Circuit EE2301: Block C Unit 129 Ideal diode example 1 solution This slide is meant to be blank Option 1: Assume diode is conducting and find the diode current direction Outcome 1: If diode current flows from anode to cathode, the assumption is true  Diode is forward biased Outcome 2: If diode current flows from cathode to anode, the assumption is false  Diode is reverse biased Option 2: Assume diode is not conducting and find the voltage drop across it Outcome 1: If voltage drops from cathode to anode, then the assumption is true  Diode is reverse biased Outcome 2: If voltage drops from anode to cathode, then the assumption is false  Diode is forward biased

30. EE2301: Basic Electronic Circuit EE2301: Block C Unit 130 Ideal diode example 1 solution This slide is meant to be blank Assume diode is conducting Forward-bias diode current (ie anode to cathode) = (10 - 12) / (5 + 10) = -2/15 A  Assumption was wrong  Diode is in reverse bias Assume diode is not-conducting Reverse-bias voltage (ie cathode referenced to anode) = 12 - 10 = 2V  Assumption was correct  Diode is in reverse bias

31. EE2301: Basic Electronic Circuit EE2301: Block C Unit 131 Ideal diode example 1 solution This slide is meant to be blank Assume diode is conducting Forward-bias diode current (ie anode to cathode) = (15 - 12) / (5 + 10) = 1/5 A  Assumption was correct  Diode is in forward bias Assume diode is not-conducting Reverse-bias voltage (ie cathode referenced to anode) = 12 - 15 = -3V  Assumption was incorrect  Diode is in forward bias

32. EE2301: Basic Electronic Circuit EE2301: Block C Unit 132 Ideal diode example 2 Problem 9.14 Find the range of V in for which D 1 is forward-biased. Assume diode is ideal The diode is ON as long as forward bias voltage is positive Now, minimum v in for v D to be positive = 2V

33. EE2301: Basic Electronic Circuit EE2301: Block C Unit 133 Offset diode model

34. EE2301: Basic Electronic Circuit EE2301: Block C Unit 134 Offset model example Problem 9.19 The diode in this circuit requires a minimum current of 1 mA to be above the knee of its characteristic. Use V γ = 0.7V What should be the value of R to establish 5 mA in the circuit? With the above value of R, what is the minimum value of E required to maintain a current above the knee

35. EE2301: Basic Electronic Circuit EE2301: Block C Unit 135 Offset model example solution This slide is meant to be blank I D = (E - V D )/R When the diode is conducting, V D = V γ I D = (5 - 0.7)/R We can observe that as R increases, I D will decrease To maintain a minimum current of 5mA, R max = 4.3/5 = 860 Ω Minimum E required to keep current above the knee (1mA), E min = (10 -3 * 860) + 0.7 = 1.56V

36. EE2301: Basic Electronic Circuit EE2301: Block C Unit 136 Rectification: from AC to DC Supply is ACDC required One common application of diodes is rectification. In rectification, an AC sinusoidal source is converted to a unidirectional output which is further filtered and regulated to give a steady DC output.

37. EE2301: Basic Electronic Circuit EE2301: Block C Unit 137 Rectifier with regulator diagram Rectifier Bi-directional input Steady DC output Filter Regulator Unidirectional output We will look at two types of rectifiers and apply the large signal models in our analysis: 1) Half wave rectifier 2) Full wave rectifier

38. EE2301: Basic Electronic Circuit EE2301: Block C Unit 138 Half-Wave Rectifier VSVS ~ RLRL On the positive cycle  Diode is forward biased  Diode conducts  V L will follow V S VSVS ~ RLRL On the negative cycle  Diode is reverse biased  Diode does not conduct  V L will remain at zero VSVS VLVL We can see that the circuit conducts for only half a cycle

39. EE2301: Basic Electronic Circuit EE2301: Block C Unit 139 Average voltage in a HW Rectifier 1 st half of period 2 nd half of period NB: This is equal to the DC term of the Fourier series

40. EE2301: Basic Electronic Circuit EE2301: Block C Unit 140 Full-Wave Rectifier  Also known as BRIDGE rectifier  Comprises 2 sets of diode pairs  Each pair conducts in turn on each half-cycle VSVS ~

41. EE2301: Basic Electronic Circuit EE2301: Block C Unit 141 Full-Wave Rectifier

42. EE2301: Basic Electronic Circuit EE2301: Block C Unit 142 Full-Wave Rectifier Half a period T/2 – time π – phase Repeats for every half a period: Integrate through half a period NB: This is equal to the DC term of the Fourier series

43. EE2301: Basic Electronic Circuit EE2301: Block C Unit 143 Full-Wave Rectifier (offset) V D-on (only one diode is on) 2V D-on (two diodes are on) With offset diodes With ideal diodes

44. EE2301: Basic Electronic Circuit EE2301: Block C Unit 144 Ripple filter Charging Discharging Anti-ripple filter is used to smoothen out the rectifier output

45. EE2301: Basic Electronic Circuit EE2301: Block C Unit 145 Ripple filter Approximation: abrupt change in the voltage From transient analysis: V M exp(-t/RC) VLVL Ripple voltage V r = V M - V L min

46. EE2301: Basic Electronic Circuit EE2301: Block C Unit 146 Ripple filter example Problem 9.40 Find the turns ratio of the transformer and the value of C given that: I L = 60mA, V L = 5V, V r = 5%, V line = 170cos(ωt) V, ω = 377rad/s Diodes are fabricated from silicon, V γ = 0.7V

47. EE2301: Basic Electronic Circuit EE2301: Block C Unit 147 Ripple filter example solution a) TURNS RATIO: To find the turns ratio, we need to find V S1 and V S2

48. EE2301: Basic Electronic Circuit EE2301: Block C Unit 148 Ripple filter example solution But V M is not equal to V S1 due to voltage drop across diodes So we now apply KVL on the secondary coil side: V S1 - V D - V M = 0 V S1 = 5.825 V (V D = 0.7V) Turns ratio, n = V line / V S1 ~ 29

49. EE2301: Basic Electronic Circuit EE2301: Block C Unit 149 Ripple filter example solution b) Value of C: Need to find the RC time constant associated with the ripple R L = V L /I L = 83.3 Ω We know it decays by V M exp(-t/RC), we now just need to know how long this lasts (t 2 ) V L-min = - V SO cos(ωt 2 ) - V D-on v so is negative at this point

50. EE2301: Basic Electronic Circuit EE2301: Block C Unit 150 Ripple filter example solution V L-min = - V SO cos(ωt 2 ) - V D-on 2 nd half of the sinusoid t 2 = (1/ω) cos -1 {-(V L-min + V D-on )/V SO } = 7.533 ms Decaying exponential: V L-min = V M exp(-t 2 /R L C)

51. EE2301: Basic Electronic Circuit Examples of Diode EE2301: Block C Unit 151

52. EE2301: Basic Electronic Circuit EE2301: Block C Unit 152

53. EE2301: Basic Electronic Circuit EE2301: Block C Unit 153 Electrical Materials Insulators Electrons are bound to the nucleus and are therefore not free to move With no free electrons, conduction cannot occur Conductors Sea of free electrons not bound to the atoms Ample availability of free electrons allows for electrical conduction Semiconductors Electrons are bound to the nucleus but vacancies are created due to thermal excitation Electrical conduction occurs through positive (called holes) and negative (electrons) charge carriers

54. EE2301: Basic Electronic Circuit EE2301: Block C Unit 154 Conduction in Semiconductors Silicon is the dominant semiconductor material used in the electronics industry. In a cubic meter of silicon, there are roughly 10 28 atoms. Among these 10 28, there will be about 1.5×10 16 vacancies at room temperature. This is known as the intrinsic carrier concentration: n = 1.5×10 16 electrons/m 3. This corresponds to 1 free electron for every 10 12 atoms. There will be same number of electrons as holes in intrinsic silicon since it is overall electrically neutral.

55. EE2301: Basic Electronic Circuit Extrinsic semiconductors EE2301: Block C Unit 155 A semiconductor material that has been subjected to the doping process is called an extrinsic material. Both n-type and p-type materials are formed by adding a predetermined number of impurity atoms to a silicon base. An n-type material is created by introducing impurity elements that have five valence electrons. In an n-type material, the electron is called the majority carrier. An p-type material is created by introducing impurity elements that have three valence electrons. In a p-type material, the hole is the majority carrier.

56. EE2301: Basic Electronic Circuit EE2301: Block C Unit 156 p-n Junction The pn junction forms the basis of the semiconductor diode Within the depletion region, no free carriers exist since the holes and electrons at the interface between the p-type and n-type recombine.

57. EE2301: Basic Electronic Circuit EE2301: Block C Unit 1 Response of the depletion region + + + - - - + + - - - - - + + + + + + - - - - + - - - - - + + + + + + + + - - - + + - -- + + - - - + + Forward biased: Voltage on the p-type side is higher than the n-type side Depletion width reduces, lowering barrier for majority carriers to move across the depletion region Large conduction current 57 Reverse biased: Voltage on the p-type side is lower than the n- type side Depletion width increases, increasing the barrier for majority carriers to move across the depletion region Very small leakage current

58. EE2301: Basic Electronic Circuit EE2301: Block C Unit 158 Analogy from tides Depletion region Forward Biased Reverse Biased