Semiconductor diode

Electronic Devices Unit I

1. Semiconductor Diode T.Ramprakash AP/ECE Ramco Institute of Technology 1

2. Semiconductor Diode PN junction diode Current equations Diffusion and Drift Current Densities Energy Band diagram Forward and Reverse bias characteristics Transition and Diffusion Capacitances Switching Characteristics Breakdown in PN Junction Diodes 2

3. Semiconductor Diode Introduction to Semiconductors PN junction diode Constructions Zero Bias - Built in Voltage Calculation and Depletion Width calculation Forward Bias Characteristics Reverse Bias Characteristics Current equations Diffusion and Drift Current Densities Energy Band diagram Transition and Diffusion Capacitances Switching Characteristics Breakdown in PN Junction Diodes 3

4. Semiconductor 4

5. Semiconductor A semiconductor is a material which has electrical conductivity to a degree between that of a metal and that of an insulator . Conductivity of Silicon  50 X 10 3 Ω –cm germanium  50 Ω -cm Semiconductors are the foundation of modern electronics including transistors , solar cells, light -emitting diodes (LEDs ), quantum dots, digital and analog integrated circuits 5

6. Silicon Vs Germanium Silicon (14) Germanium (32) 6

7. Classification of Semiconductor Intrinsic Semiconductor Extrinsic Semiconductor 7

8. Intrinsic Semiconductor A pure form of Semiconductor The concentration of electrons (n i ) in the conduction band = concentration of holes (p i ) in the valance band. ( n i = p i ) Conductivity is poor Eg . Pure Silicon, Pure Germanium (Tetravalent) 8

9. Extrinsic Semiconductor A Impure form of Semiconductor To increase the conductivity of intrinsic semiconductor, a small amount of impurity ( Pentavalent or Trivalent) is added. This process of adding impurity is known as Doping . 1 or 2 atoms of impurity for 10 6 intrinsic atoms . Electron concentration ≠ Hole concentration One type of carrier will predominate in an extrinsic semiconductor 9

10. Classification of Extrinsic Semiconductor N Type Semiconductor P Type Semiconductor 10

11. N Type Semiconductor A small amount of pentavalent impurities is added It is denoting one extra electron for conduction , so it is called donor impurity ( Donors ) + ve charged Ions Electron concentration > Hole Concentration Most commonly used dopants are Arsenic , Antimony and Phosphorus 11

12. P Type Semiconductor A small amount of trivalent impurities is added It accepts free electrons in the place of hole, so it is called Acceptor impurity ( Acceptors ) - ve charged Ions Hole concentration > Electron Concentration Most commonly used dopants are Aluminum , Boron , and Gallium 12

13. Mass Action Law Under thermal equilibrium the product of the f ree electron concentration and the free hole concentration is equal to a constant equal to the square of intrinsic carrier concentration . np = n i 2 13

14. Electrical Neutrality in Semiconductor Positive Charge Density p  Hole Concentration N D  Concentration of donor ions Negative Charge Density n  Electron Concentration N A  Concentration of Acceptor ions Total + ve charged density = Total – ve charged density p + N D = n + N A 14

15. Charge Density in a Semiconductor P Type Material N A > N D { N D ≈ 0} p p + N D = n p + N A N A = p p – n p { p p >> n p } N A = p p Mass action Law: n p p p = n i 2 N Type Material N D > N A {N A ≈ 0} p n + N D = n n + N A N D = n n – p n { n n >> p n } N D = n n Mass action Law: n n p n = n i 2 15

16. Conductivity of Semiconductor The resistivity of a semiconductor is The conductivity of a semiconductor is 16

17. Conductivity of Semiconductor The Resistivity of a semiconductor is The Conductivity of a semiconductor is q = 1.6*10 -19 coulomb 17

18. Problems Consider an Intrinsic Silicon bar of cross section 5 cm 2 and length 0.5 cm at room temperature 300 o K. An average field of 20 V/cm is applied across the ends of the silicon bar. Assume, Electron mobility = 1400 cm 2 /v-s Hole mobility = 450 cm 2 /v-s Intrinsic carrier concentration = 1.5*10 10 cm 3 Calculate, Electron hole component of current density Total current in the bar Resistivity of the Bar 18

19. PN Junction Diode anode cathode 19

20. Dopant distribution in PN Junction Diode 20

21. Dopant distribution in PN Junction Diode E n~0, and donor ions are exposed p~0, and acceptor ions are exposed Space Charge Region 21

22. 22

23. Calculation of Depletion Width 23

24. Calculation of Depletion Width 24

25. Calculation of Depletion Width Similarly, In N side region, Therefore, the total built in potential V bi W.K.T, Thermal Equilibrium, Substituting in the above equation, 25

26. Calculation of Depletion Width 26

27. PN Junction Forward bias 27

28. Forward bias Characteristics - V F ( ) 28

29. PN Junction Reverse bias 29

30. Reverse bias Characteristics + V R ( ) Reverse Saturation Current 30

31. Characteristics of PN Junction 31

32. Diode Current Equation The Diode equation relating the voltage V and current I is given by, 32

33. Diode Current Equation At x= 0 At Thermal Equilibrium At Forward Bias 33

34. Diode Current Equation 34

35. Diode Current Equation Considering Carrier generation and recombination, 35

36. Diode Current Equation At room temperature, V T = 25.8 mv Therefore, I = I o [e (40 V/ η ) -1] 36

37. Problems When a reverse bias is applied to a germanium PN junction diode, the reverse saturation current at room temperature is 0.3 micro amps. Determines the current flowing in the diode when 0.15 V forward bias is applied at room temperature. Given I = 0.3 X 10 -6 A V f = 0.15 V I = I o [e (40 V/ η ) -1] I = 0.3 * 10 -6 (e (40 * 0.15) -1) I = 0.120 mA 37

38. Problems The reverse saturation current of a silicon PN junction diode is 10 micro A. Calcualte the diode current for the forward bias voltage 0.6 V at 25 o C V T = T/ 11,600 = (273 + 25)/11600 = 0.0 257 V I = 1.174 A 38

39. Drift Current Density A/cm 2 A/cm 2 The flow of electric current due to the motion of the charge carriers under The influence of an external electric field is called Drift Current 39

40. Diffusion Current Density In a semiconductor material , the charge carriers have the tendency to move from the region of the higher concentration to that of lower concentration of the same type of charge carriers. This movement results in a current called Diffusion Current 40

41. Diffusion Current Density Concentration Gradients Diffusion Coefficients 41

42. Total Current Total Current in P type semiconductor J p = J p Drift + J p Diffusion Total Current in N type semiconductor J n = J n Drift + J n Diffusion 42

43. Energy Band Diagram – Intrinsic Semiconductor E C E V E fi E g n i = N C e [-( E c – E fi )/KT) 43

44. Energy Band Diagram – N type and P type 44

45. qV bi E 1 E 2 E = p n Energy Band Diagram V bi = |E 1 | + |E 2 | E 1 = E Fi - E Fp E 2 = E Fn - E Fi 45

46. Energy Band Diagram 46

47. Energy Band Diagram 47

48. Energy Band Diagram V bi = |E 1 | + |E 2 | 48

49. Energy Band Diagram – Forward Bias 49

50. Energy Band Diagram – Reverse Bias 50

51. Transition Capacitance The parallel layers of oppositely charged immobile ions on the two side of the junction form the capacitance C T C T is Transition or Space charge or depletion region capacitance. The Total Charge density of a p type material with area of the junction A is given by, Q = q N A W A Where dQ is the increase in charge and dV is the change in voltage. 51

52. Transition Capacitance 52

53. Transition Capacitance The relation between potential and charge density is given by the Poisson's equation. At x = W p , V = V B, q 53

54. Transition Capacitance Differentiating w.r.to V, we get . q q A q N A = A q N A q 54

55. Transition Capacitance C T = 20 pF // zero bias C T = 5 to 200 pF 55

56. Diffusion Capacitance The capacitance that exists in a forward biased junction is called a diffusion or storage capacitance (C D ) C D >> C T Where dQ represents change in the number of minority carrier when change in voltage. 56

57. Diffusion Capacitance Diffusion hole current in the N side I pn (x) At, x = 0 I pn (0) ≈ I 57

58. Diffusion Capacitance Diff. w.r.to V 58

59. Diffusion Capacitance C D increases for forward bias . C D is negligible for reverse bias. C D ranges from 10 pF to 1000pF C D is high for low frequency C D is low for high frequency 59

60. Switching Characteristics Recovery time Forward Recovery Time Reverse Recover Time 60

61. Switching Characteristics V F /R L V R /R L I O 61

62. Switching Characteristics When the applied voltage to the PN junction diode is suddenly reversed in the opposite direction, the diode response reaches a steady state after an interval of time . This is called recover time . The forward recovery time t fr , is defined as the time required for forward voltage or current to reach a specified value after switching diode from its reverse to forward biased state Forward recovery time posses no serious problem 62

63. Switching Characteristics When the PN junction diode is forward biased, the minority electron concentration in the P region is approximately linear. If the junction is suddenly reverse biased, at t1, then because of this stored electronic charge , the reverse current I R is initially of the same magnitude as the forward current The injected minority carrier have remained stored and have to reach the equilibrium state , this is called storage time (t s ) The time required for the diode for nominal recovery to reach its steady state is called transition time ( t t ) t RR = t s + t t 63

64. Switching Characteristics For commercial switching type diodes the reverse recovery time t rr ranges from less than 1 ns to as high as 1 µs . The operating frequency should be a minimum of approximately 10 times t rr . If a diode has trr of 2ns , the maximum operating frequency is f max = 1/T  1/(10*2*10 -9 )  50 MHz 64

65. Break down in PN Junction Diodes 65

66. Avalanche Break Down Thermally generated minority carriers cross the depletion region and acquire sufficient kinetic energy from the applied potential to produce new carrier by removing valence electrons from their bonds. These new carrier will in turn collide with other atoms and will increase the number of electrons and holes available for conduction. The multiplication effect of free carrier may represented by 66

67. Zener Break down Zener breakdown occurs in highly doped PN junction through tunneling mechanism In a highly doped junction, the conduction and valance bands on opposite sides of the junction are sufficiently close during reverse bias that electrons may tunnel directly from the valence band of the P side into the conduction band on the n side 67

68. Diode Ratings Maximum Forward Current Highest instantaneous current under forward bias condition that can flow through the junction. Peak Inverse Voltage (PIV) Maximum reverse voltage that can be applied to the PN junction If the voltage across the junction exceeds PIV, under reverse bias condition, the junction gets damaged. (1000 V) Maximum Power Rating Maximum power that can be dissipated at the junction without damaging the junction. It is the product of voltage across the junction and current through the junction. 68

69. Diode Ratings Maximum Average Forward Current Maximum amount of average current that can be permitted to flow in the forward direction at a special temperature (25 o C) Repetitive Peak Forward Current Maximum peak current that can be permitted to flow in the forward direction in the form of recurring pulses . Limiting value of the current is 450 mA Maximum Surge Current Maximum current permitted to flow in the forward direction in the form of nonrecurring pulses . It should not be more that a few milliseconds. (30 A for 8.3 ms ) 69

70. Diode Ratings D.C. or Static Resistance (R F ) It is defined as the ratio of the voltage to the current (V/I) in the forward bias characteristics of PN junction Diode R F = V / I A.C. or Dynamic resistance (r f ) It is defined as the reciprocal of the slope of the volt-ampere characteristics r f = change in voltage / resulting change in current r f = ∆V/ ∆ I 70

71. Reference Donald A Neaman , “Semiconductor Physics and Devices”, Fourth Edition, Tata Mc GrawHill Inc.2012. Salivahanan . S, Suresh Kumar. N, Vallavaraj.A , “Electronic Devices and circuits”, Third Edition, Tata Mc Graw - Hill Inc.2008.