1. http://www.eet.bme.hu Budapest University of Technology and Economics Department of Electron Devices Microelectronics, BSc course Operation of PN junctions: Characteristics http://www.eet.bme.hu/~poppe/miel/en/05-diode2.ppt

2. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 2 Diode characteristics ► Ideal characteristic ► Secondary effects

3. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 3 The ideal diode characteristic

4. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 4 The ideal diode characteristic I 0 is proportional with the minority carrier concentrations I0I0

5. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 5 The ideal diode characteristic Problem Saturation current of Si diode: I 0 =10 -13 A. What is U F, if I F is 10 mA? Problem How much should we increase the forward voltage if we want to increase the current 10x ?

6. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 6 Secondary effects ► Series resistance ► Generation current ► Breakdown phenomena (a bit later) ► Recombination current (just mention)

7. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 7 Secondary effects The series resistance Appears at high current levels. Reason: Solution: epitaxial structure

8. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 8 Secondary effects Calculate the series resistance according to the 100 o C characteristic! Problem The series resistance

9. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 9 Secondary effects The generation current In reverse region, in theory: that would result in pA only The experince is:

10. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 10 Secondary effects The recombination current Phenomenon appearing in the forward region Can be well described by the Shockley-Read-Hall model for semiconductors with indirect band UFUF log I F ~ exp(U/U T ) ~ exp(U/2U T ) m: non-ideality factor, between 1..2

11. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 11 Capacitances of a diode ► Space charge capacitance ► Diffusion capacitance

12. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 12 Capacitances of the diode Space charge capacitance dominates in the reverse region Diffusion capacitance in forward region only Interpretation as a differential at a given forward voltage/current charge

13. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 13 charge Capacitances of the diode The space charge capacitance

14. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 14 electron injection injected electrons recombination access holes Capacitances of the diode The diffusion capacitance Where are the opposite charges?

15. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 15 Capacitances of the diode The diffusion capacitance Harmful! Slows down the operation. Reduction: decrease , narrow base diode

16. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 16 Capacitances of the diode Problem Let us calculate the space charge capacitance of a Si diode if the width of the depletion layer is 0.33  m and the cross-sectional area is 0.02 mm 2 ! Let us calculate the diffusion capacitance in the operating point of I=1 mA if  =100 ns!

17. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 17 Capacitances of the diode Orders of magnitude: C Sp 1-10 pF C D nF-s (for a small power diode) Utilization C Sp tuning oscillators, microwave amplification C D -- CDCD C Sp

18. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 18 Operating point ► Finding the DC operating point ► Linearization in the operating point, small signal operation ► Differential resistance, capacitance ► Models

19. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 19 ► Characteristics: defines the current-voltage pairs that may occur during the operation. ► During the real operation the diode or any nonlinear element works in one point of the characteristics, that is the operating point, or quiescent point. ► This is determined also by the surrounding elements. The operating point

20. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 20 Finding the operating point The problem: a linear element and a non-linear element connected in series: Graphical solution U+U+ I U U + /R UDUD URUR I=U R /R U+U+ R UDUD

21. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 21 Finding the operating point How does the operating point change if the U + supply voltage is increased? The operating line is shifted in parallel U+U+ I U U + /R UDUD URUR U+U+ R UDUD I=U R /R

22. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 22 Finding the operating point How does the operating line change if we change R? It turns around point U + - its slope will change U+U+ I U U+/RU+/R U+/R2U+/R2 U+/R1U+/R1 U+U+ R UDUD I=U R /R

23. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 23 Large signal model of the diode The computer simulation model also looks like this. Also needed: model equations (e.g. I=I 0 (exp(U/U T )-1) model parameters (e.g.. I 0, r s, etc.) C Sp (U)+C D (U)

24. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 24 linearize For small changes we can linearize the characteristics Linearization in the operating point U+U+ I U U + /R UDUD URUR I=U R /R U+U+ R UDUD

25. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 25 O Small signal operation of diodes The operating point IOIO IOIO I U u(t) u1u1 t i1i1 i1i1 i(t) t Small signal: linearized analysis, for the alternating current component What does small signal mean? r diff operating point dependent

26. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 26 Differential resistance of the diode Forward region, I >> I 0 : If we consider the series resistance as well:

27. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 27 Differential resistance of the diode Problem The series resistance of a diode is 2 . Let us calculate its differential resistance in the I=1 mA, 10 mA, 100 mA operating points!

28. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 28 Small signal model of the diode Element values are operating point dependent! C Sp +C D Recall:

29. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 29 Temperature dependence

30. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 30 Temperature dependence ► The characteristics shows strong temperature dependence ► Reason: temperature dependence of the minority carriers  Forward voltages: V F at I F decreases with about 2mV for 1  C increase linear temperature dependence in a large range  appropriate for temperature measurements  Reverse voltages: I R at U R decreases with  7-10% for 1  C (that means doubling at each 10  C)

31. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 31 Temperature dependence Forward region: Reverese region: For a Si diode: I R ~ n i   1,15  1,075  7,5 %/ o C

32. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 32 Temperature dependence Forward region: Problem If U=700 mV, what is dU/dT for a Si diode? Compare with the characteristics!

33. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 33 Temperature dependence IOIO UDUD I U IMIM UDUD UDUD cca. -2mV/ o C In case of a forced current the forward voltage of a pn junction is an excellent temperature sensor... The sensitivity slightly depeneds on the I O current

34. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 34 The diode in switching mode

35. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 35 Diodes as rectifiers The diode was considered to be ideal! What if this is not the case? u in u out U out (t)= 0, if U in (t)  0 U in (t), if U in (t) < 0

36. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 36 Diodes as rectifiers Relation of U in and U out u in u out U in U out ideal actual Transfer characteristic

37. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 37 Diodes as rectifiers diode characteristic u in u out piecewise approximation U in U out without U d compensation witht U d compensation

38. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 38 Transient phenomena Abrupt switching from forward to reverse voltage: due to its capacitances, the diode is open for some time. This is called reverse recovery.

39. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 39 Reverse recovery Reverse recovery time t rr (2-3 ns for a quick diode)

40. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 40 Transient behavior of the diode The diffusion equation: We calculate n(x,t) from this Simplification: instead of n(x,t) we calculate with Q(t) net charge

41. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 41 Transient behavior of the diode The charge equation The current is spent on maintaining recombination depleting/supplying diffusion charge

42. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 42 Break-down phenomena ► Avalanche ► Zener

43. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 43 Break-down phenomena Reason: either of Avalanche mechanism Zener punch- throug Voltage [V]

44. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 44 Avalanche break-down M – multiplication factor U L depends on the less doped side: Electrons Holes Ionization coefficient

45. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 45 The Zener effect Physical reason: tunnelling tunnelling current of electrons

46. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 46 Break-down phenomena In case of Si: below 6V – Zener, above this – avalanche. U L ~N -1 U L ~N -0.7 Comparison of the two phenomena Zener avalanche

47. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 47 Break-down phenomena Application: a Zener-diode Voltage reference Voltage stabilizer (at low power consumption) U in U out  U in  U out

48. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 48 Break-down phenomena Temeperature dependence of Zener-diodes The best: diodes around 5V (Si diode)

49. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 49 Practical issues ► Packaging ► Data sheets

50. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 50 Samall current Large current (IC realizations will be discussed later) Actual realization of diodes diode chip glass Si chip copper lead

51. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 51 Data sheets

52. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 52 Data sheets

53. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 53 Data sheets

54. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 54 0  100 o C 6.5  1200 nA (1200/6.5)^0.01=1.054 5.4 %/ o C Data sheets

55. Budapest University of Technology and Economics Department of Electron Devices 29-09-2009 Microelectronics BSc course, Operation of PN junctions: Characteristics © András Poppe & Vladimír Székely, BME-EET 2008 55 Data sheets