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Chapter 3 Solid-State Diodes and Diode Circuits

Chapter 3 Solid-State Diodes and Diode Circuits. Microelectronic Circuit Design Richard C. Jaeger Travis N. Blalock Modified by Ming Ouhyoung. Chap 3 - 1. Chapter Goals. Understand diode structure and basic layout Develop electrostatics of the pn junction

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Chapter 3 Solid-State Diodes and Diode Circuits

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  1. Chapter 3Solid-State Diodes and Diode Circuits Microelectronic Circuit Design Richard C. JaegerTravis N. Blalock Modified by Ming Ouhyoung Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -1

  2. Chapter Goals • Understand diode structure and basic layout • Develop electrostatics of the pn junction • Explore various diode models including the mathematical model, the ideal diode model, and the constant voltage drop model • Understand the SPICE representation and model parameters for the diode • Define regions of operation of the diode (forward bias, reverse bias, and reverse breakdown) • Apply the various types of models in circuit analysis • Explore different types of diodes • Discuss the dynamic switching behavior of the pn junction diode • Explore diode applications • Practice simulating diode circuits using SPICE Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -2

  3. Diode Introduction • A diode is formed by joining an n-type semiconductor with a p-type semiconductor. • A pn junction is the interface between n and p regions. Diode symbol Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -3

  4. metallurgical junction • plane in the p-n junction at which concentration of acceptors is the same as concentration of donors. Microelectronic Circuit Design, 4E McGraw-Hill

  5. pn Junction Electrostatics Donor and acceptor concentration on either side of the junction. Concentration gradients give rise to diffusion currents. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -5

  6. Diffusion • Diffusion from a microscopic and macroscopic point of view. Initially, there are solute molecules on the left side of a barrier (purple line) and none on the right. The barrier is removed, and the solute diffuses to fill the whole container. Top: A single molecule moves around randomly. • Middle: With more molecules, there is a clear trend where the solute fills the container more and more uniformly. Bottom: With an enormous number of solute molecules, all apparent randomness is gone: The solute appears to move smoothly and systematically from high-concentration areas to low-concentration areas following Fick’s laws. Microelectronic Circuit Design, 4E McGraw-Hill

  7. Microelectronic Circuit Design, 4E McGraw-Hill

  8. Drift Currents • Diffusion currents lead to localized charge density variations near the pn junction. • Gauss’ law predicts an electric field due to the charge distribution: • Assuming constant permittivity, • Resulting electric field gives rise to a drift current. With no external circuit connections, drift and diffusion currents cancel. There is no actual current, since this would imply power dissipation, rather the electric field cancels the diffusion current ‘tendency.’ Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -8

  9. Space-Charge Region Formation at the pn Junction Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -9

  10. Potential Across the Junction Charge Density Electric Field Potential Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -10

  11. Width of Depletion Region (Example) Problem: Find built-in potential and depletion-region width for given diode Given data: On p-type side: NA = 1017/cm3 on n-type side: ND = 1020/cm3 Assumptions: Room-temperature operation with VT = 0.025 V Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -11

  12. εs = 11.7 εo, where εo = 8.85*10-14 F/cm q = 1.60*10-19 C Microelectronic Circuit Design, 4E McGraw-Hill

  13. Internal Diode Currents Mathematically, for a diode with no external connections, the total current expressions developed in Chapter 2 are equal to zero. The equations only dictate that the total currents are zero. However, as mentioned earlier, since there is no power dissipation, we must assume that the field and diffusion current tendencies cancel and the actual currents are zero. When external bias voltage is applied to the diode, the above equations are no longer equal to zero. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -13

  14. 3.2 The i-v characteristics of the diode Microelectronic Circuit Design, 4E McGraw-Hill

  15. Diode Junction Potential for Different Applied Voltages Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -15

  16. Diode i-v Characteristics The turn-on voltage marks the point of significant current flow. Is is called the reverse saturation current. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -16

  17. 3.3 The Diode Equation where IS = reverse saturation current (A) vD = voltage applied to diode (V)q = electronic charge (1.60 x 10-19 C)k = Boltzmann’s constant (1.38 x 10-23 J/K)T = absolute temperaturen = nonideality factor (dimensionless)VT = kT/q = thermal voltage (V) (25 mV at room temp.) IS is typically between 10-18 and 10-9 A, and is strongly temperature dependent due to its dependence on ni2. The nonideality factor is typically close to 1, but approaches 2 for devices with high current densities. It is assumed to be 1 in this text. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -17

  18. IS, the reverse bias saturation current for an ideal p-n diode is given by, (Schubert 2006, 61) • where • IS is the reverse bias saturation current, • e is elementary charge • A is the cross-sectional area • Dp,n are the diffusion coefficients of holes and electrons, respectively, • ND,A are the donor and acceptor concentrations at the n side and p side, respectively, • ni is the intrinsic carrier concentration in the semiconductor material, • τp,n are the carrier lifetimes of holes and electrons, respectively. Microelectronic Circuit Design, 4E McGraw-Hill

  19. Diode Voltage and Current Calculations (Example) Problem: Find diode voltage for diode with given specifications Given data: IS = 0.1 fA, ID = 300 mA Assumptions: Room-temperature dc operation with VT = 0.025 V Analysis: WithIS = 0.1 fA With IS = 10 fA With ID = 1 mA, IS = 0.1 fA Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -19

  20. 3.4 Diode characteristics under Reverse, Zero, and Forward bias Microelectronic Circuit Design, 4E McGraw-Hill

  21. Diode Current for Reverse, Zero, and Forward Bias • Reverse bias: • Zero bias: • Forward bias: Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -21

  22. Semi-log Plot of Forward Diode Current and Current for Three Different Values of IS Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -22

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  26. 3.5 Diode Temperature Coefficient Diode voltage under forward bias: Taking the derivative with respect to temperature yields Assuming iD >> IS, IS ni2, and VGO is the silicon bandgap energy at 0K. For a typical silicon diode Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -26

  27. The Nobel Prize in Chemistry 2011 is awarded to Dan Shechtman for the discovery of quasicrystals. • The achievement of Dan Shechtman is clearly not only the discovery of quasicrystals, but the realization of the importance of this result and the determination to communicate it to a skeptical scientific community. • 盡信書，不如無書 Microelectronic Circuit Design, 4E McGraw-Hill

  28. Microelectronic Circuit Design, 4E McGraw-Hill

  29. To make the discovery even more astounding, Shechtman found that by rotating the sample he could identify additional 5-fold axes as well as 3-fold and 2-fold. It became clear that the symmetry of his sample was not merely 5-fold but icosahedral (figure 4). Microelectronic Circuit Design, 4E McGraw-Hill

  30. Figure 4. Original electron diffraction images taken by Dan Shechtman. The angular relationship between the variouszones examined by Shechtman shows that the sample exhibits icosahedral symmetry6. Microelectronic Circuit Design, 4E McGraw-Hill

  31. Microelectronic Circuit Design, 4E McGraw-Hill

  32. Figure 7. Pentagonal Penrose tiling. Microelectronic Circuit Design, 4E McGraw-Hill

  33. An important discovery that helped pave the way for the understanding of the discovery ofquasicrystals was the construction and analysis by Penrose of his famous pentagonal tiling.The pentagonal Penrose tiling (figure 7) is a self-similar pattern with 5-fold symmetry, long-rangeorder and no translational periodicity. de Bruijn applied the higher dimensional approachto Penrose tiles, and Mackay subjected an image of the vertices of a Penrose tiling to opticaldiffraction and was able to show that it has a discrete diffraction diagram. These early resultswere certainly instrumental in allowing Levine and Steinhardt to accomplish their remarkableearly analysis (and resulting paper) on quasicrystals, and helped establish the credibility of thediscovery. Microelectronic Circuit Design, 4E McGraw-Hill

  34. Microelectronic Circuit Design, 4E McGraw-Hill

  35. Properties of quasicrystals • Intermetallic quasicrystals are typically hard and brittle materials with unusual transport properties and very low surface energies. Thermal and electronic transport in solid materials is normally enhanced by phonons and Bloch waves that develop as a consequence of the periodic • nature of crystals. • The low surface energy of quasicrystals make them corrosion- and adhesion-resistant and imparts them with low friction coefficients. Microelectronic Circuit Design, 4E McGraw-Hill

  36. Surface energy • Surface energy quantifies the disruption of intermolecular bonds that occur when a surface is created. In the physics of solids, surfaces must be intrinsically less energetically favorable than the bulk of a material, otherwise there would be a driving force for surfaces to be created, removing the bulk of the material. • The surface energy may therefore be defined as the excess energy at the surface of a material compared to the bulk. Microelectronic Circuit Design, 4E McGraw-Hill

  37. Reverse Bias External reverse bias adds to the built-in potential of the pn junction. The shaded regions below illustrate the increase in the characteristics of the space charge region due to an externally applied reverse bias, vD. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -37

  38. Reverse Bias (cont.) External reverse bias also increases the width of the depletion region since the larger electric field must be supported by additional charge. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -38

  39. Reverse Bias Saturation Current We earlier assumed that the reverse saturation current was constant. Since it results from thermal generation of electron-hole pairs in the depletion region, it is dependent on the volume of the space charge region. It can be shown that the reverse saturation gradually increases with increased reverse bias. IS is approximately constant at IS0 under forward bias. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -39

  40. Reverse Breakdown Increased reverse bias eventually results in the diode entering the breakdown region, resulting in a sharp increase in the diode current. The voltage at which this occurs is the breakdown voltage, VZ. 2 V < VZ< 2000 V Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -40

  41. Reverse Breakdown Mechanisms • Avalanche BreakdownSi diodes with VZ greater than about 5.6 volts breakdown according to an avalanche mechanism. As the electric field increases, accelerated carriers begin to collide with fixed atoms. As the reverse bias increases, the energy of the accelerated carriers increases, eventually leading to ionization of the impacted ions. The new carriers also accelerate and ionize other atoms. This process feeds on itself and leads to avalanche breakdown. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -41

  42. Reverse Breakdown Mechanisms (cont.) • Zener BreakdownZener breakdown occurs in heavily doped diodes. The heavy doping results in a very narrow depletion region at the diode junction. Reverse bias leads to carriers with sufficient energy to tunnel directly between conduction and valence bands moving across the junction. Once the tunneling threshold is reached, additional reverse bias leads to a rapidly increasing reverse current. • Breakdown Voltage Temperature CoefficientTemperature coefficient is a quick way to distinguish breakdown mechanisms. Avalanche breakdown voltage increases with temperature, whereas Zener breakdown decreases with temperature. For silicon diodes, zero temperature coefficient is achieved at approximately 5.6 V. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -42

  43. Breakdown Region Diode Model In breakdown, the diode is modeled with a voltage source, VZ, and a series resistance, RZ. RZ models the slope of the i-v characteristic. Diodes designed to operate in reverse breakdown are called Zener diodes and use the indicated symbol. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -43

  44. Reverse Bias Capacitance Changes in voltage lead to changes in depletion width and charge. This leads to a capacitance that we can calculate from the charge-voltage dependence. Cj0 is the zero bias junction capacitance per unit area. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -44

  45. Reverse Bias Capacitance (cont.) Diodes can be designed with hyper-abrupt doping profiles that optimize the reverse-biased diode as a voltage controlled capacitor. Circuit symbol for the variable capacitance diode (Varactor) Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -45

  46. Forward Bias Capacitance In forward bias operation, additional charge is stored in neutral region near edges of space charge region. tTis called diode transit time and ranges from 10-15 to more than 10-6 s depending on size and type of diode. Additional diffusion capacitance, associated with forward region of operation is proportional to current and becomes quite large at high currents. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -46

  47. 3.8 Schottky Barrier Diode One semiconductor region of the pn junction diode can be replaced by a non-ohmic rectifying metal contact. A Schottky contact is easily formed on n-type silicon. The metal region becomes the anode. An n+ region is added to ensure that the cathode contact is ohmic. Schottky diodes turn on at a lower voltage than pn junction diodes and have significantly reduced internal charge storage under forward bias. Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -47

  48. 3.9 Diode Spice Model Rs represents the inevitable series resistance of a real device structure. The current controlled current source models the ideal exponential behavior of the diode. Capacitor C includes depletion-layer capacitance for the reverse-bias region as well as diffusion capacitance associated with the junction under forward bias. Typical default values: Saturation current IS = 10 fA, Rs = 0 W, transit time TT = 0 seconds, N = 1 Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -48

  49. Microelectronic Circuit Design, 4E McGraw-Hill

  50. Diode Layout Microelectronic Circuit Design, 4E McGraw-Hill Chap 3 -50

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