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Lecture 17
 

Lecture 17

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    Lecture 17 Lecture 17 Presentation Transcript

    • Today’s Objectives Dielectrics and Ferroelectrics
      • What happens to a material when an electric field is applied across it (i.e., in a capacitor)? How does the electric field change, and how does the charge/area change?
      • What are the 3 primary contributions to the dielectric constant.
      • How fast would a dielectric respond?
      • Describe the 4 primary dielectric breakdown mechanisms.
      • How can the breakdown strength be improved?
      • What are ferroelectrics?
      • What are other applications of ferroelectrics?
    • Capacitance
      • Two electrodes separated by a gap define a capacitor.
      • When a bias is applied across the capacitor plates, one charges positively, the other negatively.
      • The amount of charge that the capacitor can store (Q) is proportional to the bias (V) times how good the capacitor is, the ‘capacitance’ (C).
      • The capacitance is related to the area of the plates (A), their separation (d), and the Dielectric Constant ( εε o ) of the dielectric between the plates
      • Dielectric constant of vacuum; ε o = 8.85x10 -12 F/m=55.2 Me/(V*m)
    • Why does charge built up?
      • There is generally not a built-in electric field between the plates of an unbiased capacitor.
      • When an electric field is applied, any charged carriers or species within the material will respond.
      • For a conductor or semiconductor, e - will flow to the + plate, and possibly also holes will flow to the - plate. Current is carried=no charge buildup.
      • For an insulator, there aren’t a significant number of free carriers. There are highly ionic species, however, but they aren’t very mobile at low temperatures. No appreciable current is carried=charge buildup.
    • Polarization in Insulators Positively charged species in insulators shift/rotate/align toward the negative electrode and negatively charged species shift/rotate/align towards the positive electrode; creating dipoles. The dipole moment density is termed the Polarization (P) and has the units of C/m 2 . + - Electron Cloud Electron Cloud + E Electronic polarization, occurs in all insulators - + + + - + + E Ionic polarization occurs in all ionic solids: NaCl, MgO… - - - - + - - + + - + + E Molecular polarization, occurs in all insulating molecules; oils, polymers, H 2 O… Electric Dipole Moment Polarization
    • Frequency Response (Switching Time) Electronic Polarization Ionic Polarization Molecular Polarization
    • Microwave Ovens
      • A microwave oven generates electromagnetic radiation at about 2.5 GHz. This energy is pretty good at causing H 2 O molecules to oscillate their orientation (orientational dielectric constant changes greatly).
      • 5 GHz - 100 GHz would be ideal, but then most of the energy would be absorbed by the outermost layer of the food, defeating the purpose.
      • Ice has a low dielectric constant, so not much energy is absorbed by it. Once there is a bit of melted ice, though, then you are really cooking.
      http://home.howstuffworks.com/framed.htm?parent=micriwave.htm&url=http://www.amasci.com/weird/microexp.html
    • Relative Dielectric Constants
      • Generally, the less conducting and more polar a material is, the greater will be its dielectric constant.
    • A Materials/Design Problem How can we increase the charge stored in a parallel-plate capacitor? This is an extremely important problem in solid state computer memories (RAMs, DRAMs, SDRAMs) that are based on capacitors.
      • Use a material with a higher dielectric constant ( ε ) , limited by material properties (see table next page).
      • Increase capacitor area (A) , limited by how much space you have on the IC/device. But, one can always increase the projected lateral area!!! This is a design problem.
      • Decrease plate spacing d . Limited by dielectric breakdown as the electric field across the plate increases with d .
      • Fast Read/Write speeds (typically GHz) limits the material that can be used (ionic/electronic polarization, SiO 2 , Si 3 N 4 , TiO 2 , HfO 2 …).
    • Breakdown Strength
      • You cannot charge a capacitor infinitely. Eventually, the capacitor will fail, usually catastrophically.
      • The so-called breakdown strength of a dielectric is the electric field greater than which the material breaks down.
      • The breakdown strength is separate from the dielectric properties of the material.
      • High purity, low defect densities, and low temperature are important.
      > 10 000 SiO 2 in ICs > 1 000 000 Thin films in ICs 50...900 Polymers 1800 Oiled paper 200...700 Mica 200...400 Glass, ceramics 200 Oil Critical Field Strength (kV/cm) Material
    • Dielectric Breakdown Mechanisms
      • Thermal (heat=defects=ionic conduction=more heat=…)
      • Avalanche (accelerated electrons free more electrons that accelerate and free more electrons…)
      • Discharge (fields grow enough to arc across pores, leading to erosion, leading to more arcing, …)
      • Electrolytic (conduction paths created over time due to ionic and/or environmental conduction)
    • Slight Ionic Asymmetry - Ferroelectrics
      • Properties
      • Spontaneous polarization in the absence applied electrical field.
      • Extremely high dielectric constant (~500-15,000).
      • Strong non-linear dielectric response to an applied electrical field.
      • High strain response to applied electrical field  piezoelectricity
      • Strong variation in polarization with temperature  pyroelectricity
      • Typical Perovskite Ferroelectrics
      • Pb(Zr,Ti)O 3 -PZT
      • Ba(Sr,Ti)O 3 -BST
      • KNbO 3 and LiNbO 3
      • Pb(Ca,Ti)O 3 -PCT
      • Pb(Sr,Ti)O 3 –PST
      • Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3
      Perovskite Structure
    • Ferroelectrics
    • Spontaneous Polarization
    • Electrostriction and Piezoelectricity FEs possess a spontaneous strain. This is called electrostriction . The FE crystal can be deformed by the application of an electric field or it generates a potential when there is an applied stress. This is called piezoelectricity.
    • Pyroelectricity The spontaneous polarization is strongly dependent on the temperature. It dissapears completely at the phase transformation temperature T C . The variation in the polarization with respect to the temperature is called the pyroelectric effect .
      • Non-Volatile RAMs (memory)
      • Dynamic RAMs (capacitors)
      • Tunable Microwave Devices
      • Pyroelectric Detectors/Sensors
      • Optical Waveguides
      • Piezoelectric Sensors/Actuators, MEMS
      Applications of Ferroelectrics
    • Non-Volatile RAMs (memory)
    • Non-Volatile RAMs (memory) Smart cards use ferroelectric memories. They can hold relatively large amounts of information and do not wear out from use, as magnetic strips do, because they use contactless radio frequency input/output. These cards are the size and shape of credit cards but contain ferroelectric memory that can carry substantial information, such as its bearer's medical history for use by doctors, pharmacists and even paramedics in an emergency. Current smart cards carry about 250 kilobytes of memory.
    • Dynamic RAMs (capacitors) High dielectric constant near phase transformation from the cubic to the tetragonal phase (500~15,000) Tetragonal Cubic
      • Proximity of the Curie temperature to the room temperature yields large dielectric constant in Ba x Sr 1- x TiO 3 ( x =0.5-0.7).
    • Tunable Microwave Devices / Optical Waveguides Filters Phase shifters Delay lines Oscillators   ( E =0)
    • Pyroelectric Detectors/Sensors
    • Pyroelectric Detectors/Sensors
    • Piezoelectric Sensors/Actuators, MEMS
    • Piezoelectric Sensors/Actuators, MEMS
    • Summary
      • What happens to a material when an electric field is applied across it (i.e., in a capacitor)? How does the electric field change, and how does the charge/area change?
      • What are the 3 primary contributions to the dielectric constant.
      • How fast would a dielectric respond?
      • Describe the 4 primary dielectric breakdown mechanisms.
      • How can the breakdown strength be improved?
      • What are ferroelectrics?
      • What are other applications of ferroelectrics?
      Reading for next class Magnetic properties I Chapter sections: 20.1-6