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
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.
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 …).
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) 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).