Dielectrics

1
Dielectrics

• 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?

2
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 (eeo) of the dielectric
  between the plates
• Dielectric constant of vacuum eo 8.85x10-12
  F/m55.2 Me/(Vm)

3
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 carriedno charge
  buildup.
• For an insulator, there arent a significant
  number of free carriers. There are highly ionic
  species, however, but they arent very mobile at
  low temperatures. No appreciable current is
  carriedcharge buildup.

4
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/m2.
5
Frequency Response (Switching Time)
6
Microwave Ovens

• A microwave oven generates electromagnetic
  radiation at about 2.5 GHz. This energy is pretty
  good at causing H2O 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?parentmi
criwave.htmurlhttp//www.amasci.com/weird/microe
xp.html
7
Relative Dielectric Constants

• Generally, the less conducting and more polar a
  material is, the greater will be its dielectric
  constant.

8
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
  (e), 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, SiO2, Si3N4, TiO2, HfO2).

9
DRAM SEM Micrograph
The two deep "trenches" (they are really holes)
contain the capacitors on their walls. The
dielectric (with 7 nm far too thin to be
visible) is "ONO", a triple layer of Oxide (SiO2)
Nitride (Si3N4) Oxide (SiO2). This is an
early 64 MBit DRAM (1996).
http//www.tf.uni-kiel.de/matwis/amat/elmat_en/kap
_5/illustr/i5_1_1.html
10
From the news

• Hynix Announces 512Mbit Mobile DRAM
• Hynix Semiconductor, a noted memory maker has
  said it has developed the world's fastest and
  smallest 512 MBit mobile DRAM. The new DRAM
  operates at 200 MHz and processes 1.6 GB of data
  per second. "The product will deliver the memory
  capacity and speed required for third generation
  mobile phones that provide new services, such as
  digital media broadcast (DMB), to subscribers,"
  the chip maker has said. It is expected that
  Hynix will combine this 512Mb mobile DRAM and
  Nand Flash in a multi-chip package which will
  allow mobile manufacturers to make slimmer mobile
  phones.

http//www.tech2.com/india/news
11
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.

12
Dielectric Breakdown Mechanisms

• Thermal (heatdefectsionic conductionmore
  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)