Transcript Polymer Physics Ph.D. Course
Behzad Pourabbas Sahand University of Technology Faculty of Polymer Engineering [email protected]
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IT works with DATA Creates or collects DATA (Direct Input, Through Sensors, CCD or CMOS, Receive DATA).
Process DATA (Computers: CPU). Stores DATA (Hard Discs, Data Storage devices, CD, DVD).
Reviews DATA (Paper, Display devices). Transmit DATA (Internet, Wires and Cables, Optical Fibers).
Consumes Energy (Electricity: Power, Batteries, Solar Cells , etc).
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IT works with DATA Creates or collects DATA (Direct Input, Through Sensors, CCD or CMOS, Receive DATA).
Process DATA (Computers: CPU). Stores DATA (Hard Discs, Data Storage devices, CD, DVD).
Reviews DATA (Paper, Display devices). Transmit DATA (Internet, Wires and Cables, Optical Fibers).
Consumes Energy (Electricity: Power, Batteries, Solar Cells , etc).
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IT works with DATA Sensors (Sensitive Materials, CCD or CMOS (transistors, Receive DATA, Cable or Optical Fibers).
Computers: CPU => Transistors. Hard Discs, Data Storage devices, => Semicinductors Patterning, CD, DVD, Blue Rays=> Laser Sources (Diods) and Patterning.
Display devices, LCD, LED.
Optical Fibers=> Glass or Polymeric Fibers.
Batteries, Solar Cells=> Combination of Semiconductors, Patterning and polymers.
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Materials for IT Polymers Conductive Polymers Conductivity Concept in Materials Conductivity Concept in Polymers Electrically Conductive Polymers Liquid Crystal Polymers Basic Concepts & History Main Features of LCs Advanced Applications of Polymers LCD TFT LED and OLED CCD and CMOS Sollar Cells Optical Fibers Patterning 4/25/2020 5
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Electrical conductivity of a MATTER is its ability to conduct electrons . We can measure it by measuring Resistivity very easily by several methods, Using one OHM meter for example.
There are more complicated and standard method to do this.
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Range of Conductivity in Materials
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Electrical Conductivity ( ) may occur by Electrons or Ions .
What is the Charge of Carriers? How many Carriers? And with what speed (mobility)?
qn
The ease with which the charged species will move under the influence of the applied electric field E and is usually expressed as a velocity per unit field (m 2 V -1 s -1 ) What happens in the absence of an electric field for the charge carriers?
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+ + + +
There is a drift velocity: and an average for it:
qE
.
m
whence
E
: time between scattering events
q
m
-
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Mobile Species:
Electrons and Holes : (Electronic Conductors) ▪ Electron is an Electron (Negative Charge) ▪ Hole is the an empty place of a moved electron (Positively Charged).
Cations and Anions, (Ionic Conductors).
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Theories of Conductions are aimed to explain how
n
and
depend on molecular structure, T and
E
the applied field.
In Polymers, the mobility depends on morphology as well.
There is a large range of mobity values for different materials. (The next Slide).
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Mobility values for different materials 4/25/2020 13
Conductivity in POLYMERS : They are usually insulators if: ▪ There is no charged species : (polymers are composed of covalent bonds) ▪ Careful separation of any ionic species from for example: catalysts residues; Initiators, Ionic End groups, Oxidation Products.
Conversely, One insulating polymer can made conductive by adding conductive fillers such as carbon black or metallic particles (Gold, Silver, Nickel,..) (Conductive Composites).
There are well!!!!!
substantially conductive polymers as 4/25/2020 14
Order of conductivity: superconductors, conductors, semiconductors, insulators
conductors: material capable of carrying electric current, i.e. material which has “mobile charge carriers” (e.g. electrons, ions,..) e.g. metals, liquids with ions (water, molten ionic compounds), plasma insulators: materials with no or very few free charge carriers; e.g. quartz, most covalent and ionic solids, plastics semiconductors: materials with conductivity between that of conductors and insulators; e.g. germanium Ge, silicon Si, GaAs, GaP, InP superconductors: certain materials have zero resistivity at very low temperature. 4/25/2020 15
.
some representative resistivity (
):
4/25/2020 ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ ▪ R = L/A, R = resistance, L = length, A = cross section area; resistivity at 20 o C resistivity in m resistance(in )(L=1m, diam =1mm) aluminum 2.8x10
-8 brass 8x10 -8 copper platinum 10x10 -8 silver carbon 1.7x10
-8 1.6x10
-8 3.5x10
-5 germanium 0.45 silicon 640 porcelain teflon blood fat 10 10 - 10 12 10 14 1.5
24 3.6x10
-2 10.1x10
-2 2.2x10
-2 12.7x10
-2 2.1x10
-2 44.5
5.7x10
5 6x10 8 10 16 - 10 18 10 20 1.9x10
6 3x10 7
Why materials have different conductivities
?
16
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In solid materials, electron energy levels form bands of allowed energies, separated by forbidden bands valence band = outermost (highest) band filled with electrons (“filled” = all states occupied) conduction band = next highest band to valence band (empty or partly filled) “gap” = energy difference between valence and conduction bands, = width of the forbidden band Note: ▪ electrons in a completely filled band cannot move, since all states occupied (Pauli principle); only way to move would be to “jump” into next higher band - needs energy; ▪ electrons in partly filled band can move, since there are free states to move to. ▪ ▪ Classification of solids into three types, according to their band structure: ▪ insulators : gap = forbidden region between highest filled band (valence band) and lowest empty or partly filled band (conduction band) is very wide, about 3 to 6 eV; semiconductors conductors : gap is small - about 0.1 to 1 eV; : valence band only partially filled, or (if it is filled), the next allowed empty band overlaps with it 4/25/2020 18
Atoms form a solid valence electrons interact two quantum mechanical effects.
Heisenberg's uncertainty principle : constrain electrons to a small volume their energy called
promotion
.
raises Pauli exclusion principle limits the number of electrons with the same energy.
Result: valence electrons form wide electron energy bands in a solid.
Bands separated by gaps , where electrons cannot exist .
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Fermi Energy (E F ) - highest filled state at 0 K Conduction band -partially filled or empty band Valence band – highest partially or completely filled band Semiconductors and insulators, be added (Pauli's principle).
valence band is filled, and no more electrons can Insulators > 2 eV 4/25/2020 20
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Intrinsic silicon:
DOPED SEMICONDUCTORS
:
“doped semiconductor”: (also “impure”, “extrinsic”) = semiconductor with small admixture of trivalent or pentavalent atoms; 4/25/2020 22
▪ ▪ ▪ ▪ ▪ donor (n-type) impurities: dopant with 5 valence electrons (e.g. P, As, Sb) 4 electrons used for covalent bonds with surrounding Si atoms, one electron “left over”; left over electron is only loosely bound only small amount of energy needed to lift it into conduction band (0.05 eV in Si) “
n-type semiconductor
”, has conduction electrons, no holes (apart from the few intrinsic holes) example: doping fraction of 10 -8 Sb in Si yields about 5x10 electrons per cubic centimeter at room temperature. 16 conduction 4/25/2020 23
acceptor (p-type) impurities: ▪ dopant with 3 valence electrons (e.g. B, Al, Ga, In) only 3 of the 4 covalent bonds filled vacancy in the fourth covalent bond hole ▪ “
p-type semiconductor
”, has mobile holes, very few mobile electrons (only the intrinsic ones). 4/25/2020 24
4/25/2020 ▪ ▪ ▪ ▪ Can “tune” conductivity by choice of doping fraction can choose “majority carrier” (electron or hole) can vary doping fraction and/or majority carrier within piece of semiconductor can make “p-n junctions” (diodes) and “transistors” 25
p-n JUNCTION:
▪ p-n junction = semiconductor in which impurity changes abruptly from p-type to n-type ; ▪ “diffusion” = movement due to difference in concentration, from higher to lower concentration; ▪ ▪ in absence of electric field across the junction, holes “diffuse” towards and across boundary into n-type and capture electrons; electrons diffuse across boundary, fall into holes (“
recombination of majority carriers
”); formation of a “depletion region” (= region without free charge carriers) around the boundary; ▪ charged ions are left behind (cannot move): ▪ negative ions left on p-side net negative charge on p-side of the junction; ▪ ▪ positive ions left on n-side net positive charge on n-side of the junction electric field across junction which prevents further diffusion. 4/25/2020 26
diode = “biased p-n junction”, i.e. p-n junction with voltage applied across it “forward biased”: p-side more positive than n-side; “reverse biased”: n-side more positive than p-side; forward biased diode: ▪ ▪ the direction of the electric field is from p-side towards n-side p-type charge carriers (positive holes) in p-side are pushed towards and across the p-n boundary, ▪ n-type carriers (negative electrons) in n-side are pushed towards and across n-p boundary current flows across p-n boundary 4/25/2020 27
Depletion region and potential barrier reduced
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reverse biased diode: applied voltage makes n-side more positive than p-side electric field direction is from n-side towards p-side pushes charge carriers away from the p-n boundary depletion region widens, and no current flows
diode only conducts when positive voltage applied to p-side and negative voltage to n-side
diodes used in “rectifiers”, to convert ac voltage to dc. 4/25/2020 29
Depletion region becomes wider, barrier potential higher
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Shockley, Brattain and Bardeen start working with p- and n- type germanium and silicon semiconductors in 1946 Bardeen and Brattain put together the first transistor in December 1947:
a point-contact transistor consisting of a single germanium crystal with a p- and an n- zone. Two wires made contact with the crystal near the junction between the two zones like the “whiskers” of a crystal-radio set .
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(bipolar) transistor = combination of two diodes that share middle portion, called “base” of transistor; other two sections: “emitter'' and “collector”; usually, base is very thin and lightly doped. two kinds of bipolar transistors: pnp and npn transistors “pnp” means emitter is p-type, base is n-type, and collector is p-type material; in “normal operation of pnp transistor, apply positive voltage to emitter, negative voltage to collector; 4/25/2020 33
if emitter-base junction is forward biased, “holes flow” from battery into emitter, move into base; some holes annihilate with electrons in n-type base, but base thin and lightly doped most holes make it through base into collector, holes move through collector into negative terminal of battery; i.e. “collector current” flows whose size depends on how many holes have been captured by electrons in the base; this depends on the number of n-type carriers in the base which can be controlled by the size of the current (the “base current”) that is allowed to flow from the base to the emitter; the base current is usually very small; small changes in the base current can cause a big difference in the collector current; 4/25/2020 34
transistor acts as amplifier of base current, since small changes in base current cause big changes in collector current. transistor as switch: if voltage applied to base is such that emitter-base junction is reverse-biased, no current flows through transistor -- transistor is “off” therefore, a transistor can be used as a voltage-controlled switch; computers use transistors in this way. 4/25/2020 35
“field-effect transistor” (FET)
in a pnp FET, current flowing through a thin channel of n-type material is controlled by the voltage (electric field) applied to two pieces of p-type material on either side of the channel (current depends on electric field). This is the kind of transistor most commonly used in computers. 4/25/2020 36
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Due to improvements in manufacturing, integrated circuits became smaller and smaller Gordon Moore observed that “the number of transistors on a chip seems to double every year….” Moore’s Law: the number of transistors on a chip seems to double every 18 months, while the price remains the same.
Grosch’s law for mainframes: every year, the power of computers doubles while the price is cut in half 4/25/2020 40