Outline of this Topic

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Electrical Properties

  Outline of this Topic

• 1. Basic laws and electrical properties of metals
• 2. Band theory of solids: metals, semiconductors and insulators
• 3. Electrical properties of semiconductors
• 4. Electrical properties of ceramics and polymers
• 5. Semiconductor devices _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  1

  3 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Goals of this topic:

  1. Basic laws and electrical properties of metals

• Understand how electrons move in materials: electrical
• Ohm’s Law

  conduction V = IR

• How many moveable electrons are there in a material

  (carrier density), how easily do they move (mobility) E = V / L

• Metals, semiconductors and insulators

  where E is electric field intensity

• Electrons and holes

  µ = / E where µ = the mobility ν

• Intrinsic and Extrinsic Carriers

  ν = the drift velocity

• Semiconductor devices: p-n junctions and transistors
• Resistivity
• Ionic conduction

  ρ = RA / L (Ω.m)

• Electronic Properties of Ceramics: Dielectrics,

  Ferroelectrics and Piezoelectrics

• Conductivity
• -1

  σ = 1 / ρ (Ω.m) _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  2

  4

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Materials Choices for Metal Conductors

• Electrical conductivity between different materials • Most widely used conductor is copper: inexpensive,

  abundant, very high σ varies by over 27 orders of magnitude, the greatest

• Silver has highest σ of metals, but use restricted due to cost

  variation of any physical property

• Aluminum main material for electronic circuits, transition

  to electrodeposited Cu (main problem was chemical etching, now done by “Chemical-Mechanical Polishing”)

• Remember deformation reduces conductivity, so high

  strength generally means lower σ : trade-off. Precipitation hardening may be best choice: e.g. Cu-Be.

  5 -1

• Heating elements require low

  σ (high R), and resistance to Metals: σ > 10 Ω.m)

  ( high temperature oxidation: nichrome.

• 6 5 -1

  Semiconductors: 10 σ < 10 Ω.m) < (

• 6 -1

  Insulators: σ < 10 Ω.m) ( _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  5

  7 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Conductivity / Resistivity of Metals

• Electric field causes electrons to accelerate in direction opposite

  High number of free (valence) electrons • to field

  → high σ

• Velocity very quickly reaches average value, and then remains
• Defects scatter electrons, therefore they

  constant increase ρ (lower σ).

  ρ = ρ ρ ρ • Electron motion is not impeded by periodic crystal lattice + + _{total thermal impurity deformation}

  ρ _{thermal from thermal vibrations}

• Scattering occurs from defects, surfaces, and atomic thermal

  ρ impurity from impurities vibrations

  ρ from deformation-induced point defects _deformation

• These scattering events constitute a “frictional force” that
• Resistivity increases with temperature

  causes the velocity to maintain a constant mean value: v , the d (increased thermal vibrations and point electron drift velocity defect densities)

• The drift velocity is proportional to the electric field, the

  ρ = ρ + aT _{T o} constant of proportionality is the mobility , µ . This is a measure

• Additions of impurities that form solid of how easily the electron moves in response to an electric field.

  sol: ρ Ac (1-c ) (increases ρ) • The conductivity depends on how many free electrons there _{I = i i} are, n, and how easily they move

  Two phases, α, β: • ρ ρ _{i = α α β β} V ρ +

  V _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  6

  8

• Schroedinger’s eqn (quantum mechanical equation for

  that electrons have allowed ranges of energy (energy bands) and forbidden ranges of energy (band-gaps).

  2. Band theory of solids: metals, semiconductors and insulators Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  2 /2m) + V ψ = ih’

  (-h’

  K ψ + V ψ = E ψ

  ψ δt

  2 δ

  δx

  2 ψ

  δ

  behavior of an electron)

• Solve it for a periodic crystal potential, and you will find

  11 Band Theory of Solids

  >> σ semi ^{Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering}

  8.00 σ metal

  Carrier Density N e (m

• ^-3 ) Na (m) 0.0053 2.6 x 10
²⁸ Ag (m) 0.0057 5.9 x 10 ²⁸ Al (m) 0.0013 1.8 x 10 ²⁹ Si (s) 0.15 1.5 x 10 ¹⁰ GaAs (s) 0.85 1.8 x 10 ⁶ InSb (s)

  10 (m) = Metal (s) = Semicon Mobility (RT) µ (m ² V ^-1 s ^-1 )

  Scattering events Net electron motion Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  E

  σ = n|e| µ e n : number of “free” or conduction electrons per unit volume

  9 v d = µ e E

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  12 Electrons in an Isolated atom (Bohr Model) Electron orbits defined by requirement that they contain integral number of wavelengths: quantize angular momentum, energy, radius of orbit

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 When N atoms in a solid •

• Each band can contain certain number of electrons (xN, where N is the

  are relatively far apart, they number of the atoms and x is the number of electrons in a given atomic do not interact, so electrons shell, i.e. 2 for s, 6 for p etc.). Note: it can get more complicated than this! in a given shell in different

• Electrons in a filled band cannot conduct

  atoms have same energy

• In metals, highest occupied band is partially filled or bands overlap
• As atoms come closer

• •

  Highest filled state at 0 Kelvin is the Fermi Energy, E _F

  together, they interact, Semiconductors, insulators: highest occupied band filled at 0 Kelvin: • perturbing electron energy electronic conduction requires thermal excitation across bandgap; σ↑ T↑ levels

• (At 0 Kelvin) highest filled band: valence band; lowest empty band:

  

Electrons from each atom • conduction band. E is in the bandgap

_f then have slightly different energies, producing a “band” of allowed energies _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  13

  15 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Metals, Semiconductors, Insulators _{Metals < 2 eV E > 2 eV Insulators g Semiconductors E are filled, all those above are vacant g} • At 0 Kelvin all available electron states below Fermi energy

• Only electrons with energies above the Fermi energy can _{Band gap Empty band Empty conduction E f Band gap band conduction Band gap Empty band E numbers n, l, m f Empty – Remember “Pauli Exclusion Principle” that only two electrons (spin band up, spin down) can occupy a given “state” defined by quantum}

  conduct: _{l Empty states Filled E band valence f Filled Filled E}

_{f above the Fermi energy}

_valence – So to conduct, electrons need empty states to scatter into, i.e. states _{Filled states band band} • When an electron is promoted above the Fermi level (and can thus conduct) it leaves behind a hole (empty electron state)
• – A hole can also move and thus conduct current: it acts as a “positive

  electron)

• – Holes can and do exist in metals, but are more important in

  semiconductors and insulators _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  14

  16

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 The Fermi Function Metals

  This equation represents the probability that an energy level, E, Empty is occupied by an electron and can have values between 0 and 1 states

  . At 0K, the f (E) is equal to 1 up to E and equal to 0 above E f f y g E E F F ner

  (E - E ) / kT E Electron f (E) = [1] / [e +1] f excitation

  

Filled

states

(b) (a) _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  17

  19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Semiconductors, Insulators

• In metals, electrons near the Fermi energy see empty states a very small

  energy jump away, and can thus be promoted into conducting states above

  n n o

  E very easily (temp or electric field) _f

  o ti ti nd

• nd

  High conductivity

  ba nduc ba nduc

• Atomistically: weak metallic bonding of electrons

  Co Free Co

• In semiconductors, insulators, electrons have to jump across band gap into

  electron conduction band to find conducting states above E : requires jump >> kT y _f

  g d

  E • No. of electrons in CB decreases with higher band gap, lower T F

  ner

  Electron

  E Gap Ban

  Relatively low conductivity • excitation

• An electron in the conduction band leaves a hole in the valence band, that can also conduct

  Hole in

  nce

• nce

  Atomistically: strong covalent or ionic bonding of electrons nd nd valence

  le le ba ba

  band

  Va Va

  (a) (b) _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  18

  20

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Intrinsic Semiconductors: Conductivity

  E field

  Si Si Si Si

• Both electrons and holes conduct:

  Si Si Si Si _hole

• + p|e|
_{free electron e h} σ = n|e|µ µ Si Si Si Si Si Si Si Si n: number of conduction electrons per unit volume p: number of holes in VB per unit volume

  Si Si Si Si Si Si Si Si

• In intrinsic semiconductor, n = p:

  (a) (b) σ = n|e|(µ µ ) = p|e|( µ µ + + ) e h e h

  E field

  Electrical conduction in intrinsic Si, (a) before excitation, (b) and (c) after excitation, see the Si Si Si Si _{free electron} • Number of carriers (n,p) controlled by thermal response of the electron-hole pairs to the external excitation across band gap: field. Note: holes generally have lower mobilities

  Si Si Si Si _hole than electrons in a given material (require n = p = C exp (- E /2 kT) g cooperative motion of electrons into previous

  C : Material constant Si Si Si Si hole sites)

  E : Magnitude of the bandgap _{g University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  21

  23 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Extrinsic Semiconductors

3. Electrical properties of semiconductors Semiconductors

  Semiconductors are the key materials in the electronics and • telecommunications revolutions: transistors, integrated circuits, lasers, solar cells…. • Engineer conductivity by controlled addition of Intrinsic semiconductors are pure (as few as 1 part in 10 ¹⁰ impurity atoms: Doping

• impurities) with no intentional impurities. Relatively high resistivities

  Extrinsic semiconductors have their electronic properties (electron • and hole concentrations, hence conductivity) tailored by intentional addition of impurity elements Room Temp _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  22

  24 E field _{free electron Si}

• In Si which is a tetravalent lattice, substitution of
• So N
• Impurities which produce extra conduction electrons are
• These additional electrons are in much greater numbers
• Typical values of N
• Substitution of trivalent B (or Al, Ga...) atoms in Si
• So N
• Impurities which produce extra holes are called acceptors,
• These additional holes are in much greater numbers than
• Typical values of N

  E ner g y ^{Donor state}

  (b) (a)

  Va le nce ba nd Co nduc ti o n ba nd Ban d Gap Va le nce ba nd Co nduc ti o n ba nd

  28 Semiconductors

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  4+ ^{Si 4+ Si} ₄₊ ^{Si 4+}

  27 n-type p-type ^{Si 4+} (a) ^{Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ P 5+ Si 4+ Si 4+ Si 4+ Si 4+} _{Si 4+} (a) ^{Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ B 3+ Si 4+ hole Si 4+ Si 4+ Si 4+ Si 4+} (b) ^{Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ B 3+ Si 4+ hole Si 4+ Si 4+ Si 4+ Si 4+ Si 4+} (b) ^{Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ Si 4+ P 5+}

  19 cm -3 (Many orders of magnitude greater than intrinsic carrier concentrations at RT) ^{Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering}

  16 - 10

  A ~ 10

  |e| µ h

  intrinsic hole or electron concentrations, σ ~ p|e|µ h ~ N A

  B B atoms per unit volume produce p additional holes per unit volume

  N A = N B ~ p

  produces extra holes as only three outer electrons exist to fill four bonds. Each B atom in the lattice produces one hole in the valence band.

  26 p-type semiconductors

  19 cm -3 (Many orders of magnitude greater than intrinsic carrier concentrations at RT) Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  16 - 10

  D ~ 10

  µ e

  than intrinsic hole or electron concentrations, σ ~ n|e|µ e ~ N D |e|

  called donors, N D = N As ~ n

  As As atoms per unit volume produce n additional conduction electrons per unit volume

  pentavalent As (or P, Sb..) atoms produces extra electrons, as fifth outer As atom is weakly bound (~ 0.01 eV). Each As atom in the lattice produces one additional electron in the conduction band.

  25 n-type semiconductors

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  n-type “more electrons” Free _{electrons in the conduction band} For an n-type material, excitation occurs from the donor state in which a free electron is generated in the conduction band.

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Semiconductors p-type “more holes”

  n o n ti o ti nd nd ba nduc ba nduc Co Co

  4. Electrical properties of

  y g d ner

  ceramics and polymers

  E Gap Ban _{Acceptor state the valence Hole in} nce nd nce nd le band le ba ba Va Va

  (a) (b) For an p-type material, excitation of an electron into the acceptor level, leaving behind a hole in the valence band. _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  29

  31 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Temperature Dependence of carrier Concentration and Dielectric Materials Conductivity

• Our basic equation:
• A dielectric material is an insulator which contains electric
• + p|e|

  σ = n|e|µ µ e h

  dipoles, that is where positive and negative charge are _Intrinsic • Main temperature variations separated on an atomic or molecular level

  are in n,p rather than , µ µ _Saturation e h _{p , n} • Intrinsic carrier concentration n = p = C exp (- E /2 kT) _{ln Extrinsic} g

  Extrinsic carrier concentration

• – low T (< room temp) Extrinsic
• When an electric field is applied, these dipoles align to the
_{ regime: ionization of dopants

  ∆ln p/ [∆(1/T)]}

  field, causing a net dipole moment that affects the material _{= E / 2 k g} – mid T (inc. room temp) Saturated properties.

  regime: most dopants ionized _1/T – high T Intrinsic regime: intrinsic generation dominates _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

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  32

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Capacitance Polarization

• Capacitance is the ability to store _{+ + + + + • Magnitude of electric dipole moment} charge across a potential difference.

  from one dipole:

• Examples: parallel conducting plates, _{- - - - - - p = q d} semiconductor p-n junction

  D

• Magnitude of the capacitance, C: P N • In electric field, dipole will rotate in

  C = Q / V - - - + + +

  direction of applied field: polarization

  Units: Farads

• - - - + + +
• Parallel- plate capacitor, C depends on
• - - - + + +
• The surface charge density of a geometry of plates and material capacitor can be shown to be:

  between plates D = ε ε ξ C = ε ε A / L o r r o

  A : Plate Area; L : Plate Separation

• _-12
₂ D : Electric Displacement ε : Permittivity of Free Space (8.85x10 F/m ) _o

  2

  (units Coulombs / m )

  L : Relative permittivity, = /

  ε ε ε ε _{r r o} Vac, ε = 1 _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering r}

  33

  35 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19

• Increase in capacitance in dielectric medium compared to vacuum is due to polarization of electric dipoles in dielectric.
• In absence of applied field (b), these are oriented randomly
• In applied field these align according to field (c)
• Result of this polarization is to create opposite charge Q’ on material adjacent to conducting pl
• Magnitude of dielectric constant depends upon frequency
• This induces additional charge (-)Q’ of applied alternating voltage (depends on how quickly charge within molecule can separate under applied field) on plates: total plate charge Q =

  t |Q+Q’|.

• Dielectric strength (breakdown strength): Magnitude of
• So, C = Q / V has increased electric field necessary to produce breakdown

  t _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  34

  36

• Surface density charge now

  (units Coulombs/m

  3 : Permanent Dipole Moment for T < 120 C (Curie Temperature, T c ). Above T c , unit cell is cubic, no permanent electric dipole moment

  40 Barium Titanate, BaTiO

  39 Electronic Ionic Orientation Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  P tptal = P e + P i + P o ^{Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering}

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

  ( ε r

  P = ε o

  ). It represents the total electric dipole moment per unit volume of dielectric, or the polarization electric field arising from alignment of electric dipoles in the dielectric

  2

  ξ + P

  o

  ξ = ε

  r

  ε

  o

  εξ = ε

  D =

• P is the polarization of the material
• From equations at top of page

  37

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 University of Tennessee, Dept. of Materials Science and Engineering

• -1) ξ
• Where do the electric dipoles come from?
• – Electronic Polarization: Displacement of negative electron “clouds” with respect to positive nucleus. Requires applied electric field. Occurs in all materials.
• – Ionic Polarization: In ionic materials, applied electric field displaces cations and anions in opposite directions
• – Orientation Polarization: Some materials possess permanent electric dipoles, due to distribution of charge in their unit cells. In absence of electric field, dipoles are randomly oriented. Applying electric field aligns these dipoles, causing net (large) dipole moment.

38 Origins of Polarization

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Piezoelectricity

• In some ceramic materials, application of external forces produces an electric (polarization) field and vice-versa
• Applications of piezoelectric materials microphones, strain gauges, sonar detectors
• Materials include barium titanate, lead titanate, lead zirconate
_{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

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  43 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Ionic Conduction in Ceramics Electrical Properties of Polymers

• Cations and anions possess electric charge (+,-)
• Most polymeric materials are relatively poor conductors of electrical therefore can also conduct a current if they move.

  current - low number of free electrons

• Ionic conduction in a ceramic is much less easy
• A few polymers have very high electrical conductivity - about one quarter that of copper, or about twice that of copper per unit weight.

  electron conduction in a metal (“free” electrons can move far more easily than atoms / ions) • Involves doping with electrically active impurities, similar to

  semiconductors: both p- and n-type

• In ceramics, which are generally insulators and have
• Examples: polyacetylene, polyparaphenylene, polypyrrole

  few free electrons, ionic conduction can be a significant

  Orienting the polymer chains (mechanically, or magnetically) during •

  component of the total conductivity

  synthesis results in high conductivity along oriented direction

  = σ σ σ

• total electronic ionic Applications: advanced battery electrodes, antistatic coatings, electronic devices
• Overall conductivities, however, remain very low in

  Polymeric light emitting diodes are also becoming a very important • ceramics. research field _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

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  44

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Applied Voltage D P N D P N - - - + + +

5. Semiconductor Devices and Circuits - - - + + - - - + + + + - - - + - + + - - + + + - - - + + +

  V Reverse Bias _b

• - +

  V _{b Forward Bias}

• - +

  V _o V o +|V b | V o V o -V b E c0 E c+ E F0 E c0 E ^c-

  E _F- E E _{v+ v0}

  E v0 E _v- Lower Barrier , I ↑ Higher Barrier, I ↓ _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  45

  47 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 The Semiconductor p-n Junction Diode D P N

• A rectifier or diode allows
• - - - current to flow in one
• + + + direction only. - - - + + + - - - + + +

• p-n junction diode consists of adjacent p- and n-doped

  ξ

  semiconductor regions

• Electrons, holes combine at

  n

  junction and annihilate:

  p depletion region containing

  ionized dopants

• Electric field, potential barrier

  V _h

  resists further carrier flow

  V _{e University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  46

  48

  Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 MOSFET (Metal-Oxide-Semiconductor Field Effect Transistors Transistor)

• The basic building block of the microelectronic revolution
• Can be made as small as 1 square micron
• A single 8” diameter wafer of silicon can contain as many as

  10

  11

  10 - 10 transistors in total: enough for several for every man, woman, and child on the planet

• •

  • Voltage applied from source to drain encourages carriers (in the above case

  Nowadays, the most important type of transistor.

• Cost to consumer ~ 0.00001c each.
• Achieved through sub-micron engineering of semiconductors, holes) to flow from source to drain through narrow channel.

  metals, insulators and polymers.

• Width (and hence resistance) of channel is controlled by intermediate
• Requires ~ $2 billion for a state-of-the-art fabrication facility

  voltage • Current flowing from source-drain is therefore modulated by gate voltage.

• Put input signal onto gate, output signal (source-drain current) is correspondingly modulated: amplification and switching
• State-of-the-art gate lengths: 0.18 micron. Oxide layer thickness < 10 nm _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  49

  51 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Introduction To Materials Science FOR ENGINEERS, Ch. 19 Bipolar Junction Transistor Take Home Messages

• Language: Resistivity, conductivity, mobility, drift velocity, electric field intensity, energy bands, band gap, conduction band, valence band, Fermi energy, hole, intrinsic semiconductor extrinsic semiconductor, dopant, donor, acceptor, extrinsic regime, extrinsic regime, saturated regime, dielectric, capacitance, (relative) permittivity, dielectric strength, (electronic, ionic, orientational) polarization, electric displacement, piezoelectric, ionic conduction, p-n junction, rectification, depletion region, (forward, reverse) bias, transistors, amplification.
• n-p-n or p-n-p sandwich structures. Emitter-base-collector. Base is very thin (~ 1

  Fundamental concepts of electronic motion: Conductivity, drift velocity, • micron or less) but greater than depletion region widths at p-n junctions. mobility, electric field

  Emitter-base junction is forward biased; holes are pushed across junction. Some of •

• •

  Band theory of solids: Energy bands, band gaps, holes, differences between

  these recombine with electrons in the base, but most cross the base as it so thin. They metals, semiconductors and insulators are then swept into the collector.

  Semiconductors: Dependence of intrinsic and extrinsic carrier conc. on • A small change in base-emitter voltage causes a relatively large change in emitter- • temperature, band gap; dopants - acceptors and donors. base-collector current, and hence a large voltage change across output (“load”) resistor: voltage amplification

  Capacitance: Dielectrics, polarization and its causes, piezoelectricity • The above configuration is called the “common base” configuration (base is common •

• •

  Semiconductor devices: basic construction and operation of p-n junctions,

  to both input and output circuits). The “common emitter” configuration can produce bipolar transistors and MOSFETs both amplification (V,I) and very fast switching _{University of Tennessee, Dept. of Materials Science and Engineering University of Tennessee, Dept. of Materials Science and Engineering}

  50

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