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[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt1
Bruce Mayer, PE Engineering-45: Materials of Engineering
Bruce Mayer, PELicensed Electrical & Mechanical Engineer
Engineering 45
ElectricalProperties
-2
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt2
Bruce Mayer, PE Engineering-45: Materials of Engineering
Learning Goals – Electrical Props How Are Electrical Conductance And
Resistance Characterized What Are The Physical Phenomena That
Distinguish Conductors, Semiconductors, and Insulators?
For Metals, How Is Conductivity Affected By Imperfections, Temp, And Deformation?
For Semiconductors, How is Conductivity Affected By Impurities (Doping) And Temp?
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt3
Bruce Mayer, PE Engineering-45: Materials of Engineering
SemiConductivity
Materials of Valence 4 (Grp IVA in the Periodic Table) Exhibit the property of Semiconductivity• Si, Ge in
Particular• C, Sn to a
Lesser Extent
Also Observed in Compounds
• III-V → GaAs• II-VI → InP
Semiconductivity Characterized by• Insulative Behavior at
Room Temperature– 106-1012 times LESS
conductive than metals• INCREASING
Conductivity with Increasing Temp– Opposite of
Metal Behavior
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt4
Bruce Mayer, PE Engineering-45: Materials of Engineering
Carriers in Semiconductors
At non-zero temperatures, electrons are thermally excited from the valence band to the conduction band.
The activated “free electrons” and the remaining “holes” left behind act as two “ideal gases”!!
Certain types of impurities that are grown or implanted into the SemiConductor crystal produce extra free electrons or holes.
Energy gap, Eg
Conduction band (at T = 0 K unpopulated with electrons)
Valence band (at T = 0 K totally filled with electrons)
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt5
Bruce Mayer, PE Engineering-45: Materials of Engineering
Intrinsic (Pure) Semiconductors
σ Data for Pure Silicon• Note σ↑ as T↑
Why This Temp Behavior?• Semiconductor e−
Band StructureSi electrical conductivity, σ
(S
/m)
50 100 100010-210-1
100
101102103104
pure (undoped)
T(K)
Energy
filled band
filled valence band
empty band
filled s
tate
s
GAP?
– Thermal Energy Can Allow the e- to jump the “Forbidden” Gap between the “Valence” Band and the “Conduction” Band
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt6
Bruce Mayer, PE Engineering-45: Materials of Engineering
Intrinsic (Pure) Carrier Concen
Recall Conductivity Eqn from the Metals Dicussion
Note the Exponential Increase in the Intrinsic carrier Concentration, ni or
nq
kTEi
gen Since µ Does Not
change nearly as much as ni with T
kTE ge
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt7
Bruce Mayer, PE Engineering-45: Materials of Engineering
Some BandGapsInSb 0.17 eV
Ge 0.67 eV
InN 0.7 eV
HgCdTe 0.0 - 1.5 eV
InGaAs 0.4 - 1.4 eV
Silicon 1.14 eV
InP 1.34 eV
GaAs 1.42 eV
CdTe 1.56 eV
AlGaAs 1.42 – 2.16 eV
InGaP2 1.8 eV
GaAsP 1.42-2.26eV
InGaN 0.7 - 3.4 eV
AlAs 2.16 eV
GaP 2.26 eV
AlGaInP 1.91 - 2.52 eV
ZnSe 2.7 eV
SiC 6H 3.03 eV
SiC 4H 3.28 eV
GaN 3.37 eV
Diamond 5.46 - 6.4 eV
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt8
Bruce Mayer, PE Engineering-45: Materials of Engineering
Conduction by e− & h+ Migration Concept of Electrons (e-) & Holes (h+)
• When e- moves to the Conduction Band it leaves Its Parent Atom Core, and Moves Freely
• This Leaves behind an electron “HOLE” Which Results in a POSITIVELY Charged Atom/Ion Core
• This Positive Charge can Attract an e- from an ADJACENT Atom, Thus the hole, h+, can move Left↔Right or Up↔Down– This Transfers the POSITIVE Charge-Center to the
Adjacent Atom-Core– From an electrical current perspective, the Step-by-Step
movement of the hole appears as the movement of a POSITIVELY Charged Particle; some Analogies A bubble in a Liquid moves to the high side of a sealed tube One open Spot in A parking Lots Moves Further from the Bldg
as the cars move into the Close spot in Step-By-Step Fashion
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt9
Bruce Mayer, PE Engineering-45: Materials of Engineering
e− & h+ Electrical Conduction Schematically
- +
electron hole pair creation
- +
no applied electric field
applied electric field
valence electron Si atom
applied electric field
electron hole pair migration
- + - +
In Metals, only e− Participate in Electrical Conduction (e− “sea”), But in Semiconductors HOLES also aid conduction
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt10
Bruce Mayer, PE Engineering-45: Materials of Engineering
SemiConductor Conductivity
With the Participation of Electrons and Holes• Where
– q electronic charge, 1.6x10-19 Coulomb per e- or h+
– n electron concentration, e-/m3
– p hole concentration, h+/m3
– µe electron mobility, m2/V-s
– µh hole mobility, m2/V-s
hesemi pqnq Qty Si GaAs CdTe InP
µe(m2/V-s)
0.19 0.88 0.105 0.470
µh(m2/V-s)
0.05 0.04 0.008 0.018
µe (4-30) times Greater Than µh
• Why?– Parking Garage
Analogy
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt11
Bruce Mayer, PE Engineering-45: Materials of Engineering
h+ & e-Parking Garage
Analogy n-Type
Semiconductor illustrated in (a) & (c)
p-Type Semiconductor illustrated in (b) & (d)
Thus µe >µh
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt12
Bruce Mayer, PE Engineering-45: Materials of Engineering
INtrinsic vs. EXtrinsic Conduction INtrinsic SemiConductors → n = p
• Case for “pure” Semiconductors; e.g., Si
EXtrinsic SemiConductors → n p• occurs when impurities are added with a different
no. of valence e−’s than the host (e.g., Si atoms)
N-type EXtrinsic: (n>>p) P-type EXtrinsic: (p>>n)
no applied electric field
5+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+
Phosphorus atom
no applied electric field
Boron atom
valence electron
Si atom
conduction electron
hole
3+
4+ 4+ 4+ 4+
4+
4+4+4+4+
4+ 4+enq hpq
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt13
Bruce Mayer, PE Engineering-45: Materials of Engineering
Doped SemiConductors: vs T increases w/ Doping
Reason: imperfection sites lower the activation energy needed to produce mobile e- or h+
N-Type Si, n vs T
doped 0.0013at%B
0.0052at%B
ele
ctri
cal co
nduct
ivit
y,
σ
(S/m
)
50 100 100010-210-1100101102103104
pure (undoped)
T(K)– FreezeOut → Not Sufficient
Thermal Energy to ionize either Dopants or Si
– Extrinsic → n = doping– Instrinsic → ni > doping
nd = 1021/m3
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt14
Bruce Mayer, PE Engineering-45: Materials of Engineering
FreezeOut etc.
Recall Reln for ni
The similar Reln for (N-Type) dopant Concentrations
– FreezeOut → kT << [Eg or Ed] Neither Si or Dopants are Ionized
– Extrinsic → Ed < kT < Eg
Only Dopants are (Singly) ionized and nd >> ni
– Intrinsic kT>> [Ed or Eg] nd fixed at dopant at%,
ni continues to Rise
kTEi
gen
kTEd
den Impurity
Donor Ed Acceptor Ea
P 0.044
As 0.049
Sb 0.039
B 0.045
Al 0.057
Egap 1.1 1.1
Si D
opan
t Ion
izat
ion
(eV
)nd = 1015/cc
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt15
Bruce Mayer, PE Engineering-45: Materials of Engineering
p-n Junction Physics
P and N Type Semi Matls Brought Together to form a METALLURICAL (seamless) Junction
The HUGE MisMatch in Carrier Concentrations Results in e- & h+ DIFFUSION• Remember that?
Carrier Diffusion• e- Diffuse in to the
P-Type Material• h+ Diffuse in to the
N-Type Material
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt16
Bruce Mayer, PE Engineering-45: Materials of Engineering
p-n Junction Physics cont.
In a p-n Jcn Carrier Cross-Diffusion is SELF-LIMITING• The e-/h+ Diffusion
leaves Behind IONIZED Atom Cores of the OPPOSITE Charge
• The Ion Cores set up an ELECTRIC FIELD that COUNTERS the Diffusion Gradient
For Si the Field-Filled Depletion Region• E-Field 1 MV/m• Depl Reg Width,
xd = 1-10 µm• E-fld•dx 0.6-0.7 V
– “built-in” Potential
E-Field
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt17
Bruce Mayer, PE Engineering-45: Materials of Engineering
p-n Junction Rectifier
A Rectifier is a “Check Valve” for Current flow• Current Allowed
in ONE Direction but NOT the other
Side Issue → “Bias” Voltage• A “Bias” Voltage
is just Another name for EXTERNALLY APPLIED Voltage
E-Field
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt18
Bruce Mayer, PE Engineering-45: Materials of Engineering
p-n Junction Rectifier cont
p-n junction Rectification• A small “Forward
Bias” Voltage results in Large currents
• Any level of “Reverse” Bias results in almost NO current flow
Class Q:• For Fwd Bias,
Which End is +; P or N???
A: the P end • The Applied Voltage
REDUCES the internal E-Field; This “Biases” The Junction in Favor of DIFFUSION
E-Field
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt19
Bruce Mayer, PE Engineering-45: Materials of Engineering
p-n Junction Rectifier cont.2
p-n junction No Applied Voltage
• Internal Field ENHANCED– Carriers Pulled AWAY
from Jcn; xd grows
Forward Bias
• Diffusion & E-Field in Balance, No Current Flows
Reverse Biased
• Internal Field REDUCED– Carriers PUSHED and
Diffuse to the Jcn where they are “injected” into the other side; xd Contracts
Xd
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt20
Bruce Mayer, PE Engineering-45: Materials of Engineering
Properties of Rectifying Junction
ForwardReverse
IN914 PN Diode• IF = 75 000 µA
• IR = 0.025-50 µA
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt21
Bruce Mayer, PE Engineering-45: Materials of Engineering
Transistors
Transistors are “Transfer Resistors”
Xsistors Have Three Connections • Input• Output• CONTROL
In Electronic Applications Transistors have TWO Basic Fcns
• Amplification – Both Current & Voltage
• On/Off Switching Two Main Types
• BiPolar Junction Transistor (BJT)– Good Amps
• Field Effect Transistor (FET)– Depletion Mode
Good Amps– Enhancement Mode
Good Switches
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt22
Bruce Mayer, PE Engineering-45: Materials of Engineering
BJT
The Classic pnp or npn configurations• Basically Two pn
jcns Back-to-Back
npn In “Forward-Active” mode• b-e pn jcn
FORWARD Biased• b-c pn jcn
REVERSE Biased Very Little “base”
Current Large emitter &
collector currents• Good Current-Driving
Amplifiere
b
c
e
b
c
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt23
Bruce Mayer, PE Engineering-45: Materials of Engineering
Depletion Mode - JFET
JFETs are “Normally On” Transistors
Reverse Bias on the “gate” expands the NonConducting depletion region Until the channel is “Pinched Off” and no longer conducts• Gate is Reverse
Biased → little Control-Current
• Good Depl Region modulation → good I/V amp
OPEN “Channel” Between the “source” and “drain”
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt24
Bruce Mayer, PE Engineering-45: Materials of Engineering
Enhancement Mode - IGFET Insulated Gate
Field Effect Transistors are Normally-Off devices
Applying a Positive Voltage to the Gate will attract e− to the Channel• This will eventually
“invert” a thin region below the gate to N-type, creating a conducting channel between S & D
IGFETs are Great Switches• Used in almost all
digital IC’s
Back-to-Back pn Jcns Between “source” & “drain”
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt25
Bruce Mayer, PE Engineering-45: Materials of Engineering
Ionic Materials In Metals and
Semiconductors, the atomic Ion-cores are fixed in the crystal Lattice• Although they
have the same charge as a “hole” they have almost NO “Mobility”– Thus They do
NOT contribute to Electrical Conduction
Some Small Atomic Radii impurities can be CHARGED (ionic) and MOBILE within another material• e.g., Na+ can move
fairly easily thru GLASS (SiO2)
The Total σ for Ionic Materials
ioniceletronic tot
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt26
Bruce Mayer, PE Engineering-45: Materials of Engineering
Ionic Mobility • Diffusion• E-Field
Combine These two effects into Mobility
• Where– nI Ion Valence
– DI Ion Mass Diffusion Coeff, m2/s
– q, k, T as Before
• Exercise → Find units for nI
As in the Electronic case
IIionic qN
kT
qDn III • Where
– NI Ion Concen, Ions/m3
– q electronic Charge– µI Ionic Mobility,
m2/V-s
Two Forces move The Ions
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt27
Bruce Mayer, PE Engineering-45: Materials of Engineering
Ceramics Most Ceramics have
WIDE BandGaps• SiO2 9 eV
• Si3N4 4.7eV
Thus Ceramics Tend to be VeryGood Electrical INSULATORS
But as with SemiConductors. for Ceramics nintrinsic Increases with Temperature• Thus Insulative Capacity DEGRADES at Hi-T
– e.g; mullite = 3Al2O3•2SiO2
ρ(25°C) 1012 Ω-m; ρ(500°C) 106 Ω-m
Energy
filled band
filled Valence band
empty band
fille
d s
tate
s
GAP
ConductionBand
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt28
Bruce Mayer, PE Engineering-45: Materials of Engineering
Polymers Most “Standard” Plastics are Good Insulators
• c.f. Their use as insulation on metal WIRES• Conduction Mechanism Not well understood
– Believed to be More Electronic than Ionic
A Few Polymers are Good Conductors, with σ 107 S/m• About 2X HIGHER than Cu for Conductivity/lb • Mechanism appears to be SemiConductor-like
with a doping Requirement• Discovery of these “synthetic metals” Resulted in
the 2000 Chemistry Nobel Prize for Heeger, MacDiarmid and Shirakawa
http://webpages.charter.net/dmarin/coat/#history
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt29
Bruce Mayer, PE Engineering-45: Materials of Engineering
PiezoElectric Materials
Piezoelectricity application of force pressure produces Electrical Potential
at rest compression induces voltage
applied voltage induces
expansion
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt30
Bruce Mayer, PE Engineering-45: Materials of Engineering
WhiteBoard Work
Problem 18.30• Antimony Doped
Germanium• EXtrinsic form
– All Sb Ionized
• The μ’s:– μe = 0.1 m2/V·s
– μh = 0.05 m2/V·s