Transcript W A T K I N S - J O H N S O N C O M P A N Y Semiconductor

Engineering 45
Electrical
Properties-2
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
[email protected]
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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?
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
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• 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
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Carriers in Semiconductors
Conduction band (at T = 0 K
unpopulated with electrons)
Energy gap, Eg
Valence band (at T = 0 K
totally filled with electrons)
 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.
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Intrinsic (Pure) Semiconductors
 Why This Temp
Behavior?
  Data for
Pure Silicon
• Note ↑ as T↑
Si electrical conductivity, 
 (S/m)
10 4
10 3
pure
(undoped)
10 -2
50 100
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– Thermal Energy
Can Allow the eto jump the
“Forbidden” Gap
between the
“Valence” Band
and the
“Conduction”
Band
10 2
10 1
10 0
10 -1
• Semiconductor eBand Structure
1000
T(K)
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Intrinsic (Pure) Carrier Concen
 Note the Exponential
 Recall Conductivity
Increase in the
Eqn from the Metals
Intrinsic carrier
Dicussion   nq
Concentration, ni or
ni  e
 E g kT
 Since µ Does Not
change nearly as
much as ni with T
 e
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 E g kT
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
InSb
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
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Some BandGaps
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
e- & h+ Electrical Conduction
 Schematically
valence
electron
Si atom
electron hole
pair creation
no applied
electric field
electron hole
pair migration
+ applied
electric field
- +
+
applied
electric field
- +
 In Metals, only e- Participate in Electrical
Conduction (e- “sea”), But in Semiconductors
HOLES also aid conduction
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
SemiConductor Conductivity
– µe  electron mobility,
m2/V-s
– µh  hole mobility,
m2/V-s
 With the
Participation of
Electrons and Holes
 semi  nqe  pqh
• Where
– q  electronic charge,
1.6x10-19 Coulomb
per e- or h+
– n  electron
concentration, e-/m3
– p  hole
concentration, h+/m3
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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
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
h+ & eParking Garage
Analogy
 n-Type
Semiconductor
illustrated in (a) & (c)
 p-Type
Semiconductor
illustrated in (b) & (d)
 Thus µe >µh
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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)
Phosphorus atom
  nqe
4+ 4+ 4+ 4+
4+ 5+ 4+ 4+
4+ 4+ 4+ 4+
no applied
electric field
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Boron atom
hole
conduction
electron
4+ 4+ 4+ 4+
valence
electron
4+ 4+ 4+ 4+
Si atom
4+ 3+ 4+ 4+
  pqh
no applied
electric field
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Doped SemiConductors -  vs T
10 4
10 3
(S/m)
electrical conductivity,
  increases w/ Doping
10 2
10 1
10 0
10-1
0.0052at%B
nd = 1021/m3
doped
0.0013at%B
pure
(undoped)
10-2
50 100
1000
T(K)
 Reason: imperfection
sites lower the activation
energy needed to
produce mobile e- or h+
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 N-Type Si, n vs T
– FreezeOut → Not Sufficient
Thermal Energy to ionize
Dopant or Si
– Extrinsic → n = doping
– Instrinsic → ni > doping
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
FreezeOut etc.
nd = 1015/cc
 Recall Reln for ni
ni  e
 E g kT
Si Dopant Ionization (eV)
 The similar Reln for
(N-Type) dopant
Concentrations
nd  e
 E d kT
Impurity
Donor
Ed
P
0.044
As
0.049
Sb
0.039
Acceptor
Ed
 Neither Si or Dopants are Ionized
– Extrinsic → Ed < kT < Eg
 Only Dopants are (Singly)
ionized and nd >> ni
B
0.045
Al
0.057
EgapMaterials 1.1
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of Engineering
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– FreezeOut → kT << [Eg or Ed]
1.1
– Intrinsic kT>> [Ed or Eg]
 nd fixed at dopant at%,
ni continues to Rise
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
 Carrier Diffusion
• e- Diffuse in to the
P-Type Material
• h+ Diffuse in to the
N-Type Material
• Remember that?
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
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E-Field
 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
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
p-n Junction Rectifier
 A Rectifier is a
“Check Valve” for
Current flow
E-Field
• Current Allowed in
ONE Direction but
NOT the other
 Side Issue →
“Bias” Voltage
• A “Bias” Voltage is
just Another name
for EXTERNALLY
APPLIED Voltage
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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???
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E-Field
 A: the P end
• The Applied Voltage
REDUCES the
internal E-Field; This
“Biases” The Junction
in Favor of
DIFFUSION
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
p-n Junction Rectifier cont.2
 p-n junction 
No Applied Voltage
Xd
• Diffusion & E-Field in
Balance, No Current
Flows
 Reverse Biased
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• Internal Field
ENHANCED
– Carriers Pulled AWAY
from Jcn; xd grows
 Forward Bias
• Internal Field
REDUCED
– Carriers PUSHED and
Diffuse to the Jcn
where they are
“injected” into the other
side; xd Contracts
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Properties of Rectifying Junction
Reverse
Forward
 IN914 PN Diode
• IF = 75 000 µA
• IR = 0.025-50 µA
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Transistors
 Transistors are
“Transfer Resistors”
 Xsistors Have Three
Connections
• Input
• Output
• Control
 In Electronic
Applications
Transistors have
TWO Basic Fcns
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• 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
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
BJT
 The Classic pnp or
npn configurations
• Basically Two pn
jcns back-to-Back
c
b
e
c
b
e
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 npn In “ForwardActive” mode
• b-e pn jcn
FORWARD Biased
• b-c pn jcn
REVERSE Biased
 Very Little “base”
Current
 Large emitter &
collector currents
• Good Current-Driving
Amplifier
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Depletion Mode - JFET
 JFETs are “Normally
On” Transistors
 OPEN “Channel”
Between the
“source” and “drain”
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 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
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
 Back-to-Back pn
Jcns Between
“source” & “drain”
 IGFETs are Great
Switches
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• This will eventually
“invert” a thin region
below the gate to
N-type, creating a
conducting channel
between S & D
• Used in almost all
digital IC’s
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
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
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 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
 tot   eletronic   ionic
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Ionic Mobility
 As in the Electronic
case
 ionic  N I q I
• Where
– NI  Ion Concen,
Ions/m3
– q  electronic Charge
– µI  Ionic Mobility,
m2/V-s
 Two Forces move
The Ions
Engineering-45: Materials of Engineering
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• Diffusion
• E-Field
 Combine These two
effects into Mobility
nI qDI
I 
kT
• Where
– nI  Ion Valence
– DI  Ion Mass
Diffusion Coeff, m2/s
– q, k, T  as Before
• Exercise → Find
units for nBruce
I Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
Energy
 Most Ceramics have
WIDE BandGaps
Conductio
n
Band
• SiO2  9 eV
• Si3N4  4.7eV
 Thus Ceramics Tend to be Very
Good Electrical INSULATORS
GAP
filled states
Ceramics
empty
band
 But as with SemiConductors. for Ceramics
nintrinsic Increases with Temperature
• Thus Insulative Capacity DEGRADES at Hi-T
– e.g; mullite = 3Al2O3•2SiO2
 (25°C)  1012 Ω-m; (500°C)  106 Ω-m
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
filled
Valence
band
filled
band
 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 Chem Nobel for Heeger, MacDiarmid
and Shirakawa
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
http://webpages.charter.net/dmarin/coat/#history
Polymers
Piezoelectric Materials
 Piezoelectricity – application of pressure
produces Electrical Potential
at rest
compression
induces voltage
Engineering-45: Materials of Engineering
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applied voltage
induces
expansion
Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt
WhiteBoard Work
 Problem 18.30
• Arsenic Doped
Germanium
Engineering-45: Materials of Engineering
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Bruce Mayer, PE
[email protected] • ENGR-45_Lec-09_ElectProp-Semi.ppt