Properties-2
Bruce Mayer, PE
Licensed Electrical & Mechanical Engineer
BMayer@ChabotCollege.edu
Engineering-45: Materials of Engineering
1
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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
2
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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
– 10 6 -10 12 times LESS conductive than metals
• INCREASING
Conductivity with
Increasing Temp
– Opposite of
Metal Behavior
Engineering-45: Materials of Engineering
3
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

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
4 produce extra free electrons or holes.
Bruce Mayer, PE Engineering-45: Materials of Engineering
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 σ Data for
Pure Silicon
• Note σ↑ as T↑
Si electrical conductivity, σ
104
103
102
101
100 pure
(undoped)
10-1
5
10-2
50 100
Engineering-45: Materials of Engineering
1000
T(K)
 Why This Temp
Behavior?
• Semiconductor e −
Band Structure
– Thermal Energy
Can Allow the e to jump the
“Forbidden” Gap between the
“Valence” Band and the
“Conduction”
Band
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 Recall Conductivity
Eqn from the Metals
Dicussion
  nq

 Note the Exponential
Increase in the
Intrinsic carrier
Concentration, n i n
 e

E g kT i
 Since µ Does Not or change nearly as much as n i with T
  e

E g kT
Engineering-45: Materials of Engineering
6
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

InSb
Ge
InN
HgCdTe
InGaAs
Silicon
InP
GaAs
CdTe
AlGaAs
InGaP
2
GaAsP
InGaN
0.17
0.67
0.7 eV eV eV
0.0 - 1.5 eV
0.4 - 1.4 eV
1.14 eV
1.34 eV
1.42 eV
1.56 eV
1.42 – 2.16 eV
1.8 eV
1.42-2.26
eV
0.7 - 3.4 eV
AlAs
GaP
AlGaInP
ZnSe
SiC 6H
2.16 eV
2.26 eV
1.91 - 2.52 eV
2.7 eV
3.03 eV
SiC 4H
GaN
3.28
3.37 eV eV
Diamond 5.46 - 6.4 eV
Engineering-45: Materials of Engineering
7
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

−
+
 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
Engineering-45: Materials of Engineering
8
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

−
+
 Schematically valence electron
Si atom electron hole pair creation electron hole pair migration
+ no applied electric field
- + applied electric field
- + applied electric field
 In Metals, only e
−
Participate in Electrical
Conduction (e
− “sea”), But in Semiconductors
HOLES also aid conduction
Engineering-45: Materials of Engineering
9
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt
+

 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 /m 3
– p  hole concentration, h + /m 3
– µ m e
2
 electron mobility,
/V-s
– µ m h
2
 hole mobility,
/V-s
Qty
µe
(m 2 /V-s)
µh
(m 2 /V-s)
Si
0.19
0.05
GaAs CdTe InP
0.88
0.105
0.470
0.04
0.008
0.018
 µ e

(4-30) times
Greater Than µ h
• Why?
– Parking Garage
Analogy
Engineering-45: Materials of Engineering
10
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

Engineering-45: Materials of Engineering
11
+
 n-Type
Semiconductor illustrated in (a) & (c)
 p-Type
Semiconductor illustrated in (b) & (d)
 Thus µ e
>µ h
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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)
  nq
 e
Phosphorus atom
4+ 4+ 4+ 4+
4+ 5+ 4+ 4+ hole conduction electron
4+ 4+ 4+ 4+ valence electron no applied electric field
Engineering-45: Materials of Engineering
Si atom
12
4+
4+
Boron atom
4+ 4+ 4+ 4+
3+
4+
4+
4+
4+
4+
  no applied electric field
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt
pq
 h


  increases w/ Doping
104
103
102
101
0.0052at%B doped
0.0013at%B
100 pure
(undoped)
10-1
10-2
50 100 1000
T(K)
 Reason: imperfection sites lower the activation energy needed to produce mobile e or h +
Engineering-45: Materials of Engineering
13
 N-Type Si, n vs T n d
= 10 21 /m 3
– FreezeOut → Not Sufficient
Thermal Energy to ionize either Dopants or Si
– Extrinsic → n = doping
– Instrinsic → n i
> doping
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 Recall Reln for n i

E kT n
 e g i
 The similar Reln for
(N-Type) dopant
Concentrations n d
 e

E d kT
Impurity Donor E d
Acceptor E a
P
As
0.044
0.049
Sb
B
Al
0.039
0.045
0.057
E gap
1.1
Engineering-45: Materials of Engineering
1.1
14 n d
= 10 15 /cc
– FreezeOut → kT << [E g or E d
]
 Neither Si or Dopants are Ionized
– Extrinsic → E d
< kT < E g
 Only Dopants are (Singly) ionized and n d
>> n i
– Intrinsic kT>> [E d or E g
]
 n d n i fixed at dopant at%, continues to Rise
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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?
Engineering-45: Materials of Engineering
15
 Carrier Diffusion
• e Diffuse in to the
P-Type Material
• h + Diffuse in to the
N-Type Material
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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
Engineering-45: Materials of Engineering
16
E-Field
 For Si the Field-Filled
Depletion Region
• E-Field 
1 MV/m
• Depl Reg Width, x d
= 110 µm
• 
Efld•dx 
0.6-0.7 V
– “built-in” Potential
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 A Rectifier is a
“Check Valve” for
Current flow
• Current Allowed in
ONE Direction but
NOT the other
 Side Issue →
“Bias” Voltage
17
• A “Bias” Voltage is just Another name for EXTERNALLY
APPLIED Voltage
Engineering-45: Materials of Engineering
E-Field
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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 :
18
• For Fwd Bias, Which
End is +; P or N???
Engineering-45: Materials of Engineering
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
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 p-n junction

No Applied Voltage
X d
• Internal Field
ENHANCED
– Carriers Pulled AWAY from Jcn; x d grows
 Forward Bias
• Diffusion & E-Field in
Balance, No Current
Flows
 Reverse Biased
Engineering-45: Materials of Engineering
19
• Internal Field
REDUCED
– Carriers PUSHED and
Diffuse to the Jcn where they are
“injected” into the other side; x d
Contracts
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

20
Reverse Forward
 IN914 PN Diode
• I
F
= 75 000 µA
• I
R
= 0.02550 µA
Engineering-45: Materials of Engineering Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 Transistors are
“Transfer Resistors”
 Xsistors Have Three
Connections
• Input
• Output
• CONTROL
21
 In Electronic
Applications
Transistors have
TWO Basic Fcns
Engineering-45: Materials of Engineering
• 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
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 The Classic pnp or npn configurations
• Basically Two pn jcns Back-to-Back c b e c b e
Engineering-45: Materials of Engineering
22
 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
Amplifier
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 JFETs are “Normally
On” Transistors
 OPEN “Channel”
Between the
“source” and “drain”
Engineering-45: Materials of Engineering
23
 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
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 Insulated Gate Field
Effect Transistors are Normally-Off devices
 Back-to-Back pn
Jcns Between
“source” & “drain”
Engineering-45: Materials of Engineering
24
 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
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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 (SiO
2
)
 The Total σ for

Ionic Materials tot
  eletronic
  ionic
Engineering-45: Materials of Engineering
25
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 As in the Electronic case
  ionic
N
I q

I
• Where
– N
I

Ion Concen,
Ions/m 3
– q  electronic Charge
– µ
I

Ionic Mobility, m 2 /V-s
 Two Forces move
The Ions
Engineering-45: Materials of Engineering
26
• Diffusion
• E-Field
 Combine These two effects into Mobility

I
 n
I qD
I
• Where kT
– n
I

Ion Valence
– D
I

Ion Mass
Diffusion Coeff, m 2 /s
– q, k, T  as Before
• Exercise → Find units for n
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

Energy
Conduction
Band
 Most Ceramics have
WIDE BandGaps
• SiO
2
• Si
3
N
4

9 eV

4.7eV
 Thus Ceramics Tend to be Very
Good Electrical INSULATORS
 But as with SemiConductors. for Ceramics n intrinsic
Increases with Temperature
• Thus Insulative Capacity DEGRADES at Hi-T
– e.g; mullite = 3Al
2
O
3
•2SiO
2
 ρ(25°C) 
10 12 Ω-m; ρ(500°C) 
10 6 Ω-m empty band
GAP filled
Valence band filled band
Engineering-45: Materials of Engineering
27
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt

 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 
10 7 S/m
• About 2X HIGHER than Cu for Conductivity/lb
• Mechanism appears to be SemiConductor-like with a doping Requirement
28
• Discovery of these “synthetic metals” Resulted in the 2000 Chemistry Nobel Prize for Heeger,
MacDiarmid and Shirakawa
Engineering-45: Materials of Engineering Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt



at rest compression induces voltage
Engineering-45: Materials of Engineering
29 applied voltage induces expansion
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt


• Antimony Doped
Germanium
• EXtrinsic form
– All Sb Ionized
• The μ’s:
– μ e
– μ h
= 0.1 m 2 /V·s
= 0.05 m 2 /V·s
Engineering-45: Materials of Engineering
30
Bruce Mayer, PE
BMayer@ChabotCollege.edu • ENGR-45_Lec-09_ElectProp-Semi.ppt