Transcript Lecture5.0.ppt

Lecture 5.0
Properties of Semiconductors
Importance to Silicon Chips

Size of devices
– Doping thickness/size
– Depletion Zone Size
– Electron Tunneling dimension

Chip Cooling- Device Density
– Heat Capacity
– Thermal Conductivity
Band theory of Semiconductors

Forbidden Zone – ENERGY GAP
Conduction
Band
Valence
Band
Silicon Band Structure - [Ne]3s23p2
Fermi-Dirac Probability
Distribution for electron energy, E
Probability, F(E)=
(e{[E-Ef]/kBT}+1)-1
–Ef is the
Fermi Energy
Number of Occupied States
Density of States
Fermi-Dirac
T>0
Difference between
Semiconductors and Insulators
Material
InSb
InAs
Eg(eV)
0.18
0.36
Ge
Si
GaAs
SiC
0.67
1.12
1.43
2.3
ZnS
NiO
Al2O3
3.6
4.2
8
kBT =0.0257 eV
at 298˚K
Probability of electrons in
Conduction Band

Lowest Energy in CB
 E-Ef  Eg/2
 Probability in CB
 F(E)= (exp{[E-Ef]/kBT} +1)-1 )
 = (exp{Eg/2kBT} +1)-1
  exp{-Eg/2kBT} for Eg>1 eV @ 298K
kBT =0.0257 eV
at 298˚K
Variation of Conductivity with T
=d/dT
Intrinsic Conductivity of
Semiconductor

Charge Carriers
– Electrons
– Holes
= ne e e + nh e h
 # electrons = # holes

–   ne e (e+ h)
– ne  C exp{-Eg/2kBT}
ne=2(2 m*e kBT/h2)3/2 exp(-Eg /(2kBT))
Ef=Eg/2+3/4kBT ln(m*h/m*e)
Mobilities
Semiconductor Photoelectric Effect

Light Absorption/Light Emission
(photodetector)/(photo diode laser)

Absorption max =hc/Eg
Light Emitting Diode
Photodiode Laser
Color depends on band gap, Eg
  =hc/Eg

Eg>3.0 transparent
Pb
0.37 0.27 0.33 IR
detectors
Diode Laser
Extrinsic Conductivity of Semiconductor
Donor Doping
 n-type

N=nd+ni
Acceptor Doping
p-type
p= 2(2 m*h kBT/h2)3/2 exp(-Ef/kBT)
Law of Mass Action, Nipi=ndpd or =nndn
Extrinsic Conductivity of Semiconductor

Donor Doping
Acceptor Doping
Electron Density

Dopant
Concentration
effects
 Electron
Density
 Electrical
Conductivity
Conductivity

Intrinsic Range
– Exponential with T

Extrinsic Range
– Promoted to CB
• 
– Decreasing , 
– Joins Intrinsic  

Majority/minority Carriers

= ne e e + nh e h
Majority/minority Carriers
Conductivity
 = ne e e + nh e h
 n-type ne>>nh
 Low number of holes due to
recombination.
 Law of Mass Action

– Nipi=ndpd
– (For p-type Nipi =nndn )
Extrinsic Conductivity of Semiconductor
Donor Doping
 n-type

Ed = -m*e e4/(8 (o)2 h2)
Ef=Eg-Ed/2
Acceptor Doping
p-type
Ef=Eg+Ea/2
Effective Mass
Holes
 Electrons

Wafer Sales

Following PRIME GRADE Si wafers are all
single-side polish $14.50 each for 25
wafers each $11.00 for 50 or more (we can
double side polish)
–
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–
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
4" P<111>
4" N<100>
4" P<111>
4" P<100>
4" P<111>
3.0-6.6 ohm-cm
4.0-6.0 ohm-cm
7.0-21.6 ohm-cm
12.0-16.0 ohm-cm
3.0-5.0 ohm-cm
http://www.collegewafer.com/
GaP Wafer
2" Undoped (100) $180.00 each
2" S doped (111) $180.00 each
C&ENews
1/6/03