A historical perspective
Germanium
- One of the first materials to receive wide attention for use in simiconductor device
fabrication, but it was rapidly replaced by silicon during the early 1960s.
Silicon
-The dominant material used throughout the integrated-circuit industry today.
-Silicon can easily be oxidized to form silicon dioxide:
high quality insulator
an excellent barrier layer for the selective diffusion steps
-A very abundant element in nature, a low cost, a wider bandgap than germanium,
and can therefore operate at higher temperatures
An Overview of monolithic fabrication process
Basic Process steps:
- Oxidation
Silicon dioxide can be formed by heating a silicon wafer to a high temperature
in the presence of oxygen
44% of the final oxide thick.
An Overview of monolithic fabrication process
Basic Process steps:
- Photolithography
Photolithography includes the overall process of mask fabrication and the process of transferring
patterns from the masks to the surface of the wafer
This process is critical to the production of integrated circuits, and the number of mask steps is
often used as a measure of complexity when comparing fabrication process
- Etching
- Diffusion
Shallow n- and p-type layers are formed by high temperature diffusion of donor or acceptor
impurities into silicon
- Ion Implantation
Shallow n- and p-type layers are formed by ion-implantation, in which the wafer is bombarded
with high-energy donor or acceptor atoms generated in a high voltage particle accelerator
An Overview of monolithic fabrication process
Basic Process steps:-. Evaporation and sputtering
Evaporation : metal films can be deposited through evaporation by heating the metal to its
melting point in a vacuum
Sputtering : metal and insulators may also be deposited by a sputtering
-. Chemical vapor deposition (CVD)
Thin films of silicon nitride, silicon dioxide, and polysilicon can be formed through a CVD
-. Epitaxy
Chemical Vapor-Phase Deposition
Liquid-Phase Epitaxy
Molecular Beam Epitaxy
Introduction to Micro Fabrication
An Overview of CMOS Process Flow
BEOL : Interconnection
Introduction to Micro Fabrication
1. Substrate
• Moderately high resistivity : 25-50Ocm → 1015/cm3
• In order to reproducibly manufacture wells, the background
doping needs to be significantly less than the well doping
• (100) : properties of the Si/SiO2 interface are significantly better
• Intrinsic guttering
2. Active Region Formation
• Make certain that the individual devices are electrically isolated from each other
• LOCOS (LOCal Oxidation of Silicon) / STI (Shallow Trench Isolation)
An Overview of CMOS Process Flow
LOCOS (LOCal Oxidation of Silicon)
An Overview of CMOS Process FlowIntroduction to
Micro Fabrication
2. Active Region Formation (계속) :LOCOS (LOCal Oxidation of Silicon)
Thermal oxidation (thin pad oxide) : protect silicon surface
LPCVD nitride deposition
Active area
Nitride/oxide sandwich etching except active area with hot phosphoric acid
Boron field implantation
Thermal field oxidation
Remove nitride and oxide pad
Si3N4 under tensile stress → compressive stress in Si → defect generation
Si3N4 under tensile stress + SiO2 under compressive stress → stress compensation
Introduction to Micro Fabrication
An Overview of CMOS Process Flow
2. Active Region Formation (계속):STI (Shallow Trench Isolation) The thicknesses of SiO2 and Si3N4 are approximately the same as in the LOCOS. However the
stress-related issues are relaxed because of no long high-Temperature oxidation
After patterning active area
Trench etch(∼0.5µm)
① little undercutting
② a small slope to avoid void
③ rounded top and bottom corners
Liner Oxide (Thermally grow a thin (10-20nm) oxide)
*Why thermal oxide? : better Si/SiO2 interface
help to round the corners (∼1100°C)
Trench deposition (Oxide)
CMP (here, Si3N4 servers as a polishing stop)
Introduction to Micro Fabrication
An Overview of CMOS Process Flow
2. Active Region Formation (계속):STI (Shallow Trench Isolation)
Introduction to Micro Fabrication
3. Well Implant
4. Channel Implant
Introduction to Micro Fabrication
5. Gate Formation
6. LDD Implant
7. Gate Sidewall Formation
8. SD Implant
Introduction to Micro Fabrication
An Overview of CMOS Process Flow9. CVD oxide deposition (ILD)
BPSG
P: protection against mobile ions like Na+
B: reduces the Temperature at which the glass layer “flows”
10. Contact Opening
11. Metal Deposition (Evaporation or Sputtering)-. Blanket deposition of a thin TiN layer or Ti/TiN bilayer (∵good adhesion to the SiO2 and other materials.
Effective barrier layer between the upper metal layers and the lower local interconnect layers)
-. Deposition of a blanket W by CVD
12. Pattern Metal
Introduction to Micro Fabrication
An Overview of CMOS Process Flow Introduction to Micro Fabrication
13. CVD Oxide Passivation layer (Phosphosilicate glass)
The top layer could be either SiO2 or Si3N4 and is designed to provide
some protection for the chip during the mechanical handling, as well as to protect
the chip against ambient contamination (Na+ or K+)
14. Open bonding pads
Table 1.2 A partial list of Semiconductor Material
Elemental semiconductor: group IV (Si, Ge)
Si : the most important of the semiconductors
Compound semicond.: group III & V (or II & VI)
Binary : two elements
Ternary : three elements
GaAs : superior electron transport properties and special optical propertiesex) laser diode, LED, high speed Ics
Table 1.1 A portion of the periodic table showing elements used in Semiconductor Material
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Structure1. Amorphous
2. Polycrystalline : Grain, Grain Boundary
3. Single Crystal
Fig. 1.1 Two dimensional schematics of three general types of solids:
(a) amorphous (b) polycrystalline and (c) single crystal.
Introduction to Micro Fabrication
Silicon : Silicon Material Properties
3-D Unit Cell
1. Simple Cubic (SC)
2. Body-Centered Cubic (BCC)
3. Face-Centered Cubic (FCC)
Introduction to Micro Fabrication
Silicon : Silicon Material Properties
Semiconductor Lattices 1. Diamond structure
2. Zicblend
(a) (b) (c)
Fig 1.14 (a) Diamond Structure (b) A unit cell of the diamond lattice constructed by placing
atoms 1/4,1/4,1/4 from each atom in an fcc (c) top view (along any <100> direction) of
an extended diamond lattice. The colored circles indicate one fcc sublattice and the black
circles indicate the interpenetrating fcc.
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Miller Indices
1. Intercept set in the order x,y,z
2. Invert the intercept values
3. Convert the 1/intercpt set to the smalles possible set of whole numbers
4. Enclose the whole number set in curvilinear brackets
Crystal-lattice plane
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Equivalent faces {hkl} with Crystal Plane (hkl)
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Three lattice planes in a simple cubic lattice
Three lattice direction in a simple cubic lattice
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Miller Indices
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Semiconductor Lattices : 1. Diamond structure
2. Zincblende structure : two different types of atoms in the lattice
Introduction to Micro Fabrication
Silicon : Silicon Material Properties
Imperfections in Solids
1. Lattice vibration
2. Point Defect (vacancy, interstitial, Frenkel defect)
3. Line Defect (line dislocation)
4. Area Defect (Stacking Fault)
결정 구조를 구성하는 원자 층을 각각 A, B, C라고 할 때
ABCABCA : Normal
ABCACBC : “extrinsic” faults
ABCABABC : “intrinsic” faults
5. Volume Defect
-. crucial roles : diffusion, ion implantation
-. lesser roles : oxidation kinetics
*Impurities in Solid
Substitution impurity
Interstitial impurity
Doping ( Diffusion, Ion Implantation)
Refer to Fig. 4.10 (p.44)
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Growth of Semiconductor Material1. Growth from a Melt : Czochralski Method
2. Epitaxial Growth
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Growth of Semiconductor Material1. Growth from a Melt : Czochralski Method
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Growth of Semiconductor Material1. Growth from a Melt : Czochralski Method
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
2.3 Energy-Band Theory
Fig. 2.8 (a) Schematic of an isolated silicon atoms.
(b) The splitting of the 3s and 3p states of silicon into the allowed and forbidden band.
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
2.3 Energy-Band Theory
T=0K T>0K
Fig. 2.9 Fig. 2.10
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The major difference between materials : the magnitude of the energy gap
-. Insulators: wide band gap
-. Metals: very small or no band gap
-. Semiconductors; intermediate case between insulators and metals
Ex. GaAs : EG=1.42 eV
Si : EG=1.12 eV
Ge : EG=0.66 eV
Metal, Insulator, and Semiconductor
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The k-Space Diagram
mk
mpE
22
222 h==
E-k Diagram of Free Electron
Potential function
varies through the crystal
Indirect Band GapDirect Band Gap
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
2.5 Statistical Mechanism고전 통계 물리
1. Maxwell-Boltzmann : Boson의 분포 함수로
여기서 각 입자는 구별 가능하며 각 양자상태에 허용되는 입자의 수는 제한이 없음
2. Bose-Einstein : Boson의 분포 함수로
여기서 각 입자는 구별 불가능하며 각 양자상태에 허용되는 입자의 수는 제한이 없음
3. Fermi-Dirac : fermion(파울리의 배타 원리를 만족시키는 입자)의 분포 함수로
입자는 구별 불가능하며 각 양자상태에 허용되는 입자의 수에 제한이 있음
1
1)(−
=kT
EBEBe
Ef
1
1)(+
=kT
EFDCe
Ef
kTE
MB AeEf −=)(
양자 통계 물리
1)(1)( <<==≈→>>−−− EfAee
CEfeC MB
kTE
kTE
FDkT
E
1)(1)( <<==≈→>>−−− EfAee
BEfeB MB
kTE
kTE
BEkT
E
어떤 에너지상태를 점유할 확률이 1보다 매우 작은 경우
양자 역학적 분포 함수가 고전적인 분포 함수에 접근
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
2.5 Statistical Mechanism
The Fermi-Dirac Distribution Function and the Fermi Level
The Fermi Function f(E) :the probability that an available state at an energy E will be occupied
by an electron under equilibrium conditions
The Fermi-Dirac probability function
versus energy at T=0K
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
2.5 Statistical Mechanism
The Fermi-Dirac Distribution Function and the Fermi Level
Maxwell-Boltzmann
Approximation/
Boltzmann
Approximation
f(E)≈1
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
an Introduction to Semiconductor Devices
3.1 Charge Carriers in Semiconductors
The distribution of electrons in the conduction band Material
The distribution of holes in the valence band Material
)()()( EfEgEn Fc=
( ))(1)()( EfEgEp Fv −=
dEEfEgnEc
Fc∫∞
= )()(0
( )dEEfEgpV
bot
E
EFv∫ −= )(1)(0
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Intrinsic semiconductor : pure (undoped) semiconductor
Efi : The Fermi energy level for the intrinsic semiconductor
]
In intrinsic semiconductor under equilibrium conditions
The electron and hole concentrations are equal.
The Intrinsic Carrier Concentration
( )⎥⎦
⎤⎢⎣
⎡ −−==
kTEENnn Fic
Ci exp0
Si atom density: 5×1022 cm-3,
total bonds: 2×1023 cm-3
ni~ 1.5 × 1010cm-3 @ 300 K
( )⎥⎦
⎤⎢⎣
⎡ −−==
kTEENpp VFi
Vi exp0
Band gap energy
3.1 Charge Carriers in Semiconductors
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.1 Charge Carriers in Semiconductors
The Intrinsic Carrier Concentration
In thermal equilibrium condition,
The Intrinsic Fermi-Level Position
( ) ( )⎥⎦
⎤⎢⎣
⎡ −−=⎥
⎦
⎤⎢⎣
⎡ −−kT
EENkT
EEN VFiV
FiCiC expexp
( ) ( ) ⎟⎟⎠
⎞⎜⎜⎝
⎛++=⎟⎟
⎠
⎞⎜⎜⎝
⎛++= *
*
ln43
21ln
21
21
n
pVC
C
VVCFi m
mkTEE
NNkTEEE
Emidgap Therefore, ⎟⎟⎠
⎞⎜⎜⎝
⎛=− *
*
ln43
n
pmidgapFi m
mkTEE
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.2 Dopant Atoms and Energy Levels
Intrinsic Silicon LatticeSilicon Lattice
Doped with phosphorus
(Donor Impurity)
N-type Silicon
Silicon Lattice
Doped with Boron
(Acceptor Impurity)
P-type Silicon
Extrinsic Semiconductor
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3.3 Carrier Distributions in the Extrinsic Semiconductor
Extrinsic Semiconductor : Terminology
1. Extrinsic semiconductor: doped semiconductor
cf) Intrinsic semiconductor: undoped semiconductor
2. Dopants: specific impurity atoms that are added to semiconductors in controlled amounts
for the purpose of increasing either the electron or the hole concentration
3. Donor: impurity atom that increases the electron concentration
cf) n-type material: a donor-doped material
4. Acceptor: impurity atom that increases the hole concentration
cf) p-type material: a acceptor-doped material
5. Majority carrier: the most abundant carrier in a given semiconductor sample; electron in
an n-type material and hole in a p-type materials
6. Minority carrier: the least abundant carrier in a given semiconductor sample; holes in an
n-type material and electrons in a p-type material
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.3 Carrier Distributions in the Extrinsic Semiconductor
N-type Silicon
( )⎥⎦
⎤⎢⎣
⎡ −−=
kTEENp VF
Vexp0
P-type Silicon
( )⎥⎦
⎤⎢⎣
⎡ −−=
kTEENn Fc
Cexp0
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.3 Carrier Distributions in the Extrinsic Semiconductor
( ) ( ) ( )
( ) ( )
( )⎥⎦
⎤⎢⎣
⎡ −=
⎥⎦
⎤⎢⎣
⎡ −⎥⎦
⎤⎢⎣
⎡ −−=
⎥⎦
⎤⎢⎣
⎡ −+−−=⎥
⎦
⎤⎢⎣
⎡ −−=
kTEEn
kTEE
kTEEN
kTEEEEN
kTEENn
FiFi
FiFFicC
FiFFicC
FcC
exp
expexp
expexp0
( )⎥⎦
⎤⎢⎣
⎡ −−=
kTEEpp FiF
i exp0
In similar way,
Therefore, for the semiconductor in thermal equilibrium
( ) ( ) 200 expexpexp i
gVC
VFFcVC n
kTE
NNkT
EEkT
EENNpn =⎥⎦
⎤⎢⎣
⎡−=⎥
⎦
⎤⎢⎣
⎡ −−⎥⎦
⎤⎢⎣
⎡ −−=
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.3 Carrier Distributions in the Extrinsic Semiconductor
Degerate and Nondegenerate Semmiconductor
Simplified energy-band diagram for degenerately doped (a) n-type and (b) p-type
semiconductors. The Fermi level is in the conductor band and valence band, respectively
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.5 Carrier Concentrations-Effects of Doping
Compensated Semiconductor
-. Contains both donor and acceptor impurity atoms in the same region
-. N-type : Nd > Na
-. P-type : Na > Nd
-. Completely compensated semiconductor : Na=Nd (the same characteristics with intrinsic semiconductor)
Equilibrium Electron and Hole Concentration
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.5 Carrier Concentrations-Effects of Doping
Equilibrium Electron and Hole Concentration
( ) ( )
( )( ) ( ) 2
2
0
20
20
0
2
0
00
00
00
22,
0
min
iadad
iad
i
da
ddaa
da
nNNNNntherefore
nnNNn
nnpwith
NpNn
ionizationcompletegassu
nNppNnor
NpNn
+⎟⎠⎞
⎜⎝⎛ −
+−
=
=−−−
=
+=+
−+=−+
+=+ +−Charge Neutrality Condition
Refer to example 3.10 (p.104)
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.5 Carrier Concentrations-Effects of Doping
Equilibrium Electron and Hole Concentration
( )⎥⎦
⎤⎢⎣
⎡ −−=
kTEENn Fic
Ci exp
A typical majority-carrier concentration versus temperature plot constructed assuming a
phosphorus doped ND=5×1014 cm-3 Si.
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.5 Carrier Concentrations-Effects of Doping
Carrier Concentration Temperature Dependence
1. The increased ionization of dopant sites and the associated increase in the majority
carrier concentration when the temperature of a semiconductor is raised from near T=0 K
toward room temperature
• Below 100 K or so, in the freeze-out temperature region, n drops significantly below ND
and approaches zero as T = 0 K. At temperatures T → 0 K the thermal energy available in
the system is insufficient to release the weakly bound fifth electron on donor sites and
totally insufficient to excite electrons across the band gap
2. In the intrinsic temperature region, n rises above ND, approaching ni with increasing T
3. The wider the band gap, the greater the energy required to excite electrons from the
valence band into the conduction band
Equilibrium Electron and Hole Concentration
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.5 Carrier Concentrations-Effects of Doping
Equilibrium Electron and Hole Concentration (Summary)
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.6 Position of Fermi Energy Level-Effects of Doping and Temperature
( )
⎟⎟⎠
⎞⎜⎜⎝
⎛=−
⎟⎟⎠
⎞⎜⎜⎝
⎛=⎟⎟
⎠
⎞⎜⎜⎝
⎛=−
⎥⎦
⎤⎢⎣
⎡ −−=
−
a
VVF
d
ctypen
cFc
FcC
NNkTEE
waysimilarIn
NNkT
nNkTEE
kTEENnBecause
ln
lnln
exp
0
0
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
3.6 Position of Fermi Energy Level-Effects of Doping and TemperatureFermi level positioning in Si at 300 K as a function of the doping concentration
Fermi level positioning as a function of temperature for various doping concentration
<참고>
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Carrier Drift
-. Charged-particle motion in response to an applied electric field
-. An electric field (E) tends to accelerate the +q charged holes in the direction of the
electric field and the –q charged electrons in the direction opposite to the electric field
-. Repeated periods of acceleration and subsequent decelerating collisions
-. Measurable quantities are macroscopic observables that reflect the average or overall
motion of the carriers
-. The thermal motion of the carriers is completely random and therefore averages out to
zero on a macroscopic scale, does not contribute to current transport
4.1 Carrier Drift
4.1.1 Drift Current Density
Typical random behavior of a hole or
electron in a semiconductor with no
applied electric field
Behavior of a electron in a
semiconductor with applied electric
field
Macroscopic behavior of a hole in a
semiconductor with applied electric field
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an Introduction to Semiconductor Devices
4.1 Carrier Drift
4.1.1 Drift Current Density
Drift Current
the charge per unit time crossing an arbitrarily chose plane of observation oriented normal to
the direction of current flow
driftpdp
CarrierHole
ddrift JvepvJ |)( === ρ
eEamF p == *
Ev pdp µ=
Due to collisions (scattering events)
Average drift velocity
pEevepJ dpdpdriftp µ== )(|Therefore, EnpeJ dndpdrift )( µµ +=
nEevenJ dndndriftn µ== )(|In similar way,
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
Mobility
Mobility is obviously very important parameter in characterizing electron and hole transport due to drift
-. Unit: cm2/Vs
-. Carrier mobility varies inversely with the amount of scattering taking place within the semiconductor
-. Dominant scattering mechanisms:
Lattice scattering involving collisions with thermally agitated lattice atoms: Phonon Scattering
Ionized impurity (donor or acceptor) scattering
-. lower m* gives higher µ
Ex. µn (GaAs) >> µ n (Si))
4.1 Carrier Drift
4.1.2 Mobility Effect
***
ppp m
eEtveEdtdvmamF =→===
***| 21
p
cpdpp
p
cpd
velocitydriftaverage
p
cppeakd m
eEv
Eme
vEme
vτ
µττ
==→=→=
where τcpis the mean free time between collisions
Considering collisions,
*n
cndnn m
eEv τµ ==
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4.1 Carrier Drift
4.1.2 Mobility Effect : Phonon Scattering
For Phonon Scattering
23−∝ TLµ
Figure. 4.2
The temperature dependence of electron mobilities
in silicon. In lightly doped semiconductors, lattice
scattering dominates.
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4.1.2 Mobility Effect : Ionized Impurity Scattering
4.1 Carrier Drift
For ionized impurity scattering
II N
T 23
∝µ
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4.1 Carrier Drift
4.1.2 Mobility Effect : Total Mobility
Doping dependence: At low doping concentrations, below approximately 1015 cm-3 in Si, the carrier
mobilities are essentially independent of the doping concentration. For dopings in excess of ~1015 cm-3, the
mobilities monotonically decrease with increasing NA or ND
Temperature dependence: Decreasing system temperature causes an ever-decreasing thermal agitation
of the semiconductor atoms, which in turn decreases the lattice scattering. Ionized impurities become more
and more effective in deflecting the charged carriers as the temperature and hence the speed of the carriers
decreases
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4.1 Carrier Drift
4.1.3 Semiconductor Conductivity and Resistivity
Resistivity
Resistivity (ρ) is defined as the proportionality constant between the electric field impressed across
a homogeneous material and the total particle current per unit area
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4.1 Carrier Drift
4.1.3 Semiconductor Conductivity and Resistivity
Resistivity
an Introduction to an Introduction to Semiconductor DevicesSemiconductor Devices
4.1 Carrier Drift
The vd is proportional to E at low electric fields, while at high electric fields vd
saturates and becomes independent of E
4.1.4 Velocity Saturation
Ev pdp µ=
uo
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4.2 Carrier Diffusion
Diffusion
-. Diffusion is a process whereby particles tend to spread out or redistribute as a result of
their random thermal motion, migrating on a macroscopic scale from regions of high particle
concentration into region of low particle concentration
-. On a macroscopic scale the net effect of diffusion is precisely the same within both the
hypothetical system and semiconductors; there is an overall migration of particles from
regions of high particle concentration to regions of low particle concentration
-. Within semiconductors the mobile particles – the electrons and holes – are charged, and
diffusion related carrier transport gives rise to particle currents
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4.2 Carrier Diffusion
Using Fick’s law,
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4.2 Carrier Diffusion
Total Current Density
Total Particle Current Density
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4.3 Graded Impurity Distribution
Relating Diffusion coefficients/Mobilities
-. For interrelation of the D’s and the m’s, it is necessary to establish the
connecting formula known as the Einstein relationship
-. In deriving the Einstein relationship, we consider a nonuniformly doped
semiconductor maintained under equilibrium conditions
-. Constancy of the Fermi Level: nonuniformly doped n-type semiconductor as an
example dEF/ dx =dEF /dy= dEF /dz =0
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Current flow under equilibrium conditions: the total current is identically zero
Einstein relationship: Under equilibrium conditions, and focusing on the electrons,
4.3 Graded Impurity Distribution
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With dEF/dx=0,
4.3 Graded Impurity Distribution
Substituting
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4.4 Carrier Generation and Recombination
-. Recombination-generation (R-G) is nature’s order-restoring mechanism, the
means whereby the carrier excess or deficit inside the semiconductor is stabilized
Recombination: a process whereby electrons and holes (carriers)
are annihilated or destroyed
Generation: a process whereby electrons and holes are created
*Generation Processes
1. light with an energy > EG → photogeneration
2. thermal energy > EG → direct thermal generation
3. The thermally assisted generation of carriers with R-G centers
4. Impact ionization (the inverse of Auger recombination)
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4.4 Carrier Generation and Recombination
1. Band-to-band Recombination
The direct annihilation of a conduction band electron and a valence band hole
→ the production of a photon (light)
Near-midgap energy levels
introduced by some common
impurities in Si