Transcript Chapter 1 Semiconductors: A General Introduction

Semiconductor Device Physics
Lecture 1
Dr.-Ing. Erwin Sitompul
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Textbook and Syllabus
Textbook:
“Semiconductor Device Fundamentals”,
Robert F. Pierret, International Edition,
Addison Wesley, 1996.
Syllabus:
Chapter 1: Semiconductors: A General Introduction
Chapter 2: Carrier Modeling
Chapter 3: Carrier Action
Chapter 5: pn Junction Electrostatics
Chapter 6: pn Junction Diode: I–V Characteristics
Chapter 7: pn Junction Diode: Small-Signal Admittance
Chapter 8: pn Junction Diode: Transient Response
Chapter 14: MS Contacts and Schottky Diodes
Chapter 9: Optoelectronic Diodes
Chapter 10: BJT Fundamentals
Chapter 11: BJT Static Characteristics
Chapter 12: BJT Dynamic Response Modeling
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Final Grade = 10% Homework + 20% Quizzes +
30% Midterm Exam + 40% Final Exam +
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Grade Policy
Semiconductor Device Physics
Homework 2
Ito Chen
009201700008
21 March 2021
D6.2. Answer: . . . . . . . .
• Heading of Homework Papers (Required)
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Greek Alphabet
—new
—zz-eye
—pie
—taw
—fie
—k-eye
—sigh
—mew
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Chapter 1
Semiconductors: A General Introduction
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Chapter 1
Semiconductors: A General Introduction
What is a Semiconductor?
 Low resistivity
 “conductor”
 High resistivity
 “insulator”
 Intermediate resistivity  “semiconductor”
 The conductivity (and at the same time the resistivity) of
semiconductors lie between that of conductors and insulators.
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Chapter 1
Semiconductors: A General Introduction
What is a Semiconductor?
 Semiconductors are some of the purest solid materials in
existence, because any trace of impurity atoms called “dopants”
can change the electrical properties of semiconductors
drastically.
 Unintentional impurity level:
1 impurity atom per 109 semiconductor atom.
 Intentional impurity ranging from 1 per 108 to 1 per 103.
No recognizable
long-range order
Entire solid is made up of
atoms in an orderly
three- dimensional array
Completely ordered
in segments
polycrystalline amorphous crystalline
 Most devices fabricated today employ crystalline
semiconductors.
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Chapter 1
Semiconductors: A General Introduction
Semiconductor Materials
Elemental:
Si, Ge, C
Compound:
IV-IV
III-V
II-VI
Alloy:
Si1-xGex
AlxGa1-xAs
As
Cd
Se
Ga
SiC
GaAs, GaN
CdSe
: Arsenic
: Cadmium
: Selenium
: Gallium
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Chapter 1
Semiconductors: A General Introduction
From Hydrogen to Silicon
# of Electrons
1
2
3
Z Name 1s 2s 2p 3s 3p 3d
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Notation
1
1H
1
1s
2 He
2
1s 2
3 Li
2
1
1s 2 2s 1
4 Be
2
2
1s 2 2s 2
5B
2
2
1
1s 2 2s 2 2p1
6C
2
2
2
1s 2 2s 2 2p2
7N
2
2
3
1s 2 2s 2 2p3
8O
2
2
4
1s 2 2s 2 2p4
9F
2
2
5
1s 2 2s 2 2p5
10 Ne
2
2
6
1s 2 2s 2 2p6
11 Na
2
2
6
1
1s 2 2s 2 2p6 3s 1
12 Mg
2
2
6
2
1s 2 2s 2 2p6 3s 2
13 Al
2
2
6
2
1
1s 2 2s 2 2p6 3s 2 3p1
14 Si
2
2
6
2
2
1s 2 2s 2 2p6 3s 2 3p2
15 P
2
2
6
2
3
1s 2 2s 2 2p6 3s 2 3p3
16 S
2
2
6
2
4
1s 2 2s 2 2p6 3s 2 3p4
17 Cl
2
2
6
2
5
1s 2 2s 2 2p6 3s 2 3p5
18 Ar
2
2
6
2
6
1s 2 2s 2 2p6 3s 2 3p6
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Chapter 1
Semiconductors: A General Introduction
The Silicon Atom
 14 electrons occupying the first 3 energy levels:
 1s, 2s, 2p orbitals are filled by 10 electrons.
 3s, 3p orbitals filled by 4 electrons.
 To minimize the overall energy, the 3s and
3p orbitals hybridize to form four
tetrahedral 3sp orbital.
 Each has one electron and is capable of
forming a bond with a neighboring atom.
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Chapter 1
Semiconductors: A General Introduction
The Si Crystal
• Each Si atom has 4 nearest
neighbors.
• Atom lattice constant
(length of the unit cell side)
° 1A=10
°
–10m
a = 5.431A,
• Each cell contains:
8 corner atoms
6 face atoms
4 interior atoms
a
“Diamond Lattice”
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Chapter 1
Semiconductors: A General Introduction
How Many Silicon Atoms per cm–3?
 Number of atoms in a unit cell:
 4 atoms completely inside cell
 Each of the 8 atoms on corners are shared among 8 cells
 count as 1 atom inside cell
 Each of the 6 atoms on the faces are shared among 2 cells
 count as 3 atoms inside cell
Total number inside the cell = 4 + 1 + 3 = 8
 Cell volume = (.543 nm)3 = 1.6 x 10–22 cm3
 Density of silicon atom
= (8 atoms) / (cell volume)
= 5 × 1022 atoms/cm3
• What is density of
silicon in g/cm3?
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Chapter 1
Semiconductors: A General Introduction
Compound Semiconductors
 “Zincblende” structure
 III-V compound semiconductors: GaAs, GaP, GaN, etc.
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Chapter 1
Semiconductors: A General Introduction
Crystallographic Notation
Miller Indices
Notation
Interpretation
(hkl)
crystal plane
{hkl}
equivalent planes
[hkl]
crystal direction
<hkl>
equivalent directions
h: inverse x-intercept of plane
k: inverse y-intercept of plane
l: inverse z-intercept of plane
(h, k and l are reduced to 3
integers having the same ratio.)
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Chapter 1
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Crystallographic Planes
(632) plane
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(221) plane
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Chapter 1
Semiconductors: A General Introduction
Crystallographic Planes
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Chapter 1
Semiconductors: A General Introduction
Crystallographic Planes of Si Wafers
 Silicon wafers are usually cut along a {100} plane with a flat or
notch to orient the wafer during integrated-circuit fabrication.
 The facing surface is polished and etched yielding mirror-like
finish.
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Chapter 1
Semiconductors: A General Introduction
Crystal Growth Until Device Fabrication
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Chapter 1
Semiconductors: A General Introduction
Crystallographic Planes of Si
Unit cell:
View in <111> direction
View in <100> direction
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View in <110> direction
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Chapter 2
Carrier Modeling
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Chapter 2
Carrier Modeling
Electronic Properties of Si
 Silicon is a semiconductor material.
 Pure Si has a relatively high electrical resistivity at room
temperature.
 There are 2 types of mobile charge-carriers in Si:
 Conduction electrons are negatively charged,
e = –1.602  10–19 C
 Holes are positively charged,
p = +1.602  10–19 C
 The concentration (number of atom/cm3) of conduction
electrons & holes in a semiconductor can be influenced in
several ways:
 Adding special impurity atoms (dopants)
 Applying an electric field
 Changing the temperature
 Irradiation
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Chapter 2
Carrier Modeling
Bond Model of Electrons and Holes
 2-D Representation
Si
Si
Si
Si
Si
Si
Si
Si
Si
Hole
 When an electron breaks
loose and becomes a
conduction electron, then a
hole is created.
Si
Si
Si
Si
Si
Si
Si
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Si
Si
Conduction
electron
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Chapter 2
Carrier Modeling
What is a Hole?
 A hole is a positive charge associated with a half-filled covalent
bond.
 A hole is treated as a positively charged mobile particle in the
semiconductor.
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
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
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




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Chapter 2
Carrier Modeling
Conduction Electron and Hole of Pure Si
• Covalent (shared e–) bonds exists
between Si atoms in a crystal.
• Since the e– are loosely bound,
some will be free at any T,
creating hole-electron pairs.
ni = intrinsic carrier
concentration
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ni ≈ 1010 cm–3 at room temperature
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Chapter 2
Carrier Modeling
Si: From Atom to Crystal
Energy states
(in Si atom)
Energy bands
(in Si crystal)
• The highest mostly-filled
band is the valence band.
• The lowest mostly-empty
band is the conduction band.
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Chapter 2
Carrier Modeling
Energy Band Diagram
Electron energy
Ec
EG, band gap energy
Ev
• For Silicon at 300 K, EG = 1.12 eV
• 1 eV = 1.6 x 10–19 J
 Simplified version of energy band model, indicating:
 Lowest possible conduction band energy (Ec)
 Highest possible valence band energy (Ev)
 Ec and Ev are separated by the band gap energy EG.
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Chapter 2
Carrier Modeling
Measuring Band Gap Energy
 EG can be determined from the minimum energy (hn) of photons
that can be absorbed by the semiconductor.
 This amount of energy equals the energy required to move a
single electron from valence band to conduction band.
Electron
Ec
Photon
photon energy: hn = EG
Ev
Hole
Band gap energies
Semiconductor
Band gap (eV)
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Ge
0.66
Si
1.12
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GaAs
1.42
Diamond
6.0
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Chapter 2
Carrier Modeling
Carriers
 Completely filled or empty bands do not allow current flow,
because no carriers available.
 Broken covalent bonds produce carriers (electrons and holes)
and make current flow possible.
 The excited electron moves from valence band to conduction
band.
 Conduction band is not completely empty anymore.
 Valence band is not completely filled anymore.
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Chapter 2
Carrier Modeling
Band Gap and Material Classification
Ec
Ev
Ec
EG= ~8 eV
Ec
Ev
SiO2
EG = 1.12 eV
Si
Ec
Ev
Ev
Metal
 Insulators have large band gap EG.
 Semiconductors have relatively small band gap EG.
 Metals have very narrow band gap EG .
 Even, in some cases conduction band is partially filled,
Ev > Ec.
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Chapter 2
Carrier Modeling
Carrier Numbers in Intrinsic Material
 More new notations are presented now:
 n : number of electrons/cm3
 p : number of holes/cm3
 ni : intrinsic carrier concentration
 In a pure semiconductor, n = p = ni.
 At room temperature,
ni = 2  106 /cm3 in GaAs
ni = 1  1010 /cm3 in Si
ni = 2  1013 /cm3 in Ge
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