Characteristics of Semiconductor Materials - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch2...
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1 TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials Characteristics of Semiconductor Materials Chapter 2 : Semiconductor Manufacturing Technology by M. Quirk and J. Serda Chapter 3.1 and 3.2 : Semiconductor Science by Tudor E. Jenkins Saroj Kumar Patra, Department of Electronics and Telecommunication, Norwegian University of Science and Technology ( NTNU )
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Transcript of Characteristics of Semiconductor Materials - NTNUfolk.ntnu.no/jonathrg/fag/TFE4180/slides/Ch2...
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TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Characteristics of Semiconductor MaterialsChapter 2 : Semiconductor Manufacturing Technology by M. Quirk and J. Serda
Chapter 3.1 and 3.2 : Semiconductor Science by Tudor E. Jenkins
Saroj Kumar Patra,Department of Electronics and Telecommunication,
Norwegian University of Science and Technology ( NTNU )
2
Electron Shells in Atoms
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Figure 2.2 Quirk & Serda
K = 2
L = 8
M = 18
N = 32
O = 32
P = 10
Q = 2
3
Elementary Model of Carbon Atom
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Carbon atom: The nucleus contains an equal number of protons (+) and neutrons (6 each). Six electrons (-) orbit around the nucleus.
Valence electron
Electron(negative charge)
Neutron(neutral charge)
Atomic number(number of protons)
Nucleus (center of atom; contains protons and neutrons)
Orbital shell
Proton(positive charge)
Valence shell (outer shell of atom)
C 6
+
+NN+
N N
++
N+
N
-
-
-
-
-
-
Figure 2.1 Quirk & Serda
4
Group IV A Elemental semiconductors
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Semiconductors
Group IVA
C, Carbon 6
Si, Silicon 14
Ge, Germanium 32
Sn, Tin 50
Pb, Lead 82
Figure 2.18 Quirk & Serda
5
Covalent Bonding of Pure Silicon
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Si
Si SiSi
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Silicon atoms share valence electrons to form insulator-like bonds.
Figure 2.19 Quirk & Serda
6
Crystal Structure of Si and GaAs
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
7
Crystal Structure
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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Classifying Materials
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
• Conductors
• Insulators
• Semiconductors
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Energy Bandgaps
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Semiconductor
Conduction Band
Valence Band
Energy Gap
Electron Energy
Conduction Band
Valence Band
Insulator
Energy Gap
Electron Energy
Conduction Band
Valence Band
Overlapping bands - little energy is needed for conduction
Conductor
Electron Energy
Figure 2.4 Quirk & Serda
10
Energy Band Diagram for III-V Semiconductor
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
11
Covalent Bonding in Pure Silicon
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Si
Si SiSi
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si Si
Si
Si
Si
Si
Silicon atoms share valence electrons to form insulator-like bonds.
Figure 2.19 Quirk & Serda
12
Doping of Silicon
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
PSi Si
Si Si
Si Si PP
PP
dopant dispenser
wafer
dopant layerdiffusion of dopant
atoms through silicon
Deposition Step Drive-in & Diffusion Step
wafer substrate ActivationStep
Figure 2.21 Quirk & Serda
13
Silicon Dopants
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Group III (p-type)
Boron 5
Aluminum 13
Gallium 31
Indium 49
Group IV
Carbon 6
Silicon 14
Germanium 32
Tin 50
Group V (n-type)
Nitrogen 7
Phosphorus 15
Arsenic 33
Antimony 51
Acceptor Impurities Donor ImpuritiesSemiconductor
* Items underlined are the most commonly used in silicon-based IC manufacturing.
Figure 2.22 Quirk & Serda
14
Electrons in n-type Silicon with Phosphorus Dopants
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Donor atoms provide excess electrons to form n-type silicon.
Phosphorus atom serves as n-type dopant
Excess electron (-)
Si
Si Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
P
P
P
Figure 2.23 Quirk & Serda
15
Conduction in n-type Silicon
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Free electrons flow toward positive terminal.
Positive terminal from power supply
Negative terminal from power supply
Figure 2.24 Quirk & Serda
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Holes in p-type Silicon with Boron Dopant
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Acceptor atoms provide a deficiency of electrons to form p-type silicon.
+ Hole
Boron atom serves as p-type dopant
Si
Si Si
Si
Si
Si
Si Si
Si
Si
Si
Si
Si
Si
Si Si
B Si
SiSi
Si
Si
Si
B
B
Figure 2.25 Quirk & Serda
17
Conduction in p-type Silicon
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Positive terminal from voltage supply
Negative terminal from voltage supply
+Holes flow toward negative terminal
-Electrons flow toward positive terminal
Figure 2.26 Quirk & Serda
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Impurity States
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
p-typen-type
19
Density of Impurity States (for amorphous semiconductor)
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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Density of Electrons and Hole in a Semiconductor
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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How Size affects Resistance
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
High ResistanceLow Resistance
LAR =
Figure 2.12 Quirk & Serda
22
Electrical Conductivity and Mobility
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
• The simplest picture of electrical conductivity and mobility can be understood byconsidering the electrons in a semiconductor as a classical gas in the body of thematerial.
• Then using the Maxwell-Boltzmann distribution function we can get the relation,
• At a temperature of 300 K and using free electron mass for ‘m’, we have
• The random nature of electron velocities means that the time average current that flowsis zero.
• On application of an electric field to the semiconductor, the electrons will drift in theopposite direction of the field, so that there is now a net flow of charge and hence a flowof current. Therefore the equation of motion using drift velocity vd can be written as:
τ
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Electrical Conductivity and Mobility(continued)
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
• The second term in the equation is introduced to prevent ever increasing electronvelocity with the electric field on. Therefore electrons are accelerated until a time τ andthen suffer scattering within the system and their velocity randomized.
• In the absence of Electric field the equation becomes:
τ
τ
• The relaxation time represents the tendency of scattering in the semiconductor to returnthe electron distribution to thermal equilibrium.
• Therefore in the steady state the ‘d/dt’ term will not be there.
• The solution can be written as:
τ
τ
24
Electrical Conductivity and Mobility(continued)
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
• The current density ‘J’ is therefore given as:
• The mobility is expressed as :
• Where ‘n’ is the number of electrons per unit volume of the semiconductor. This isOhm’s Law
τ
τσ
στ
Or Electrical Conductivity
μ| || |
τMobility
25
Silicon Resistivity vs. Dopant Concentration
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Redrawn from VLSI Fabrication Principles, Silicon and Gallium Arsenide, John Wiley & Sons, Inc.
Dop
ant C
once
ntra
tion
(ato
ms/
cm3 )
Electrical Resistivity (ohm-cm)
1021
1020
1019
1018
1017
1016
1015
1014
101310-3 10-2 10-1 100 101 102 103
n-type p-type
Figure 2.27 Quirk & Serda
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Cross-section of planar pn-junction
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
p-type Si n-type Si
Figure 2.28 Quirk & Serda
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Flow of Free Electrons in Copper
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Copper atom
One electron in the valence ring
-
-
-
-
-
-
- -
- -
- -
--
-Cu 29Cu 29
-
-
-
-
-
-
--
- -
-
-
--
K
LMN
Shell #KLMN
Total #
Maximum # e- per shell
281832 60
Actual # e- per shell
28181 29
Figure 2.11 Quirk & Serda
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Alternative Semiconductor Materials
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
Comparison of Some Physical Properties for Semiconductor MaterialsProperty Si Ge GaAs SiO2
Melting point(C) 1412 937 1238
1700(approx.)
Atomic Weight 28.09 72.60 144.63 60.08Atomic Density(atoms/cm3) 4.99 x1022 4.42x1022 2.21x1022 2.3x1022
Energy BandGap (eV) 1.11 0.67 1.40
8(approx.)
Table 2.3 Quirk & Serda
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Resistivity vs. Impurity Concentration for Si and GaAs
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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Electronic Properties of some Semiconductors
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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Physical Constants
TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
32
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TFE4180 Semiconductor Manufacturing Technology, Characteristics of Semiconductor Materials
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