Semiconductor Devices and
Nanoelectronics
Qiliang Li
Dept. of Electrical and Computer Engineering
George Mason University, Fairfax, VA
qli6@gmu.edu
Email: qli6@gmu.edu 1
Content Outline
Semiconductor materials and the carriers in semiconductors;
Semiconductor fabrication and devices physics for pn junction, metal-semiconductor junction and MOS structure;
MOSFET, its basic circuits (inverter, NAND and NOR logic) and memory devices (Flash, SRAM, DRAM and other NVM)
Concepts in Nanoelectronics
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What is semiconductors?
Their electrical conductivity is between that of metals (e.g., Al, Au, ) and insulators (e.g.,
SiO2, Al
2O3
and HfO2);
Semiconductors are the foundation of modern electronic circuits
Important concepts: pn junction, transistor (BJT and MOSFET), solar cell, Light-emitting diode,
digital and analog integrated circuits
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The Common Semiconductors
Conventional semiconductors: Silicon (Si), germanium (Ge), GaAs, GaN, SiC
One dimensional semiconductor: nanowires and nanotubes
Two-dimensional semiconductors, e.g., MoS2
we are always looking for new functional semiconductor materials
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Chapter 1. Electrons and Holes in
Semiconductors
1.1 Si Crystal Structure
Unit cell of Si is cubic
Each Si atom has 4 nearest neighbors
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5.3 A
1.2 Bond Model of electrons and holes
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Si Si Si
Si Si Si
Si Si Si
Si Si Si
Si Si Si
Si Si Si
Si Si Si
Si Si
Si Si Si
Si Si Si
Si Si
Si Si Si
As B
Intrinsic Si
Doped Si
As: group V
B: group III
EION
= 50 mV
Very low
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1.3 Energy Band Model
2p
2s
(a) (b)
conduction band)(
(valence band)
Filled lower bands
} Empty upper bands
}
3P
3S
The highest filled band is the valence band
The lowest empty band is the conduction band
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1.3 Energy Band Model
Conduction band Ec
Ev
Eg
Band gap
Valence band
Energy band diagram shows the bottom edge of
conduction band, Ec
, and top edge of valence band, Ev
.
Ec
and Ev
are separated by the band gap energy, Eg
.
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1.4 Energy Band structure
Si band structure
Indirect band gap
6 minimum at
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1.4 Energy Band structure
Ge band structure
Indirect band gap
8 minimum at
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1.4 Energy Band structure
GaAs band structure
Direct band gap
Common methods:
Slater-Koster tight-binding method Semi Empirical extended Huckel method
(using Huckel molecular orbital theory)
Density functional theory (DFT) Local-Density Approximation (LDA) method
Density functional theory (DFT) Generalized Gradient Approximations (GGA) method
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1.5 Calculate the band structure
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1.5 Calculate the band structure
Use MoS2 monolayer as example:
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1.5 Calculate the band structure
We used Virtual Nanolab ATK software to calculate it.
Welcome collaboration on the research!
MoS2 band structure
calculated by using
DFT-GGA method
Direct band gap
Eg = 1.79 eV
Effective mass:
ml = 0.59 m0
mt = 0.50 m0
mdos = (6)2/3(mlmtmt)
1/3
= 1.75 m0
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Chapter 2. Device Fabrication and Physics
2.1 Device
Fabrication
Technology
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2.1 Device Fabrication Technology
VLSI (Very Large Scale Integration)ULSI (Ultra Large Scale Integration)GSI (Giga-Scale Integration)
Variations of this versatile technology are
used for flat-panel displays, micro-electro-
mechanical systems (MEMS), and chips for
DNA screening...
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2.1 Device Fabrication Technology
Wafer
Oxidation
Lithography
Etching
Annealing &Diffusion
AlSputtering
(0)
Positive resist SiO2
P-Si
P-Si
SiO2P-Si
Mask
UV
SiO2 SiO2P-Si
(1)
(2)
(3)
SiO2
UV
Lithography
SiO2 SiO2
SiO 2 SiO2
PN+
SiO2 SiO2
PN+
P-Si
SiO2 SiO2
PN+
Mask
Al Res is t
(4)
Arsenic implantation
Al
UV
(7)
(5)
(6)
Al
UV
Ion
Implantation
* An example from Modern Semiconductor Devices for Integrated Circuits (C. Hu)
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Metal etching
CVDnitridedeposition
Lithographyand etching
Back Side milling
Back side metallization
Dicing, wire bonding,and packaging
SiO2 SiO2
PN+
(8)
(9)
SiO2 SiO2
PN+
SiO2 SiO2
PN+
(10)
SiO2 SiO 2
PN
+
(11)
Al
Si3N
4
Si3N
4
Si3N
4
Al
Al
Al
Photoresist
SiO2 SiO 2
PN+
(12)
SiO2 SiO2
PN
+
(13)
Si3N
4
Si3N
4
Al
Al
Au
Au
wire
Plastic package
metal leads
2.1 Device Fabrication Technology
* An example from Modern Semiconductor Devices for Integrated Circuits (C. Hu)
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2.2 pn Junction
On the P-side of the depletion layer, = qNa
On the N-side, = qNd
s
aqN
dx
d
=E
)()(1
xxqN
CxqN
xP
s
a
s
a =+=E
)()(N
s
d xxqN
x -=E
N P
Depletion Layer Neutral Region
xn
0 xp
x x
p xn
qNd
qNa
x
E
xn xp
0
Neutral Region
N
N
N P
P
P
Electric Field
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2.2 pn Junction
On the P-side,
Arbitrarily choose the voltage at x = xP as V = 0.
On the N-side,
2)(2
)( xxqN
xV Ps
a =
2)(2
)( Ns
d xxqN
DxV =
2)(2 Ns
dbi xx
qN =
x
E
xn xp
Ec
Ef Ev
bi, built-in potential
0
xn
xp
x
biV
N
N
P
P
Electric Field and potential
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V is continuous at x = 0
If Na >> Nd , as in a P+N junction,
What about a N+P junction?
wheredensity dopant lighterNNN ad
1111 +=
+==
da
bisdepNP NNq
Wxx112
Nd
bisdep xqN
W = 2
qNW bisdep 2=
N P
Depletion Layer Neutral Region
xn
0 xp
Neutral Region
0=adNP
NNxx| | | |
PN
2.2 pn JunctionDepletion layer width
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(b) reverse-biased
qV
Ec
EcEfn
Ev
qbi + qV EfpEv
2.2 pn Junction
qN
barrier potential
qN
VW srbisdep
=+= 2|)|(2
+ V
N P
Reverse-Biased
dep
sdep W
AC=Reverse biased PN
junction is a capacitor.
222
2
2
)(21
AqN
V
A
W
C S
bi
s
dep
dep
+==
Vr
1/Cdep
2
Increasing reverse bias
Slope = 2/qNsA2
bi
Capacitance data
How to minimize the
junction capacitance?
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2.2 pn Junction - breakdown2/1
|)|(2
)0(
+==
rbis
p VqN EEEEEEEE
Peak electric field and
breakdown voltage: bi
critsB
qNV
=2
2EEEE
Empty StatesFilled States -
Ev
Ec
V/cm106=
critp EE
peG J / H=
Tunneling Breaking
EcEfn
Ec
Ev
Efp
originalelectron
electron-holepair generation
Impack ionization avalanche breakdown
daB N
1
N
1
N
1V +=Basis for tunneling FET for smaller
subthreshold swing
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2.2 pn Junction forward bias
Minority carrier injection
)1()()( 00 =kTqV
PPPP ennxnxn
)1()()( 00 =kTqV
NNNN eppxpxp
( )P
LxxkTqVP xxeenxn
nP = ,)1()( //0
L: diffusion length ~ 10 um, depending on N
xJ
enL
Dqp
L
DqxJxJ kTVqP
n
nN
p
pPnPNpN
allat
)1()()(current Total 00
=
+=+=
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2.2 pn Junction Solar Cell
)/ln( 2idpoc nGNq
kTV =
GAqLAJI ppsc == )0(
NP+
Isc
0x
* Modern Semiconductor Devices for Integrated Circuits (C. Hu)
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2.2 pn Junction LED
)(
24.1
energy photon
24.1 m) ( h wavelengtLED
eVEg=
Direct band gap
Example: GaAs
Direct
recombination is
efficient as k
conservation is
satisfied.
Indirect band gap
Example: Si
Direct
