ece685 nanoelectronics – semiconductor devices
DESCRIPTION
ECE685 Nanoelectronics – Semiconductor Devices. Lecture given by Qiliang Li. Silicon Structure. Unit cell of silicon crystal is cubic. Each Si atom has 4 nearest neighbors . Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Si. Dopants, Electrons and holes. As. B. - PowerPoint PPT PresentationTRANSCRIPT
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ECE685 Nanoelectronics – Semiconductor Devices
Lecture given by Qiliang Li
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• Unit cell of silicon crystal is cubic.
• Each Si atom has 4 nearest neighbors.
Silicon Structure
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Si Si Si
Si Si
Si Si Si
Si Si Si
Si Si
Si Si Si
As B
Dopants, Electrons and holes
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N-type
P-type
Relationship between Resistivity and Dopant Density
= 1/
DOPA
NT D
ENSI
TY c
m-3
RESISTIVITY
(cm)
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GaAs, III-V Compound Semiconductors, and Their Dopants
As AsGa
Ga
· GaAs has the same crystal structure as Si.· GaAs, GaP, GaN are III-V compound semiconductors, important for optoelectronics.· Wich group of elements are candidates for donors? acceptors?
GaAs
AsGa Ga
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Energy Band Model
· Energy states of Si atom (a) expand into energy bands of Si crystal (b).· The lower bands are filled and higher bands are empty in a semiconductor.· The highest filled band is the valence band.· The lowest empty band is the conduction band .
2p
2s
(a) (b)
conduction band)(
(valence band)
Filled lower bands
} Empty upper bands
}
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Energy Band Diagram
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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Donor and Acceptor in the Band Model
Conduction Band Ec
EvValence Band
Donor Level
Acceptor Level
Ed
Ea
Donor ionization energy
Acceptor ionization energy
Ionization energy of selected donors and acceptors in silicon
AcceptorsDopant Sb P As B Al In
Ionization energy, E c –E d or E a –E v (meV) 39 44 54 45 57 160
Donors
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Device Fabrication
Oxidation
Lithography &Etching
Ion Implantation
Annealing & Diffusion
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Side View Top View
Beginning from a silicon wafer
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Side View Top View
Thermal Oxidation
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Side View Top View
Spin-on Photo Resist (PR)
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Side View Top View
Alignment, UV Expose and Develop Photo Resist (PR)
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Side View Top View
Oxide Etched
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Side View Top View
Remove Photo Resist (PR)
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Side View Top View
Doping (implantation or diffusion)
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Side View Top View
Grow Field Oxide (wet/dry) and dopant diffusion
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Side View Top View
Spin-on Photo Resist (PR)
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Side View Top View
Alignment, UV Expose and Develop Photo Resist (PR)
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Side View Top View
Oxide Etched
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Side View Top View
Remove Photo Resist (PR)
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Side View Top View
Grow Gate Oxide (dry)
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Side View Top View
Spin-on Photo Resist (PR)
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Side View Top View
Alignment, UV Expose and Develop Photo Resist (PR)
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Side View Top View
Field Oxide Etched
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Side View Top View
Field Oxide Etched
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Metal (e.g., Aluminum) deposition
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Side View Top View
Spin-on Photo Resist (PR)
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Side View Top View
Alignment, UV Expose and Develop Photo Resist (PR)
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Side View Top View
Aluminum Etched
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Side View Top View
Remove Photo Resist (PR), annealing - complete
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PN junction is present in perhaps every semiconductor device.
N P
VI
– +
PN Junction
V
I
Reverse bias Forward bias
Donor ions
N-type
P-type
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Energy Band Diagram of a PN Junction
A depletion layer exists at the PN junction where n 0 and p 0.
Ef is constant at equilibrium
Ec and Ev are smooth, the exact shape to be determined.
Ec and Ev are known
relative to Ef
N-region P-region(a) Ef
(c)
Ec
Ev
Ef
(b)
Ec
Ef
Ev
Ev
Ec
(d)
Depletionlayer
Neutral P-region
NeutralN-region
Ec
Ev
Ef
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Light emitting diodes (LEDs)•LEDs are made of compound semiconductors such as InP and GaN.• Light is emitted when electron and hole undergo radiative recombination.
Ec
Ev
Radiative recombination
Non-radiative recombination through traps
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LED Materials and Structure
)(24.1
energy photon24.1 m) ( h wavelengtLED
eVEg
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Common LEDs
AlInGaP Quantun Well
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V
I
Reverse bias Forward bias
V = 0
Forward biased
Reverse biased
Schottky Diodes
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MOS: Metal-Oxide-Semiconductor
SiO2
metal
gate
Si body
Vg
gate
P-body
N+
MOS capacitor MOS transistor
Vg
SiO2
N+
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Surface Accumulation
fbgox VVV
)( fbgoxacc VVCQ
oxsox CQV /
oxaccox CQV /Gauss’s LawVg <Vt
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ox
ssa
ox
depa
ox
dep
ox
sox C
qNC
WqNCQ
CQV
2
Surface Depletion ( )gV > Vfb
Ec, Ef
Ev
Ec
Ef Ev
M O S
qVg
depletion region
qs
Wdep
qVox
- - --SiO
2
gate
P-Si body
+ + + + + + - - - - - - -
V - - - - - - -
depletion layer charge, Q dep
- - - - - - -
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Threshold Condition and Threshold Voltage
Threshold (of inversion):ns = Na , or
(Ec–Ef)surface= (Ef – Ev)bulk , or
A=B, and C = D
i
aBst n
Nq
kT ln22
i
a
a
v
i
vbulkvf
gB n
Nq
kTNN
qkT
nN
qkTEE
Eq lnlnln|)(
2
Ec, Ef
M O S
Ev
Ef
Ei
Ec
A
B
C = qB
Ev
DqVg
= qVt
st
ox
BsaBfbgt C
qNVthresholdatVV
222
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Threshold Voltage
ox
BssubBfbt C
qNVV
222 + for P-body,
– for N-body
(a)
(b)
Tox = 20nm
V t(V), N
+ gat
e/P-bo
dy
V t(V), P
+ ga
te/N-
body
Body Doping Density (cm-3)
Body Doping Density (cm-3)
V t(V), P
+ ga
te/N-
body
V t(V), N
+ ga
te/P-b
ody
Tox = 20nm
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Strong Inversion–Beyond Threshold
Ec, Ef
Ev
Ec
Ef Ev
M O S
qVg
-
-
- - --
a
Bsdmaxdep qN
WW 22Vg > Vt
SiO2
gate
P - Si substrate
++++++++++
V
Vg > Vt
- - - - - --- - - - - - - -
Q dep Qinv
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Basic MOSFET structure and IV characteristics
+ +
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)(0 dstgsnsoxeds
ds mVVVCL
WdVdI
m
VVV tgs
dsat