Principles of Semiconductor Devices-L34
Transcript of Principles of Semiconductor Devices-L34
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Alam ECE-606 S09 1
www.nanohub.org
NCN
ECE606: Solid State DevicesLecture 34: MOSCAP Frequency Response
Muhammad Ashraful [email protected]
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Outline
Alam ECE-606 S09 2
1. Background
2. Small signal capacitances
3. Large signal capacitance
4. Conclusion
Ref: Sec. 16.4 of SDF
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Topic Map
Alam ECE-606 S09 3
Equilibrium DC Small
signal
Large
Signal
Circuits
Diode
Schottky
BJT/HBT
MOSCAP
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Small Signal Equivalent Circuit
Alam ECE-606 S09 4
p
G0
CD
CJ
CJ/Co
1
VG
For insulated devices,consider only majority carrier
junction capacitance CJ
High
frequency
Low
frequency
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Junction Capacitance
Alam ECE-606 S09 5
p-Si
+-~
sinG S
V t υ ω + C G ≡
dQ G
dV G
=d −Q S ( )dV
G
SGS
OC
V Qψ = −
( ) ( )
1G
S
S
OSC
dV d
d Q d Q
ψ = +
− −
11 1
S OG C C C = +
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Junction Capacitance
Alam ECE-606 S09 6
1 1 1
SG OC C C = +
( )S
S
S
Q
d C
d
ψ
−≡
V G
O
C
SC
( )SSQ ψ
which we already
understand!
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Definition of m for later use
Alam ECE-606 S09 7
( )1 S Om C C = + V
G
OC
C S
ψ S
O GS G
O S
C V V
C C mψ ≡=
+
‘body effect coefficient’
( )01 S O T m x W κ κ = +
1.1 ≤ m ≤ 1.4
in practice:
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Outline
Alam ECE-606 S09 8
1. Background
2. Small signal capacitances
3. Large signal capacitance
4. Conclusion
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Junction Capacitance in accumulation
Alam ECE-606 S09 9
0
, 00
ox
j accC C x
κ ε ≈ ≡
,
,
0
, ,
s acc
s
s
j acc s acac a
o
o cc
cc
C C W C
C
C
C κ ε = ≡
+
Wacc
-Q
+Q
Low frequencyC/Co
1
+Q
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Junction Capacitance in depletion
Alam ECE-606 S0910
+Q
VG -Q
0 0
0,
01 j d s
e
s
s p
C
C
C C
C C C C = =+ +
0
1 1o G
s
W
x
V
V δ
κ
κ = + −
2
0 0
02
A AG
o s
qN qN W V x
W
κ ε κ ε
= +
1
o
G
C
V
V δ
=
+
Low frequencyC/Co
V G
1
0
0
0
01 so
W
C
x
ε κ κ ε =
+
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Junction capacitance in inversion
Alam ECE-606 S09 11
-Q
+Q
Electrons
Exposed
Acceptors
VG
VG’ >VT’
0, 0
0
s j invC C x
κ ε
≈ ≡
0
,
o inv s j inv inv
o inv inv
C C C C
C C W
κ ε = ≡
+
Low frequency
C/Co
VG
1
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Equivalent Oxide Thickness
Alam ECE-606 S09 12
Q i = −C G V G −V T ( )
,
ino
G j inv o
v
v oin
C
C
C C C C
C = = <
+
s onv
inv
iC W
κ ε ≡
0
o ooC
x
κ ε =
ox O
e c
G
le EOT C κ ε = ox O
O Oinelec
s
v
O
x x E W OT κ ε
κ ε = + >
‘Equivalent oxide thickness - electrical ’
Low frequency
C/C ox
1
V G
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High frequency curve at inversion
Alam ECE-606 S09 13
-ΔQ
ΔQ
WT
What about high frequency part of the curve?
0, 0
0
s j invC C
xκ ε ≈ ≡
High frequency
C/Co
1
V GV TH
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Response Time
Alam ECE-606 S09 14
VG
VG’ >VT’
VGVG
Dielectric Relaxation
0s
στ =
κ ε
SRH Recombination-Generation
2
1 1( ) ( )
i i
n p n p
np n n R
p p n n
− −= →
τ + + τ + τ + τ
Ref. Lecture no. 15
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High frequency response in MOS-C
Alam ECE-606 S09 15
High frequency
Low frequency
C/Co
1
VG
-ΔQ
ΔQ
WT
Low Frequency
-ΔQ
ΔQ
WT
High Frequency
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Ideal vs. Real C-V Characteristics
Alam ECE-606 S09 16
Flat band voltage …
Threshold voltage …
C/CoC/Co
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Outline
Alam ECE-606 S09 17
1. Background
2. Small signal capacitances
3. Large signal capacitance
4. Conclusion
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Topic Map
Alam ECE-606 S09 18
Equilibrium DC Small
signal
Large
Signal
Circuits
Diode
Schottky
BJT/HBT
MOS
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Large Signal Deep Depletion
19
0 0
,
0
0
1
s j dep
oxs
s
C C C C
W C C
x
κ
κ
= =+ +
1
o
G
C
V
V δ
=
+
(even beyond threshold)High frequency
Low frequency
C/Co
VG
-ΔQ
ΔQ
Wdm
NA
x0
ρ(x)
-ΔQ
ΔQ Wdm
NA
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Relaxation from Deep Depletion
20
High frequency
Low frequency
C/C ox
1
Deep depletion
-ΔQ
ΔQ
Wdm
NA
x0
ρ(x)
-ΔQ
ΔQ
Wdm
NA
x0
ρ(x)
-ΔQ
ΔQ
Wdm
ρ(x)
Depending on the measurementfrequency, it will either merge
with low-freq. or high-freq. curve.
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Ideal vs. Real C-V Characteristics
Alam ECE-606 S09 21
Flat band voltage …
Threshold voltage …
C/CoC/Co
VGVG
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Low or High frequency?
Alam ECE-606 S09 22
typically observe hi-
frequency CV
G = n i 2τ
p-Si
typically observe low-frequency CV
No deep-depletion as well
p-Si
n+-Si n+-Si
What happens if I shine light on a MOS capacitor?
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Summary
Alam ECE-606 S09 23
1) Since current flow through the oxide is small, we are
primarily interested in the junction capacitance of the
MOS-capacitor.
2) High frequency of MOS-C is very different than low-
frequency C-V. In MOSFET, we only see low frequency
response.
3) Deep depletion is an important consideration for MOS-
capacitor that does not happen in MOSFETs.