EE-612:
Lecture 16:
MOSFET Leakage
Mark Lundstrom
Electrical and Computer Engineering
Purdue University
West Lafayette, IN USA
Fall 2008
NCN
www.nanohub.org
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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leakage components
4)
0
n+ 1)
3) VD
n+
2)
p-Si
1) subthreshold current
2) junction leakage
3) gate-induced drain leakage (GIDL)
4) gate-leakage
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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drain leakage current
I D ,VD
0
n+
n+
p-Si
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junction leakage current components
equilibrium
VD = VDD
1) diffusion current
EC
EF
EC
2) generation current
FP
EV
3) avalanche current
qVDD
4) Zener tunnel current
FN
EV
ID > 0
p-substrate n+ drain
p-substrate
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n+ drain
6
diffusion current
VD = VDD
EC
FP
EV
(
Dn ni2 qVD
Jn = q
e
Ln N A
qVDD
FN
kB T
)
−1
ID
p-substrate n+ drain
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generation current
VD = VDD
EC
J n = qWeff (VD )
FP
EV
(
ni qVD
e
2τ
2 kB T
)
−1
FN
ID
p-substrate n+ drain
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avalanche current
VD = VDD
dJ n = α n J n dx + α p J p dx
EC
empirical expression:
FP
EV
α = Ae−b / E
the peak electric field is the
most important
FN
α decreases as T increases
ID
p-substrate n+ drain
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BTBT in the drain-substrate junction
VD = VDD
expect J(BTBT) to vary as:
also should vary as:
e−WDEPL
e− EG
B
FP
e− C E
qVDD
FN
EC
BTBT
EV
p-substrate n+ drain
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A
BTBT in the drain-substrate junction
k-space
VD = VDD
k
FP
k
X
k
conserve crystal
momentum by phonon
emission or absorption
BTBT
qVDD
FN
EC
EV
defect-assisted tunneling
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BTBT (iii)
tunneling probability should involve the barrier
height (EG) and the barrier width (depletion layer)
JB− B =
E=
⎡ 4 2m * EG3/ 2 ⎤
2m * q 3 E VDD
exp ⎢ −
⎥
3 2 1/ 2
4 π h EG
3qEh ⎥⎦
⎢⎣
eqn. (2.27) of
Taur and Ning
2qN A (VDD + Vbi )
ε Si
for NA = 5 x 1018 cm-3, VDD=1V, JB-B ~ 1A/cm2
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BTBT and halos
n+
n+
‘strong’ halos can lead to
high leakage
p-Si
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BTBT: effect on I-V
VDS = 1.1V
log I D
VDS = 0.05V
VD ↑
eq (VGS −VT )/ mkB T
VGS
drain junction
leakage
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BTBT (iv)
BTBT trends
BTBT is becoming increasingly important as channel doping
densities increase.
It is also a concern for alternative channel materials with
smaller bandgaps.
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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gate-induced BTBT
VG ≤ 0
0
VD
n+
n+
BTBT can occur at the surface!
p-Si
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gate-induced BTBT: effect on I-V
VDS = 1.1V
log I D
VDS = 0.05V
e(VGS −VT )/ mkB T
VGS
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gate-induced drain leakage (GIDL)
VG < VDD
0
VDD
n+
n+
x
depletion region in the n+ drain
p-Si
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GIDL (iii)
W
BTBT
FN
EC
EV
x
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GIDL (iii)
VG < VDD
0
VDD
n+
n+
p-Si
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GIDL: example
M. Okuno, et al., “45-nm Node CMOS Integration with a Novel STI Structure and
Full-NCS/Cu Interlayers for Low-Operation-Power (LOP) Applications,” IEDM,
Washington DC. Dec. 5-7, 2005
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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gate leakage
φOX = 3.17 eV
EFG
EC
EC
FP
EV
EV
φOX
′ ≈ 4.5 eV
tOX > 4 nm
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Fowler-Nordheim tunneling
ΔVOX > φOX
EC
φOX
FP
EV
EFG
EC
EV
tOX > 4 nm
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Fowler-Nordheim tunneling (ii)
2
OX
J FN / C1E
⎡ C2 ⎤
= exp ⎢ −
⎥
E
⎣ OX ⎦
EOX = 8 MV/cm
J FN = 5 × 10 −7 A/cm 2
⎤
⎥ eqn. (2.209) Taur and Ning
⎥⎦
log [JMEAS / EOX2]
J FN
3/2
2
⎡ 4 2m *φOX
q 3 EOX
=
exp ⎢ −
2
16π hφOX
3hqEOX
⎢⎣
1 / EOX
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gate leakage (thin oxides)
φOX = 3.17 eV
EFG
EC
EC
FP
EV
EV
tOX = 1 − 2 nm
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direct tunneling
ΔVOX
EC
φOX
FP
J DT ~ e−TOX /t0 × e− φOX /φ0
EV
EFG
EC
EV
1 − 2 nm
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direct tunneling in practice
Lo, Buchanan, and Taur, “Modeling and characterization of quantization,
polysilicon depletion, and direct tunneling effects in MOSFETs with ultrathin
oxides,” IBM J. Res. Develop., 43, pp. 327-337, 1999.
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gate leakage at the 2008 node
is gate leakage a problem at the 2008 (59 nm) node?
AGATE = WLGATE = 1000 nm × 22 nm = 2.2 × 10 −10 cm 2
EOT ( LSTP ) = 1.6 nm
VDD = 1.1 V
(2007 Ed. ITRS)
J G (LSTP) = 10 A/cm 2 (from plot on previous slide)
I GATE ≈ 2200 pA/μ m
I SD,leak (LSTP) = 30 pA/μ m
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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ITRS 2007 Ed.
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oxide scaling
[2] EOT: for a gate dielectric of thickness Td and relative
dielectric constant κ, EOT is defined by: EOT = Td / (κ
/3.9), where 3.9 is the relative dielectric constant of
thermal silicon dioxide.
It is projected that high-κ gate dielectric will be required
by 2008 to control the gate leakage.
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high-k gate dielectrics
Cox =
ε ox
t ox
Cins =
ε ins
tins
ε ox ε ins
ε ox
ε ox
=
=
=
ε ox tins (ε ox tins / ε ins ) EOT
J DT ~ e− tins / t0 × e−φins /φ0
If κINS >> κOX, then we can use a thicker tINS, get a
higher CINS, and lower JDT
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high-k gate dielectrics (ii)
“45nm High-k + Metal Gate Strain-Enhanced Transistors”
Intel Technology Journal, 12 (2), June 17, 2008
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gate leakage
[5] Jg,limit is the maximum allowed gate leakage current
density at 25C, and it is measured with the gate biased
to Vdd and the source, drain, and substrate all set to
ground.
SiO2 --> SiON
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total leakage
[8] Isd,leak: subthreshold leakage current is defined as the
NMOSFET source current per micron of device width, at
25C, with the drain bias set equal to Vdd and with the
gate, source, and substrate biases set to zero volts.
Total NMOS off-state leakage current (Ioff) is the sum of
the NMOS subthreshold, gate, and junction leakage
current (which includes band-to-band tunneling and gate
induced drain leakage [GIDL]) components).
For LSTP, meeting the Isd,leak target of ~30pA/μm is the
key scaling goal.
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total leakage; 2008
LSTP Isd, leak target:
30pA/μm
LSTP, junction leakage target:
10pA/μm
LSTP, gate leakage target:
8.1e-02 A/cm2
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outline
1) MOSFET leakage components
2) Band to band tunneling
3) Gate-induced drain leakage
4) Gate leakage
5) Scaling and ITRS
6) Summary
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leakage challenges
1) Gate leakage has become a major problem
because of tox scaling and is leading to the
replacement of SiO2.
(2007 Ed.
ITRS, PIDS
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EE-612Chapter)
F08
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leakage challenges
1) Gate leakage has become a major problem
because of tox scaling and is leading to the
replacement of SiO2.
2) Band-to-band tunneling (and GIDL) are also
concerns.
3) And don’t forget about the sunthreshold channel
current!
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