Transcript Lecture_6_Leakage_24

Leakage in MOS devices
Mohammad Sharifkhani
Reading
• Text book, Chapter III
• K. Roy’s Proc. of IEEE paper
Introduction
• What is leakage?
– IOFF (drain current when
transistor is supposed
to be off)
• Including gate leakage
• Why is it important?
– Stand-by power; energy
consumption for no
work
Introduction
• How bad is it?
– 1nA/um @0.25um @30
degree C
– 1uA/um @0.1um @80
degree C
• Each generation for a
15mm2 chip
– I off increase by 5x
– Total Width increase by 50%
–  Total leakage current on a
chip 7.5x
–  Leakage power 5x
Introduction
MOS Leakage behavior
Leakage components
• 6 leakage components
– I1: PN junction reversed
bias
– I2: Subthreshold
leakage
– I3: Gate tunneling
– I4: Hot carrier injection
– I5: GIDL
– I6: Punchthrough
PN junction reverse bias current
• Minority carrier drift/diffusion
– Near the edge of depletion
region
– The direct band-to-band
tunnelling model (BTBT)
• Describes the carrier generation
in the high field region without
any influence of local traps.
• Electron-hole generation in
depletion region
• Band to band tunneling
(BTBT) is dominant
PN junction reverse bias current
• Tunneling current density
increases exponentially with
doping:
– Na, Nd
– Vapp (drops too, minor effect)
• Doping increases with scaling
• For typical devices it is
between 10pA – 500pA at
room temperature; For a die
with million devicesoperated
at 5 V, this results in 0.5mW
power consumption  rather
small
• For 0.25 μm CMOS: J = 10100 pA/ μm2 at 25 deg C.
Subthreshold leakage
• Most important among all
• Weak inversion
– Minority carriers in the
channel is small but not zero
– Small Vds; drops across the
reversed-bias pn; small field
– small field, carrier 
current is due to diffusion
rather than drift (base in
BJT)
• Wdm: maximum width of
depletion layer; m<2
Subthreshold leakage
• When Vth is small 
Vgs = 0 does not turn
‘off’ the MOS
Subthreshold leakage
• Exponential relationship with
Vgs and Vth
– 255mV Vth variation  3
orders of magnitude in leakage
• St; milivolts/decade
– Threshold voltage variation
effect on leakage
– About 70-120mV/dec
– Smaller St: sharper slope
• Less voltage variation for 10x
leakage increase
Subthreshold leakage (DIBL)
• Drain Induced Barrier Lowering
• Short channel devices
• Depletion region of drain interacts
with source near channel surface
• Voltage at the drain lowers the
potential barrier at the source
– Lowers VTh
– Increases subthreshold current
without any change onS
• Causes source to inject carriers into
channel surface independent of the
gate voltage
• More DIBL at higher VD and shorter
Leff
• Moves curve up, to right, as VD
increases
Subthreshold leakage (Body Effect)
• Vth roll off
– Increase of Vth with
reduction of Channel Length
• Reverse body bias
– Widens depletion region
• Length ↓, Vth↑
• Bulk doping ↑  Vth
substrate sensitivity ↑
• Reverse body bias ↑ 
Vth substrate sensitivity ↓
• Slope St remains the
same
Subthreshold leakage (Narrow
Width Effect)
• Isolations
– Local Oxide Isolation (LOCOS)
– Trench isolation
• In LOCOS, the fringing field
causes the gate-induced
depletion region to spread
outside the channel width and
under the isolations
– Gate has to work more to
create the channel (inversion)
– More substantial (comparable)
as the channel width
decreases
•  Increase of Vth due to
narrow-channel effect
• Kicks in for W<0.5um
Subthreshold leakage (Narro Width
Effect)
• Trench isolated
technologies:
– Vt decreases for effective
channel widths W ≤ 0.5 μm
NMOS
• For PMOS: A much more
complex behavior
– reduction of the width first
decreases the until the width
is 0.4 m. The width reduction
below 0.4 um causes a sharp
increase
Subthreshold leakage (Channel
Length Effect)
• Short-channel devices:
source-to-drain distance
comparable to depletion
width in vertical direction
• Source and drain depletion
regions penetrate more into
channel length.
• Part of the channel being
already depleted.
•  Gate voltage has to
invert less bulk charge to
turn a transistor on.
Subthreshold leakage
(Temperature Effect)
• 23 fA/um to 8 pA/um
– Factor of 356
• Smaller St:
– Sharper transition (worse sensitivity)
• Two parameters increase the
subthreshold leakage as temperature
is raised:
– 1) Vth linearly increases with
temperature
– 2) the threshold voltage decreases.
• The temperature sensitivity of was
measured to be about 0.8 mV C.
Gate Leakage
• Tox ↓  Eox ↑
• Two mechanisms of electron
tunneling
– Fowler–Nordheim Tunneling:
electrons tunnel into conduction
band of oxide layer
• Very high field strength; usually
not present in products
– Direct Tunneling: electrons from
the inverted silicon surface to
the gate through the forbidden
energy gap of the SiO2 layer
Hot Carrier Injection
• In a short-channel transistor, due to high electric
field near the Si–SiO2 interface, electrons or
holes can gain sufficient energy from the electric
field to cross the interface potential barrier and
enter into the oxide layer
– Reliability risk! (Electrons can trap into or destroy
oxide)
• Increases as L drops (unless VDD drops
accordingly)
Gate-Induced Drain Leakage
(GIDL)
• GIDL is due to high field effect in
the drain junction of an MOS
transistor
• Vg<0  Thins out the depletion
region between drain to well PN
junction
– Effect of new electric field on the old
PN depletion region  holes tunnel
to substrate from drain
– Since the substrate is at a lower
potential for minority carriers, the
minority carriers that have been
accumulated or formed at the drain
depletion region underneath the gate
are swept laterally to the substrate,
completing a path for the GIDL
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Gate-Induced Drain Leakage
(GIDL)
• The effect of GIDL is
more visible at higher
VDD and lower Vg
• Thinner oxide thickness
and higher VDD (higher
potential between gate
and drain) enhance the
electric field and therefore
increase GIDL
• Increase from 4nA 
36nA (for VD from 2.7V to
4V)
Gate-Induced Drain Leakage
(GIDL)
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•
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•
•
•
•
•
Increasing current for negative VG values
• Localized along channel width between gate and drain
• Major problem in Ioff current:
• Contributes to standby power, so must control this by
increasing oxide thickness, increasing drain doping, or
eliminating traps.
• For high performance device (low Vth), is not a major
issue.
Punchthrough
• When Source and Drain depletion region
“touch” each other deep in the channel.
•  Less gate influence on the current
– Channel is created deeper in substrate
– Higher St
• Varies quadratically with VD and with VS
Leakage component contribution
• In each region, the
last term dominates