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7/27/2019 VLSI lect 2
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Short channel effects in MOST
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Short-channel device: channel length is comparable to
depth of drain and source junctions and depletion
width
In general, short channel effects visible when L 1m
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If we shrink all length scales, a number of physical effects
can become relevant that are unimportant in largerMOSFETs:
Channel shortening, Punch-through, Tunneling,
Thermionic leakage, Threshold voltage variation with
drain bias, Velocity saturation, Field-dependent
mobility, Avalanche breakdown, Oxide failure ..
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1) Channel shortening
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Happens when VDS approaches VDSsat = VGS - VT
In long channel devices, we get pinch-off and saturation
ofID
In short devices, L can be a significant fraction ofLElectric field is large over L - thats where most of thesource-drain voltage is dropped.
Drift is enhanced by high electric field there: result is a
boost in ID as VDS is increased beyond VDSsat 5
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2) Threshold voltage variation
In threshold voltage equation, channel depletion region
is assumed to be created by gate voltage only -
Depletion regions around source and drain neglected:
valid if channel length is much larger than depletion
region depths
In short-channel devices, depletion regions from drainand source extend into channel
As channel length L decreases, threshold voltage
decreases 6
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Source
depletionregion
Drain
depletion
regionGate-induced depletion region
N+
source
N+
drain
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Drain-induced barrier lowering (DIBL)
As VDS increases, threshold voltage decreases
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3) Carrier velocity saturation
Field along channel - As channel length is reduced,
electric field increases
Electron drift velocity is proportional to electric field
only for small field values - For large electric field,
velocity saturates
9
If the velocity of carriers becomes large enough,
they can lose energy through inelastic processes.
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10
If L is large compared to the inelastic scattering length,
one sees velocity saturation
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Results:
There is a significant reduction in current.
Saturation current now depends linearlyon VGS - VT
rather than quadratically in longer devices.
IDsat = W Cox (VGSVT) vsat
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Ballistic Devices
If the channel is short enough (below the mean free path
length scales), then the carriers can travel from source to
drain without going through any significant scattering
events. This is called ballistic transport.
Carriers can often gain an average velocity over vsat .
This phenomena is known as velocity overshoot.
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4) Avalanche breakdown
At high enough energies/fields (in regions with large
electric fields, like drain pinch-off area), carriers can
produce electron-hole pairs through collisions.
These pairs may not be bound, and can also get
accelerated, leading to more pairs.
Result is runaway ID not controlled by VG.
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5) Thermionic leakage / tunneling
Both processes can be relevant.
Tunneling matters more for smaller devices
Thermionic emission matters more at higher
temperatures.
Biggest problem is that these can lead to substantial
off-currents and power dissipation
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Tunneling Leakage Current
For SiO2 films thinner than 1.5 nm, tunneling leakage
current has become the limiting factor.
HfO2 has several orders lower leakage for the same EOT.15
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Unless a new gate dielectric (other than SiO2) is
developed, leakage current due to tunneling will forceminimum transistor dimensions to be 25-50 nm.
HfO2 has a relative dielectric constant of ~ 24, six times
larger than that of SiO2 .
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Fowler-Nordheim Tunneling
For very thin gate oxide, electrons can tunnel through
the gate oxide, resulting in current from gate to drain orsource
oxE
E
oxFN eWLECI
0
2
1
E0, C1 constants
Eox = electric field across
oxide
Gate leakage
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To keep VT variations under control, the gate oxide
thickness is reduced. Eventually this leads to tunneling
of electrons from the gate to the silicon substrate which
results in leakage current.
Gate oxide considerations
The maximum acceptable leakage current for a device
with Vdd = 1V is about 1A/cm2.
This corresponds to an oxide thickness of roughly 2 nm.
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Leakage
Power consumed even when circuit is inactive - Leakage
power raises temperature of chip - Can cause
functionality problem in some circuits
Reducing transistor leakage
Long-channel devices
Small drain voltage
Large threshold voltage VT19
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Leakage
Leakage vs. performance tradeoff:
For high-speed, need small VT and L
For low leakage, need high VT
and large L
Process scaling
VT reduces with each new process (historically)
Leakage increases ~10X!20
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6) Mobility degradation
In short-channel devices, n and p are not constant -As vertical electric field increases, surface mobility
decreases
TGS
VV
1
0
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As VDS is increased, drain depletion region gets
deeper and extends further into channel
For very large VDS, source and drain depletion
regions can meet punch-through
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7) Punch-through
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8) Hot-carrier effect
Increased electric fields causes increased electron
velocity
High energy electrons can tunnel into gate oxide
This changes the threshold voltage (increases VT
for NMOS)
Can lead to long-term reliability problems
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High-velocity electrons can also impact the drain,
dislodging holes
Holes are swept towards the substrate cause
substrate current impact ionization
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Both devices have same W/L ratio
Short-channel device has ~ 40% lesser current
Linear dependence of current on VGS in short-channel
deviceit is quadratic in long channel device 25
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MOS ID-V
GSCharacteristics
0
1
2
3
4
5
6
0 0.5 1 1.5 2 2.5
VGS (V)
(for VDS = 2.5V, W/L = 1.5)
X 10-4
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9) Subthreshold conduction
When VGS < VT, transistor is off
However, small drain current ID still flows -
subthreshold leakagecurrent
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10) Oxide failure
High energy electrons accelerated by large fields can
break bonds - can effectively introduce enough defect
states in gap to permit sufficient conduction to get
runaway failure.
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Trends:
Shorter channel lower threshold voltage
Higher VD lower threshold voltage.
Thinner oxide higher threshold voltage.
To keep up with source-drain field, we must scale oxide
to be thinner.
Thinner oxide higher gate field enhanced
surface scattering at channel-oxide interface
lower effective mobility. 29
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MOSFET Technology Scaling
New technology node every three years or so.
Defined by minimum metal line width.
All feature sizes, e.g. gate length, are ~70% of previous
node.
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MOSFET Scaling
Constant Field
Ideal, helps reliability
Constant Voltage
Traditional, board-level compatible
Hybrid - practical
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Constant Field Scaling
Uniformly scale all linear dimensions by factor of s > 1
Also reduce supply voltage by s
Preserves field strength
Also known as full scaling
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Before
Scaling
After Scaling
Length L L/s
Width W W/s
Oxide Thickness tox tox/sJunction Depth Xj Xj/s
Supply Voltage VDD VDD/s
Threshold Voltage VT VT/s
Doping Densities NA,ND sNA,sND
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oxscaledox sCC ,
s
CC
g
scaledg ,
Capacitance:
s
I
s
V
s
V
sL
sW
stI
DTGS
ox
ox
scal edD
2
,2
Current:
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Power:2s
P
s
I
s
VP
DDS
scal ed
AP
sLsW
sP
A
P
scaled
scaled
2
Power density :
Delay: ssI
sVsCscaled
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Constant Voltage scaling
Before Scaling After Scaling
Length L L/s
Width W W/s
Oxide Thickness tox tox/s
Junction Depth Xj Xj/s
Supply Voltage VDD VDD
Threshold Voltage VT VT
Doping Densities NA,ND s2NA,s
2ND36
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(for velocity non-saturated devices)
oxscaledox sCC ,s
CC
g
scaledg ,
Capacitance:
Current:
DTGSox
ox
scal edDsIVV
sL
sW
stI
2
,2
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2ssI
VsCscaled
Delay:
Power: sPsIVPDDSscal ed
Power density:
APs
sLsWsP
APscaled
scaled /3
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Current:
Power:
DscaledD II ,
PIVP DDSscaled
(for velocity-saturated devices)
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sI
VsCscaled
Delay:
Power density:
APs
sLsW
P
A
P
scaled
scaled /2
(velocity-saturated case)
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Hybrid scaling
Scale voltage, but not as fast as process
Some circuits need a minimum voltage (band-gap,
analog circuits, etc) - Low thresholds have leakage
problems
Result is somewhere between constant field and
constant voltage - delay ~ 1/s, higher power than
constant field but less than constant voltage
41
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0
5
10
15
20
25
30
35
40
45
0.65um 0.5um 0.35um 0.25um 0.18um 0.13um 0.1um
gate
Cu interconnect
Al interconnect
Cu + gateAl + gate
Delay in ps
Wire - 43um long & 0.8um high