Transcript ppt

ESE370:
Circuit-Level
Modeling, Design, and Optimization
for Digital Systems
Day 11: September 22, 2014
MOS Transistor Operating Regions
Part 2
1
Penn ESE370 Fall2014 -- DeHon
Today
• Operating Regions (continued)
– Resistive
– Saturation
– Velocity Saturation
– Subthreshold
• Drain Induced Barrier Lowering
2
Penn ESE370 Fall2014 -- DeHon
Last Time…
3
Penn ESE370 Fall2014 -- DeHon
Channel Evolution
Increasing Vgs
4
Penn ESE370 Fall2014 -- DeHon
Threshold
• Voltage where strong inversion occurs
N A 
 threshold voltage
F  T ln 
– Around 2ϕF
 ni 
– Engineer by controlling doping (NA)

5
Penn ESE370 Fall2014 -- DeHon
Resistive Region
COX 
• VGS>VT, VDS small
OX
tOX

IDS
2 
W 
VDS
 nCOX  VGS VT VDS 

 L 
2 
6
Penn ESE370 Fall2014 -- DeHon
Channel Field
• When voltage gap VG-Vxdrops below VT,
drops out of inversion
– Occurs when: VGS-VDS< VT
– Channel is “pinched off”
– Current will flow, but cannot increase any
further
7
Penn ESE370 Fall2014 -- DeHon
Saturation
• VDS> VGS-VT
IDS
2 

VGS  VT 
W 
2


 n COX  VGS  VT  
 L 
2



IDS 

n COX W 
2
  VGS  VT 
 L 
2

8
Penn ESE370 Fall2014 -- DeHon
Saturation Region
Blue curve
marks transition
from Linear
to Saturation
9
Penn ESE370 Fall2014 -- DeHon
New Stuff
10
Penn ESE370 Fall2014 -- DeHon
Preclass 1
• What is electrical field in channel?
– Leff=25nm, VDS=1V
– Field = VDS/Leff
• Velocity: v=F*μ
– Electron mobility: μn = 500 cm2/(Vs)
• What is electron velocity?
11
Penn ESE370 Fall2014 -- DeHon
Same Phenomena
• I = (1/R) × V
• I increases
linearly in V
• What’s I?
 DQ/Dt
– Speed at which
charge moves
• F=V/L
• v=×F=(/L) ×V
• Velocity increases
linearly in V
• What’s a moving
electron?
12
Penn ESE370 Fall2014 -- DeHon
Short Channel
S
• Model assumes carrier velocity
increases with field
– Increases with voltage
• Are there limits to how fast things
(including electrons) can move?
13
Penn ESE370 Fall2014 -- DeHon
D
Short Channel
S
• Model assumes carrier velocity increases
with field
– Increases with voltage
• There is a limit to how fast carriers can
move
– Limited by scattering
to 105m/s < speed-of-light
• How relate to preclass 1b velocity?
• Encounter when channel short
– Modern processes, L is short enough
Penn ESE370 Fall2014 -- DeHon
14
D
Velocity Saturation
• At what voltage do we hit the speed
limit? (preclass 1c)
• This is Vdsat
– The voltage at which velocity (current)
saturates
15
Penn ESE370 Fall2014 -- DeHon
Velocity Saturation
• Once velocity saturates:
IDS

VDSAT 
  satCOX W VGS  VT 


2 

VDSAT 
L sat
n
16
Penn ESE370 Fall2014 -- DeHon
S
Velocity Saturation
• Once velocity saturates:
IDS
VDSAT 
L sat
n

VDSAT 
  satCOX W VGS VT 


2 
• Can still increase current with parallelism

– W – make it wider
– Vgs-Vt – make it deeper
17
Penn ESE370 Fall2014 -- DeHon
D
Velocity Saturation
• How does this Vdsat compare to the
threshold we have been using?
18
Penn ESE370 Fall2014 -- DeHon
Velocity Saturation
19
Penn ESE370 Fall2014 -- DeHon
Subthreshold
20
Penn ESE370 Fall2014 -- DeHon
Below Threshold
• Transition from insulating to
conducting is non-linear, but
not abrupt
• Current does flow
– But exponentially dependent on
VGS
21
Penn ESE370 Fall2014 -- DeHon
Subthreshold
IDS
W 
 IS  e
 L 
 VGS 


nkT / q 
 VDS 

kT / q 
1  e  1 VDS 


22
Penn ESE370 Fall2014 -- DeHon
Subthreshold
S
• W/L dependence follow from resistor
behavior (parallel, series)
– Not shown explicitly in text
• λ is a channel width modulation effect
IDS
W 
 IS  e
 L 
 VGS 


nkT
/
q


 VDS 

kT / q 
1  e  1 VDS 


23
Penn ESE370 Fall2014 -- DeHon
D
Steady State
• What current flows in steady state?
• What causes (and determines)
the magnitude of current flow?
• Which device?
24
Penn ESE370 Fall2014 -- DeHon
Leakage
• Call this steady-state current flow
leakage
• Idsleak
25
Penn ESE370 Fall2014 -- DeHon
Subthreshold Slope
• Exponent in VGS determines how steep
the turnoff is
kT 
– Every S Volts
S  n ln 10
– Divide IDS by 10
 q 
IDS
W 
 IS  e
 L 
 VGS 


nkT
/
q


 VDS 

kT / q 

1 V 
1

e

DS



26
Penn ESE370 Fall2014 -- DeHon
Subthreshold
Slope
• Exponent in VGS determines how steep the
turnoff is
– Every S Volts (S not related to source)
kT 
– Divide IDS by 10
S  n ln 10
 q 
• n – depends on electrostatics
– n=1  S=60mV at Room Temp. (ideal)
– n=1.5  S=90mV 
– Single gate structure showing S=90-110mV
27
Penn ESE370 Fall2014 -- DeHon
IDS vs. VGS
28
Penn ESE370 Fall2014 -- DeHon
Subthreshold Slope
• If S=100mV and Vth=300mV,
what is Ids(Vgs=300mV)/Ids(Vgs=0V) ?
• What if S=60mV?
IDS
W 
 IS  e
 L 
kT 
S  n ln 10
 q 
 VGS 


nkT / q 
 VDS 

kT / q 

1 V 
1

e

DS



29
Penn ESE370 Fall2014 -- DeHon
Threshold
30
Penn ESE370 Fall2014 -- DeHon
Threshold
• Describe VT as a constant
• Induce enough electron collection to
invert channel
31
Penn ESE370 Fall2014 -- DeHon
VDS impact
• In practice, VDS impacts state of channel
32
Penn ESE370 Fall2014 -- DeHon
VDS impact
• Increasing VDS, already depletes
portions of channel
33
Penn ESE370 Fall2014 -- DeHon
VDS impact
• Increasing VDS, already depletes
portions of channel
• Need less charge, less voltage to invert
34
Penn ESE370 Fall2014 -- DeHon
Drain-Induced Barrier
Lowering (DIBL)
VT
VDS
35
Penn ESE370 Fall2014 -- DeHon
DIBL Impact
36
Penn ESE370 Fall2014 -- DeHon
In a Gate?
• What does it impact most?
– Which device, has large Vds?
• Vin=Vdd ?
• Vin=Gnd ?
– How effect
operation?
• Speed of switching?
• Leakage?
37
Penn ESE370 Fall2014 -- DeHon
In a Gate
• VDS largest for off device
– Easier to turn on
IDS

VDSAT 
  satCOX W VGS  VT 


2 
38
Penn ESE370 Fall2014 -- DeHon
In a Gate
• VDS largest for off device
– Easier to turn on
– Leak more
IDS
W 
 IS  e
 L 
 VGS 


nkT / q 
 VDS 

kT / q 
1  e  1 VDS 


39
Penn ESE370 Fall2014 -- DeHon
PMOS
• Similar phenomena to NMOS
• Signs different
– Negative Vth
– turn on when less than this
• Reason based on carriers
40
Penn ESE370 Fall2014 -- DeHon
Approach
• Identify Region
• Drives governing equations
• Use to understand operation
41
Penn ESE370 Fall2014 -- DeHon
Big Idea
• 3 Regions of
operation for
MOSFET
– Subthreshold
– Resistive
– Saturation
• Pinch Off
• Velocity Saturation
– Short channel
42
Penn ESE370 Fall2014 -- DeHon
Admin
• Text 3.3.2 – highly recommend read
– Finish it up on Wednesday
• Office Hours: M, T, W
• HW4 due Thursday
• Midterm 1 next Monday
– Midterms from 2010, 2011, 2012, 2013
• All online … both with and without answers
• Suggest start without answers
43
Penn ESE370 Fall2014 -- DeHon