ECE 875: Electronic Devices

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ECE 875Electronic Devices

• Prof. Virginia Ayres
• Electrical Computer Engineering
• Michigan State University
• ayresv_at_msu.edu

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Lecture 40, 18 Apr 14
Chp 06 MOSFETs Aspects of realistic MOSFET
operation (n-channel p-substrate) Subthreshold
swing definition 01 example definition
02 example
Chp 14 Sensors Chemical ion sensors Temperature
sensors Mechanical sensors
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Motivation It is hard to turn a device OFF. ID
when the MOSFET is supposed to be OFF is called
leakage current.
How do you turn the MOSFET OFF
(n-channel) Reduce VG below threshold. No
inversion layer. Want ID 0. But leakage
current is still a problem why diffusion and
EHP formation cant be stopped. Subthreshold
swing S is a metric for turning a MOSFET device
OFF Test conditions use a small VDS to get a
current ID running. Then see how that current
responds to changes in VG
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If the bottom distortion is examined carefully,
it is small but it doesnt exactly 0. Small
currents matter.
Definition 01 for Subthreshold swing S S
DVG/decade ID
1/slope, not slope
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bias
S 100 mV/decade ID DVG 10210-3 V 0.1
V Decade ID starts where (ID, VG) ID
10-7A, when VG VT 0.5 V Actual ID
finishes where (ID, VG) Find finish ID ??
A, when VG 0 V Goal ID finishes
where (ID, VG) ID 10-13 A, when VG 0
V Find start VG VT DVT ??
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(No Transcript)
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bias
Example try changing device to a MOSFET
with NA 5 x 1015 cm-3 d 10 nm 100
Angstroms Effective insulator charge 4 x 1010 q
C cm-2 Keep S 100 mV/decade ID DVG 10210-3
V 0.1 V Set up answer to same questions ID
10-13 A, when VG 0 V Find start VG VT DVT
?? Find finish ID ?? A, when VG 0 V Find
first what else??
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Answer
New MOSFET
New VT
bias
S 100 mV/decade ID DVG 10210-3 V 0.1
V Decade ID Find starts where (ID, VG) ID
10-7A, when VG VT ?? V Actual ID
finishes where (ID, VG) Find finish ID ??
A, when VG 0 V Goal ID finishes
where (ID, VG) ID 10-13 A, when VG 0
V Find start VG VT DVT ??
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Find VT
n-channel
-


-
signs
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Find VT
-0.95 V
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Streetman and Banerjee, Chp. 06, p. 286
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New MOSFET
New VT
bias
S 100 mV/decade ID DVG 10210-3 V 0.1
V Decade ID Find starts where (ID, VG) ID
10-7A, when VG VT -0.215 V Actual ID
finishes where (ID, VG) Find finish ID ??
A, when VG 0 V Goal ID finishes
where (ID, VG) ID 10-13 A, when VG 0
V Find start VG VT DVT ??
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VT -0.215 V
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VT -0.215 V
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New MOSFET
New VT
bias
S 100 mV/decade ID DVG 10210-3 V 0.1
V Decade ID Find starts where (ID, VG) ID
10-7A, when VG VT -0.215 V Actual ID
finishes where (ID, VG) Find finish start
ID 10-5 A, when VG 0V Find finish ID
10-13 A, when VG - V Goal ID finishes
where (ID, VG) ID 10-13 A, when VG 0
V Find start VG VT DVT ??
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VT-new -0.215 V DVT
VT -0.215 V
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Lecture 40, 18 Apr 14
Chp 06 MOSFETs Aspects of realistic MOSFET
operation (n-channel p-substrate) Subthreshold
swing definition 01 example definition
02 example
Chp 14 Sensors Chemical ion sensors Temperature
sensors Mechanical sensors
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Definition 01 for Subthreshold swing S S
DVG/decade ID
Definition 02 for Subthreshold swing S
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Definition 02 incorporates analysis of an
important problem in turning a MOSFET
OFF stopping diffusion current
few e-
Lots of e-
Lots of e-
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Two points
1. few e- in depletion region, not charge
sheet x-dependence
Lots of e-
Lots of e-
2. VDS extra depletion region in real OFF
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Investigate the diffusion current issue
These are the source and drain ends of the
channel, not the n regions
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Investigate the diffusion current issue
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Investigate the diffusion current issue
Diffusion current
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Diffusion current ID f(ys) ? VG
ID
Surface potential is related to VG
Examine change in ID per change in VG starting
with change in ys per change in VG
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Influence of interface traps
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New MOSFET
New VT
bias
Consistent? S 100 mV/decade ID DVG 10210-3 V
0.1 V Decade ID Find where (ID, VG) ID
10-7A, when VG VT -0.215 V Actual ID
finishes where (ID, VG) Find start ID
10-5 A, when VG 0V Find finish ID 10-13 A,
when VG - V Goal ID finishes
where (ID, VG) Find finish ID 10-13 A,
when VG 0 V Find start VG VT DVT
V
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Did Cox and/or CD change?
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Did Cox and/or CD change? Yes. Example Find S
for this device. Assume room temperature
operation at 300 K.
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Answer
3.45 x 10-7 F cm-2
2.5 x 10-8 F cm-2
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Answer
3.45 x 10-7 F cm-2
2.5 x 10-8 F cm-2
S
3.45 x 10-7 F cm-2
0.0259 eV e
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Units
Definition 01 for Subthreshold swing S S
DVG/decade ID
Definition 02 for Subthreshold swing S
S
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Lecture 40, 18 Apr 14
Chp 06 MOSFETs Aspects of realistic MOSFET
operation (n-channel p-substrate) Subthreshold
swing definition 01 example definition
02 example
Chp 14 Sensors Chemical ion sensors Temperature
sensors Mechanical sensors
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What has changed from a conventional MOSFET?
Choices Gate Insulator Channel Substrate
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What has changed from a conventional MOSFET
Gate Insulator Channel Substrate
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