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 36, 09 Apr 14

• Chp 06 MOSFETs
• Aspects of realistic MOSFET operation (n-channel
  p-substrate)
• Comment on 2D mobility m
• Use of field oxide in CMOS
• Short channel effects on ON operation
• high E (y) gt velocity saturation gt lower IDS
• micron-scale worst
• nano-scale not so bad
• scaling
• Good test for future ON/OFF operation
  sub-threshold (not fully ON) swing

VM Ayres, ECE875, S14
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In the charge sheet and constant mobility
approximation
ID
Mobility is assumed to be constant However It
is a different value than 3D bulk mobility
VM Ayres, ECE875, S14
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Lecture 36, 09 Apr 14

• Chp 06 MOSFETs
• Aspects of realistic MOSFET operation (n-channel
  p-substrate)
• Comment on 2D mobility m
• Use of field oxide in CMOS
• Short channel effects on ON operation
• high E (y) gt velocity saturation gt lower IDS
• micron-scale worst
• nano-scale not so bad
• scaling
• Good test for future ON/OFF operation
  sub-threshold (not fully ON) swing

VM Ayres, ECE875, S14
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Use CMOS (complementary metal-oxide-semiconductor
FET) or NMOS design to reduce power consumption.
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Use CMOS (complementary metal-oxide-semiconductor
FET) or NMOS design to reduce power consumption.
Wikipedia - A high voltage on the gates will
cause the nMOSFET to conduct and the pMOSFET to
not conduct while a low voltage on the gates
causes the reverse. This arrangement greatly
reduces power consumption and heat
generation. However, during the switching time
both MOSFETs conduct briefly as the gate voltage
goes from one state to another. This induces a
brief spike in power consumption and becomes a
serious issue at high frequencies.
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Another issue Coulomb forces are distance
dependent and spatially they last forever. So
charges on a 1st gate can influence the channel
under the 2nd gate.
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Block with an oxide called the field oxide
more SiO2
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CMOS
OFF
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ON
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Working tools
Channel/ Drain to Source
Linear, saturation and intermediate forms
ID
Channel/ Drain to Source
Potential associated with E-field across oxide
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1st term in VFB
Given both gates are n poly Si, which acts like
a metal
-0.98 V
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2nd term in VFB
Qinsulator Qm Qot Qf Qit Given Qf/q
1011 Assume Qm Qot Qit 0 for this
insulator under these operating
conditions Therefore Qinsulator 1011 x 1.6 x
10-19 C And
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Next term 2yB
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Next term QD/Cox
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Goal want minimum isolation VT 20 V
VM Ayres, ECE875, S14
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Goal want minimum isolation VT 20 V
Set
VM Ayres, ECE875, S14
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Lecture 36, 09 Apr 14

• Chp 06 MOSFETs
• Aspects of realistic MOSFET operation (n-channel
  p-substrate)
• Comment on 2D mobility m
• Use of field oxide in CMOS
• Short channel effects on ON operation
• high E (y) gt velocity saturation gt lower IDS
• micron-scale worst
• nano-scale not so bad
• scaling
• Good test for future ON/OFF operation
  sub-threshold (not fully ON) swing

VM Ayres, ECE875, S14
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Goal

• More MOSFETs/CMOSs more computer capability.
  Therefore make each MOSFET /CMOS smaller.
  Current CMOS 22 nm node.

Node
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Problem make channel length L shorter and MOSFET
device performance deteriorates (see RHS). Why
is this?
Small L short channel
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Problem is with mobility/velocity
Charge sheet constant mobility approximation
Linear, saturation and intermediate forms
ID
Constant mobility assumption
ltvelgt m E (y)
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m
ltvelgt
E
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Problem is with mobility/velocity
We assumed that ltvelgt m E (y) meant a linear
function of E (y). This is only true for low VDS
(low bias). Realistically
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Problem is with mobility/velocity
ECE 474 velocity saturation
ltvelgt
105 V/cm
E ext
E (y) that increases along L
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At high E (y)-field near Drain, have a velocity
saturation effect that has nothing to do with
pinch in the channel.
ltvelgt
E (y) that increases along L
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Result of velocity saturation RHS
velocity saturation occurs at lower VDS than that
required for the channel pinch due to development
of the depletion regions at the Drain end
Small L short channel
High E -field in channel ? large VDS drop across
a physically small L
VM Ayres, ECE875, S14
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What to use for mobility m now
ltvelgt
n 2 ? empirical velocity saturation of
electrons in Si channel for moderate doping
E (y) that increases along L
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What to use for mobility m now
ltvelgt
n 1 ? empirical holes in Si channel for
moderate doping n 1 is also a mathematically
simpler approximation for the n 2 curve for
electrons
E (y) that increases along L
VM Ayres, ECE875, S14
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What to use for mobility m now
ltvelgt
Two straight lines and avoid the point where they
cross is another simpler approximation for the n
2 curve for electrons
E (y) that increases along L
VM Ayres, ECE875, S14