17 March 2011 J. F. Wager 1
Oxide ThinOxide Thin--Film Transistor Film Transistor
AssessmentAssessment2011 China FPD Conference
Shanghai, China
17 March 2011
John F. Wager
OREGON STATE UNIVERSITY
School of EECS
Corvallis, OR 97331-5501
U. S. A.
17 March 2011 J. F. Wager 2
TFT OperationTFT OperationSOURCE DRAIN
TCOTCO
Channel
Glass
TCO
Insulator
GATE
+V
+Vinjection extraction
transport
trapping
‘M’-I-’S’
control
AOS
ONLY!
17 March 2011 J. F. Wager 3
TFT OperationTFT OperationSOURCE DRAIN
TCOTCO
Channel
Glass
TCO
Insulator
GATE
+VG
+VD
ID(VGS,VDS)
IG,LEAKAGE
IC,SHUNT
ID(VGS,VDS) = ID,IDEAL+ IC,SHUNT + IG,LEAKAGE
IG(VGS,VDS) = -IG,LEAKAGE
ID,IDEAL
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Gate
VG VD
Source
Drain
ID ≡ ID(VG,VD)
TFT ITFT I--V ModelingV Modeling
ID,IDEAL
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TFT ModelingTFT Modeling
STATICDYNAMIC INTRINSICIG,LEAKAGE IC,SHUNT
�
�
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Basic Measurable Quantities Basic Measurable Quantities
ID(VGS,VDS) = ID,IDEAL+ IC,SHUNT + IG,LEAKAGE
IG(VGS,VDS) = -IG,LEAKAGE
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Basic TFT MeasurementsBasic TFT Measurements
5 types:
• Output curve
• Transfer curve (linear)
• Transfer curve (saturation)
• VT transfer curve (linear)
• VT transfer curve (saturation)
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Basic TFT MeasurementsBasic TFT Measurements
Output curve
• ID-VDS
�D- or E-mode?
� ~VT
�Saturation?
� RSERIES?
� ILEAKAGE?
ID
?
OUTPUTINPUT VDSVGS
IG
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Basic TFT MeasurementsBasic TFT Measurements
Transfer curve (linear)
• log(ID)-VGS
• VDS<< VGS-VT (VDS = 0.1 V)
� VON
�S
� μ(VGS)
� Hysteresis?
� ILEAKAGE?
LINEAR!?
ID
OUTPUTINPUT VDSVGS
IG
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Basic TFT MeasurementsBasic TFT Measurements
Transfer curve (saturation)
• log(ID)-VGS
• VDS/tG ≥ ~2 MV/cm
� ID(on-off)
SATURATION!
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Basic TFT MeasurementsBasic TFT Measurements
VT transfer curve (linear)
• ID-VGS
• VDS<< VGS-VT (VDS = 0.1 V)
� VT
LINEAR!
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Basic TFT MeasurementsBasic TFT Measurements
VT transfer curve (saturation)
• [ID]1/2-VGS
• VDS/tG ≥ ~2 MV/cm
� VT
� μSAT
SATURATION!
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OutputOutput
CurveCurve
AssessmentAssessment
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Output Curve AssessmentOutput Curve Assessment
pre-saturation
saturationID
VDScut-off�
�
�
�
��
VDSAT = VGS-VT
VGS↑
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Output Curve AssessmentOutput Curve Assessment
ID
VDS
�
pre-saturation
saturation
linear (wrt VDS)
ID,PRE =Z
LCI µ VGS − VT −
VDS
2
VDS� �
ID,SAT =Z
2LC
Iµ VGS − VT( )
2
� �
� �ID,LIN =
Z
LC
Iµ VGS − VT[ ]
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Output Curve AssessmentOutput Curve Assessment
VGS @ ID = 0 V � VT
ID
VDS
VT = + (E-mode)
VT = - (D-mode)
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Output Curve AssessmentOutput Curve Assessment
ID
VDS
‘Soft’ saturation (bad!)
‘Hard’ saturation (good!)
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Output Curve AssessmentOutput Curve Assessment
IC,SHUNT
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Output Curve AssessmentOutput Curve Assessment
ID
VDS
RSERIES!
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Output Curve AssessmentOutput Curve Assessment
RSERIES
�
�
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Output Curve AssessmentOutput Curve Assessment
ID
VDS
ILEAKAGE!
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Output Curve AssessmentOutput Curve Assessment
IG,LEAKAGE
�
�
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TransferTransfer
CurveCurve
(linear)(linear)
AssessmentAssessment
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Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID
VON
S
hysteresis?
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Transfer Curve (linear)Transfer Curve (linear)
S�
How to modify ID,IDEAL?
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Transfer Curve (linear)Transfer Curve (linear)
Wager, Keszler, & Presley, Transparent Electronics (2008).
Subthreshold current augmentation procedure:
1=m : uniform subthreshold trap density
G
IT
C
Cm +=1 : uniform trap densityID( )
−1= ID,SUB( )
−1
+ID,PRE( )
−1
m
−×
−
Tk
qV
B
DS
e1
( )
TkC
C1
VVq
2
BGSUBD,
BG
IT
ONGS
eq
TkµC
L
WI
+
−
=
ID,PRE =W
LµCG VGS − VON( )VDS −
VDS
2
2
�
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Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID log IG
GOOD!
leakage?
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Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID log IG
BAD!
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Output Curve AssessmentOutput Curve Assessment
IG,LEAKAGE
�
�
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Transfer Curve (linear)Transfer Curve (linear)
R. L. Hoffman, J. Appl. Phys. 95, 5813-5819 (2004).
• Mobility (average):
• Identical to μeff, except for VON.• Corresponds to average channel mobility.
(LINEAR)where
(LINEAR)0VDS
GSDGSD
DS
V
)(VI)(VG
→
=
0VONGSG
GSDGSAVG
DS
)V(VCL
Z
)(VG)(Vµ
→
−
=
VT
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Transfer Curve (linear)Transfer Curve (linear)• Mobility (incremental):
• Identical to μFE, except defined differentially.• Corresponds to incremental channel mobility.
(LINEAR)
R. L. Hoffman, J. Appl. Phys. 95, 5813-5819 (2004).
0V
G
GS
GSD
GSINC
DS
CL
Z
V
)(VG
)(Vµ
→
∂
∂
=
μINC → physics
μAVG →
circuits
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Transfer Curve (linear)Transfer Curve (linear)
2 ways to overestimate μμμμ:
• Test a leaky TFT
• Test a TFT with a non-patterned
channel
DON’T DO THIS!
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TransferTransfer
Curve Curve
(saturation)(saturation)
AssessmentAssessment
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Transfer Curve (saturation)Transfer Curve (saturation)
VGS
log ID
ID(on-off)
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Transfer Curve (saturation)Transfer Curve (saturation)
ID(on-off)
�
�
�
IG,LEAKAGE
μ
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Transfer Curve (saturation)Transfer Curve (saturation)
VGS
log ID
Increasing
leakage!
Decreasing
mobility!
ID(on-off)
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VVTT Transfer Transfer
CurveCurve
AssessmentAssessment
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VVTT Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
WHERE IS VT?
?
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VVTT Transfer Transfer
CurveCurve
(linear)(linear)
AssessmentAssessment
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ID
VGS
VVTT (linear) Curve Assessment(linear) Curve Assessment
VT-VDS/2
ID =Z
LC
Iµ VGS − VT −
VDS
2
VDS
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VVTT Transfer Transfer
CurveCurve
(saturation)(saturation)
AssessmentAssessment
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VVTT (saturation) Curve (saturation) Curve
AssessmentAssessment
VT
�
�
[ID]1/2
VGS
ID,sat =Z
2LC
Iµ VGS − VT( )
µSAT
=
d ID,SAT
dVGS
Z
2LCI
Intercept!
Slope!
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TransferTransfer
CurveCurve
(linear)(linear)
AssessmentAssessment
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Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
No hysteresis!
equilibrium
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Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
CW hysteresis!
non-equilibrium, steady-state
IF
curves
retrace!
Steady-state wrt sweep rate!
Hysteresis!
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Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
CCW hysteresis!
non-equilibrium, steady-state
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Transfer Curve AssessmentTransfer Curve Assessment
VGS
log IDnon-equilibrium, non-steady-state
Curves
do
NOT
retrace!
CW or CCW hysteresis!
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Transfer Curve AssessmentTransfer Curve Assessment
Hysteresis? Equilibrium? Retrace? Steady state?
no yes yes yes
CW no yes yes
CCW no yes yes
CW or CCW no no no
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Hysteresis PhysicsHysteresis Physics
Hysteresis arises from charge migration!Hysteresis arises from charge migration!
CCII = insulator capacitance density= insulator capacitance density
QQMIGRATIONMIGRATION = migrating charge density= migrating charge density
= change in charge centroid= change in charge centroid
= 0 (all charge @ IG interface)= 0 (all charge @ IG interface)
= 1 (all charge @ CI interface)= 1 (all charge @ CI interface)
∆VON
=− ∆γ QMIGRATION
CI
∆γγγ
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Hysteresis PhysicsHysteresis Physics
VGS
log ID
+
-
e- channel
insulator
gate
VGS = +∆γ = −� �
∆γ = +� �
∆γ = −� �
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Hysteresis PhysicsHysteresis Physics
Specie? Charge? Δγ? ΔVON Hysteresis?
e- - - + CW
+ion + + - CCW
-ion - - - CCW
CW hysteresis CW hysteresis �������� electronselectrons
CCW hysteresis CCW hysteresis �������� ionsions
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Hysteresis PhysicsHysteresis Physics
VGS
log ID
+
-
e- channel
insulator
gate
VGS = +CW = e-
CCW = +ionCCW = -ionΔVON= -
ΔVON= -ΔVON= +
17 March 2011 J. F. Wager 53
Ion migration
• field-aided diffusion
• unintentionally incorporated species
Electron migration• electron trapping of accumulated e-
• e- emission from trap states
Consider e- trapping in more detail…
Hysteresis PhysicsHysteresis Physics
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Oxide TFT AssessmentOxide TFT Assessment
e-
EA
trap
trap
trapx
EC
semiconductorinsulator
EC
τn,cap
=1
σn
vth
n(a)(b)
(c)Tk
E-
B
A
enth
vnσ
1
capn,τ =
xα-enth
vnσ
1
capn,τ =
FAST!
Slow…
Slow…
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Hysteresis SummaryHysteresis Summary
Equilibrium
• ideal case
Non-equilibrium, steady state
• e- traps (cw hysteresis)
Non-equlibrium, non-steady state
• e- traps, ion migration, leakage…
• Alice’s restaurant: “you can get
anything you want…”
WRT sweep rate!
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• TFT assessment is based on:
– ID(VGS,VDS)
– IG(VGS,VDS)
• 5 basic assessment methods
–– Output curveOutput curve
–– Transfer curve (linear, multiTransfer curve (linear, multi--sweep)sweep)
• Classification scheme
–– EquilibriumEquilibrium
–– NonNon--equilibrium, steadyequilibrium, steady--state (state (cwcw))
–– NonNon--equilibrium, nonequilibrium, non--steady statesteady state
ConclusionsConclusions
17 March 2011 J. F. Wager 574 April 2011 J. F. Wager 57
Collaborators and FundingCollaborators and Funding
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