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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
17 March 2011 J. F. Wager 4
Gate
VG VD
Source
Drain
ID ≡ ID(VG,VD)
TFT ITFT I--V ModelingV Modeling
ID,IDEAL
17 March 2011 J. F. Wager 5
TFT ModelingTFT Modeling
STATICDYNAMIC INTRINSICIG,LEAKAGE IC,SHUNT
�
�
17 March 2011 J. F. Wager 6
Basic Measurable Quantities Basic Measurable Quantities
ID(VGS,VDS) = ID,IDEAL+ IC,SHUNT + IG,LEAKAGE
IG(VGS,VDS) = -IG,LEAKAGE
17 March 2011 J. F. Wager 7
Basic TFT MeasurementsBasic TFT Measurements
5 types:
• Output curve
• Transfer curve (linear)
• Transfer curve (saturation)
• VT transfer curve (linear)
• VT transfer curve (saturation)
17 March 2011 J. F. Wager 8
Basic TFT MeasurementsBasic TFT Measurements
Output curve
• ID-VDS
�D- or E-mode?
� ~VT
�Saturation?
� RSERIES?
� ILEAKAGE?
ID
?
OUTPUTINPUT VDSVGS
IG
17 March 2011 J. F. Wager 9
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
17 March 2011 J. F. Wager 10
Basic TFT MeasurementsBasic TFT Measurements
Transfer curve (saturation)
• log(ID)-VGS
• VDS/tG ≥ ~2 MV/cm
� ID(on-off)
SATURATION!
17 March 2011 J. F. Wager 11
Basic TFT MeasurementsBasic TFT Measurements
VT transfer curve (linear)
• ID-VGS
• VDS<< VGS-VT (VDS = 0.1 V)
� VT
LINEAR!
17 March 2011 J. F. Wager 12
Basic TFT MeasurementsBasic TFT Measurements
VT transfer curve (saturation)
• [ID]1/2-VGS
• VDS/tG ≥ ~2 MV/cm
� VT
� μSAT
SATURATION!
17 March 2011 J. F. Wager 13
OutputOutput
CurveCurve
AssessmentAssessment
17 March 2011 J. F. Wager 14
Output Curve AssessmentOutput Curve Assessment
pre-saturation
saturationID
VDScut-off�
�
�
�
��
VDSAT = VGS-VT
VGS↑
17 March 2011 J. F. Wager 15
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[ ]
17 March 2011 J. F. Wager 16
Output Curve AssessmentOutput Curve Assessment
VGS @ ID = 0 V � VT
ID
VDS
VT = + (E-mode)
VT = - (D-mode)
17 March 2011 J. F. Wager 17
Output Curve AssessmentOutput Curve Assessment
ID
VDS
‘Soft’ saturation (bad!)
‘Hard’ saturation (good!)
17 March 2011 J. F. Wager 18
Output Curve AssessmentOutput Curve Assessment
IC,SHUNT
17 March 2011 J. F. Wager 19
Output Curve AssessmentOutput Curve Assessment
ID
VDS
RSERIES!
17 March 2011 J. F. Wager 20
Output Curve AssessmentOutput Curve Assessment
RSERIES
�
�
17 March 2011 J. F. Wager 21
Output Curve AssessmentOutput Curve Assessment
ID
VDS
ILEAKAGE!
17 March 2011 J. F. Wager 22
Output Curve AssessmentOutput Curve Assessment
IG,LEAKAGE
�
�
17 March 2011 J. F. Wager 23
TransferTransfer
CurveCurve
(linear)(linear)
AssessmentAssessment
17 March 2011 J. F. Wager 24
Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID
VON
S
hysteresis?
17 March 2011 J. F. Wager 25
Transfer Curve (linear)Transfer Curve (linear)
S�
How to modify ID,IDEAL?
17 March 2011 J. F. Wager 26
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
�
17 March 2011 J. F. Wager 27
Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID log IG
GOOD!
leakage?
17 March 2011 J. F. Wager 28
Transfer Curve (linear)Transfer Curve (linear)
VGS
log ID log IG
BAD!
17 March 2011 J. F. Wager 29
Output Curve AssessmentOutput Curve Assessment
IG,LEAKAGE
�
�
17 March 2011 J. F. Wager 30
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
17 March 2011 J. F. Wager 31
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
17 March 2011 J. F. Wager 32
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!
17 March 2011 J. F. Wager 33
TransferTransfer
Curve Curve
(saturation)(saturation)
AssessmentAssessment
17 March 2011 J. F. Wager 34
Transfer Curve (saturation)Transfer Curve (saturation)
VGS
log ID
ID(on-off)
17 March 2011 J. F. Wager 35
Transfer Curve (saturation)Transfer Curve (saturation)
ID(on-off)
�
�
�
IG,LEAKAGE
μ
17 March 2011 J. F. Wager 36
Transfer Curve (saturation)Transfer Curve (saturation)
VGS
log ID
Increasing
leakage!
Decreasing
mobility!
ID(on-off)
17 March 2011 J. F. Wager 37
VVTT Transfer Transfer
CurveCurve
AssessmentAssessment
17 March 2011 J. F. Wager 38
VVTT Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
WHERE IS VT?
?
17 March 2011 J. F. Wager 39
VVTT Transfer Transfer
CurveCurve
(linear)(linear)
AssessmentAssessment
17 March 2011 J. F. Wager 40
ID
VGS
VVTT (linear) Curve Assessment(linear) Curve Assessment
VT-VDS/2
ID =Z
LC
Iµ VGS − VT −
VDS
2
VDS
17 March 2011 J. F. Wager 41
VVTT Transfer Transfer
CurveCurve
(saturation)(saturation)
AssessmentAssessment
17 March 2011 J. F. Wager 42
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!
17 March 2011 J. F. Wager 43
TransferTransfer
CurveCurve
(linear)(linear)
AssessmentAssessment
17 March 2011 J. F. Wager 44
Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
No hysteresis!
equilibrium
17 March 2011 J. F. Wager 45
Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
CW hysteresis!
non-equilibrium, steady-state
IF
curves
retrace!
Steady-state wrt sweep rate!
Hysteresis!
17 March 2011 J. F. Wager 46
Transfer Curve AssessmentTransfer Curve Assessment
VGS
log ID
CCW hysteresis!
non-equilibrium, steady-state
17 March 2011 J. F. Wager 47
Transfer Curve AssessmentTransfer Curve Assessment
VGS
log IDnon-equilibrium, non-steady-state
Curves
do
NOT
retrace!
CW or CCW hysteresis!
17 March 2011 J. F. Wager 48
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
17 March 2011 J. F. Wager 49
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
17 March 2011 J. F. Wager 50
Hysteresis PhysicsHysteresis Physics
VGS
log ID
+
-
e- channel
insulator
gate
VGS = +∆γ = −� �
∆γ = +� �
∆γ = −� �
17 March 2011 J. F. Wager 51
Hysteresis PhysicsHysteresis Physics
Specie? Charge? Δγ? ΔVON Hysteresis?
e- - - + CW
+ion + + - CCW
-ion - - - CCW
CW hysteresis CW hysteresis �������� electronselectrons
CCW hysteresis CCW hysteresis �������� ionsions
17 March 2011 J. F. Wager 52
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
17 March 2011 J. F. Wager 54
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…
17 March 2011 J. F. Wager 55
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!
17 March 2011 J. F. Wager 56
• 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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