oxide thin -film transistor assessmentserver.semiconchina.org/downloadfile/1304663756406.pdf ·...

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.


TFT OperationTFT OperationSOURCE DRAIN

TCOTCO

Channel

Glass

TCO

Insulator

GATE

+V

+Vinjection extraction

transport

trapping

‘M’-I-’S’

control

AOS

ONLY!


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


Gate

VG VD

Source

Drain

ID ≡ ID(VG,VD)

TFT ITFT I--V ModelingV Modeling

ID,IDEAL


TFT ModelingTFT Modeling

STATICDYNAMIC INTRINSICIG,LEAKAGE IC,SHUNT

�

�


Basic Measurable Quantities Basic Measurable Quantities

ID(VGS,VDS) = ID,IDEAL+ IC,SHUNT + IG,LEAKAGE

IG(VGS,VDS) = -IG,LEAKAGE


Basic TFT MeasurementsBasic TFT Measurements

5 types:

• Output curve

• Transfer curve (linear)

• Transfer curve (saturation)

• VT transfer curve (linear)

• VT transfer curve (saturation)



Output curve

• ID-VDS

�D- or E-mode?

� ~VT

�Saturation?

� RSERIES?

� ILEAKAGE?

ID

?

OUTPUTINPUT VDSVGS

IG



Transfer curve (linear)

• log(ID)-VGS

• VDS<< VGS-VT (VDS = 0.1 V)

� VON

�S

� μ(VGS)

� Hysteresis?

� ILEAKAGE?

LINEAR!?

ID

OUTPUTINPUT VDSVGS

IG



Transfer curve (saturation)

• log(ID)-VGS

• VDS/tG ≥ ~2 MV/cm

� ID(on-off)

SATURATION!



VT transfer curve (linear)

• ID-VGS

• VDS<< VGS-VT (VDS = 0.1 V)

� VT

LINEAR!



VT transfer curve (saturation)

• [ID]1/2-VGS

• VDS/tG ≥ ~2 MV/cm

� VT

� μSAT

SATURATION!


OutputOutput

CurveCurve

AssessmentAssessment


Output Curve AssessmentOutput Curve Assessment

pre-saturation

saturationID

VDScut-off�

�

�

�

��

VDSAT = VGS-VT

VGS↑



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[ ]



VGS @ ID = 0 V � VT

ID

VDS

VT = + (E-mode)

VT = - (D-mode)



ID

VDS

‘Soft’ saturation (bad!)

‘Hard’ saturation (good!)



IC,SHUNT



ID

VDS

RSERIES!



RSERIES

�

�



ID

VDS

ILEAKAGE!



IG,LEAKAGE

�

�


TransferTransfer

CurveCurve

(linear)(linear)



Transfer Curve (linear)Transfer Curve (linear)

VGS

log ID

VON

S

hysteresis?



S�

How to modify ID,IDEAL?



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

�



VGS

log ID log IG

GOOD!

leakage?



VGS

log ID log IG

BAD!



IG,LEAKAGE

�

�



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


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



2 ways to overestimate μμμμ:

• Test a leaky TFT

• Test a TFT with a non-patterned

channel

DON’T DO THIS!


TransferTransfer

Curve Curve

(saturation)(saturation)



Transfer Curve (saturation)Transfer Curve (saturation)

VGS

log ID

ID(on-off)



ID(on-off)

�

�

�

IG,LEAKAGE

μ



VGS

log ID

Increasing

leakage!

Decreasing

mobility!

ID(on-off)


VVTT Transfer Transfer

CurveCurve



VVTT Transfer Curve AssessmentTransfer Curve Assessment

VGS

log ID

WHERE IS VT?

?



CurveCurve

(linear)(linear)



ID

VGS

VVTT (linear) Curve Assessment(linear) Curve Assessment

VT-VDS/2

ID =Z

LC

Iµ VGS − VT −

VDS

2

VDS



CurveCurve

(saturation)(saturation)



VVTT (saturation) Curve (saturation) Curve


VT

�

�

[ID]1/2

VGS

ID,sat =Z

2LC

Iµ VGS − VT( )

µSAT

=

d ID,SAT

dVGS

Z

2LCI

Intercept!

Slope!


TransferTransfer

CurveCurve

(linear)(linear)



Transfer Curve AssessmentTransfer Curve Assessment

VGS

log ID

No hysteresis!

equilibrium



VGS

log ID

CW hysteresis!

non-equilibrium, steady-state

IF

curves

retrace!

Steady-state wrt sweep rate!

Hysteresis!



VGS

log ID

CCW hysteresis!

non-equilibrium, steady-state



VGS

log IDnon-equilibrium, non-steady-state

Curves

do

NOT

retrace!

CW or CCW hysteresis!



Hysteresis? Equilibrium? Retrace? Steady state?

no yes yes yes

CW no yes yes

CCW no yes yes

CW or CCW no no no


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



VGS

log ID

+

-

e- channel

insulator

gate

VGS = +∆γ = −� �

∆γ = +� �

∆γ = −� �



Specie? Charge? Δγ? ΔVON Hysteresis?

e- - - + CW

+ion + + - CCW

-ion - - - CCW

CW hysteresis CW hysteresis �� electronselectrons

CCW hysteresis CCW hysteresis �� ionsions



VGS

log ID

+

-

e- channel

insulator

gate

VGS = +CW = e-

CCW = +ionCCW = -ionΔVON= -

ΔVON= -ΔVON= +


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…



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…


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!


• 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

oxide thin -film transistor assessmentserver.semiconchina.org/downloadfile/1304663756406.pdf ·...

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