Towards Predictable Compact Model Descriptions for Organic Thin-Film Transistors S. Mijalković, D. Green, A. Nejim Silvaco Technology Centre, St Ives,

Towards Predictable Compact Model Descriptions for Organic Thin-Film Transistors

S. Mijalković, D. Green, A. NejimSilvaco Technology Centre, St Ives, Cambridgeshire, UK

A. Rankov, E. Smith, T. Kugler, C. Newsome, J. HallsCambridge Display Technologies, Godmanchester, UK

MOS-AK / GSA Workshop, 3 April 2009, IHP, Frankfurt / Oder

Organic Electronics: “Harry Potter’’ and “Star Trek” Technologies are Coming your Way!

o Fast development of organic electronics is supported by applications that require low cost electronic circuits covering mechanically flexible large areas.

o These include e-skin, e-paper, e-nose, smart-fabrics, flexible displays, printed electronics or radio frequency identification tags (RFID).

o Organic electronics can be fabricated using faster and cheaper low temperature processes.o Basis of the future organic electronic circuits are organic TFTs (OTFTs).

e-nose

e-skin

Smart fabrics

Flexible display


Organic Solar Cells

Organic Solar Cells

Organic Electronics: Forecast and Opportunities

o The organic electronics will be a $30 billion business in 2015 mainly due to logic, displays and lighting.

o It will be a $250 billion business in 2025, with at least ten billion dollars sales from logic/memory, OLED displays for electronic products, OLED billboard, signage etc, non-emissive organic displays, OLED lighting, batteries and photo-voltaics, with sensors almost at that level.

o Organic lighting will severely dent sales of both incandescent and fluorescent lighting in the second decade from now.

o Organic electronics in the form of electronic billboards, posters, signage and electronic books will revolutionize the conventional printing and publishing industry.

o The future organic market will be newly created without replacing much from the inorganic semiconductors in existing electronics products.


Organic Electronics:Challenge for Electronic Design Automation (EDA)

o Inorganic semiconductor industry relies extensively on EDA software to support the iterative cycles of process, device and circuit technology improvements.

o To further develop organic electronics industry, equivalent design tools are needed.

o EDA tools essentially depend on numerical and analytical device models which are, in case of OSCs, not yet matured and quite sparsely implemented in commercial EDA tools.

o Cambridge Display Technology (CDT) and Silvaco Europe have joined forces together in a TSB funded project entitled PMOS to enhance EDA tools for use in the organic electronics and to help move organic transistor technology from the lab to the shop floor.MOS-AK / GSA Workshop, 3 April 2009, IHP, Frankfurt / Oder

UK Technology Strategy Board (TSB) Project: Physical Modelling of Organic Semiconductors (PMOS)

• Cambridge Display Technology (CDT)– Expert in polymer light emitting diode (PLED) technologies.– Leader in development of solution processable (printable) organic.

semiconductors for display fabrication.– Expertise in development of PLED materials and deposition processes.

• Silvaco– Leading provider of TCAD and EDA software for IC design– Provides established products for TCAD process and device simulation, spice

parameter extraction, circuit simulation, custom IC design and verification.

Project activities• Design of OTFT devices using physical TCAD modelling.• OTFT spice modelling and parameter extraction.• Measurements and modelling of device reliability and aging effects. • The focus is on display device (OLED) drivers as these will be the first

large scale organic semiconductor products.

Project partners


OFET Architectures and Peculiar Features

o OSC morphology varies from amorphous to (poly)-crystalline.

o OFETs are commonly realized using an OSC layer without a deliberate doping.

o The carriers that contribute to the charge distribution and transport in OFETs must be injected from the metallic contacts.

o Without particular semiconductor type of the OSC layer, OFET can operate in the electron or hole carrier accumulation mode depending on polarity of the gate voltage and capabilities of the contacts to inject particular carrier type.

o The source and drain have no junction isolation. A drain/source leakage current is limited by intrinsic OSC conductivity and contact resistance rather then reverse junction current.

o Contact resistances often dominate the OFET performance and represent a bottleneck to achieve full potential of the intrinsic transistor effect.

o OFETs are typically characterized with much lower carrier intrinsic carrier mobility having different bias physical origin then their inorganic counterparts.


Density of States and Carrier Concentration

E

dEEfEgn AA )()(

1. Intrinsic DOS from molecular LUMO (HOMO) energy levels ranging from delocalized conduction and valence bands in molecular crystals to localized trap-like energy distributions amorphous OSCs.

2. DOS from localized in-gap trap energy levels.

HOMO

LUMO

E

TRAPS

CB

VB


Silvaco Atlas: OFET Example


Variable Range Hopping Conductivity ModelPercolation Theory

8.2

)(

)(

dEEEkTsEg

dEdEdRssEg

N

NsB

Fc

jiijijc

s

bcc

)exp(0cn s

qn

cij

cc

jiFjFiijij

ijij

GG

sGGkT

EEEEEE

a

Rs

sGG

exp2

2

exp

0

0


Sheet Channel Conductivity

TT

kT

Vqn

B

TT Ci

c

/exp

2

)/( 0

003

30

0

Vissenberg and Matters, Phys. Rev. B, 1998.


0

exp)(kT

VqTnn C

i

Exponential DOS distribution

00

exp)(kT

EEq

kT

NEg CA

A

Carrier concentration

)/sin(

/)(

0

00 TT

TTnTn ii

Local channel conductivity

Sheet Channel Conductivity (and Mobility)

Ci

CCC

CCC

VC

Q

QG

0

TT

B

TTTTn

qn

T

T

C

nkTV

c

i

iC

i

SiC

/

2

)/sin()/(

12

2

0

30

400

0

00

0

00


Exact

Model

Channel Sheet Charge: Surface Potential (SP) Model

sFBGiC

i

is

t

CstsFBG

VVCQ

xxxh

C

TnqG

V

VhGVVV

1)exp()(

)(2

0

20

2-25 -20 -15 -10 -5 0 5 10 15 20

-25

-20

-15

-10

-5

0

5

Surface Potential (GAMMA=1e-4)

(Vg-Vfb-Vc) [V]

(Psi

-Vc)

[V]

-25 -20 -15 -10 -5 0 5 10 15 20

-25

-20

-15

-10

-5

0

5

Surface Potential (GAMMA=1)

(Vg-Vfb-Vc) [V]

(Psi

-Vc)

[V]

0

exp)(kT

VqTnn C

i

Exponential DOS carrier concentration gives

SP equation in accumulation operation mode:

There is an approximate analytical solution (Gildenblat, et al., IEEE J. SSC, 2004)


Channel Sheet Charge: Unified Charge Control Model (UCCM)

0

0

0

2

ln

2

11

tiCP

CTGiCP

CtiCCP

t

CC

CCi

C

i

GVs

C

iC

C

GVC

s

VCQ

VVVCQ

QVCQQ

V

QC

dVCC

Qd

CQ

CC

C

V

GV

n

s

iC

CC

0

0

2

)2ln(exp1ln

)2ln(

2

t

CTGitC V

VVVCVQ


Intrinsic Drain-Source Current

x

y

zW

L

S Dd

D

S

V

V

nCDS dGL

WI

)(

21

)(

0

sC

it

s

n

V

Vs

s

nsCDS

Q

CV

d

d

dd

dG

L

WI

D

S

Surface-Potential Based Model

C

t

iC

n

C

Q

Q C

nCCDS

Q

V

CQd

d

QdQd

dQG

L

WI

CD

CS

01

)(

Charge Based Model


12

2

11

0diff

22drift

diffdrift

DCSCtDS

i

DCSCDS

DSDSDS

QQ

L

WVI

C

QQ

L

WI

III

Parameter Extraction in UTMOST IV


Model Verification: Surface Potential Based

-40 -35 -30 -25 -20 -15 -10 -5 0-8

10

-7

10

-6

10

-5

10

-4

10

Vg [V]

-Ids

[A]

-40 -35 -30 -25 -20 -15 -10 -5 0-0.07

-0.06

-0.05

-0.04

-0.03

-0.02

-0.01

0

Vds [V]Id

s [m

A]

Comparison between simulated (lines) and measured (circles and squares) transfer characteristics of the OTFT

Comparison between simulated (lines) and measured (circles and squares) output characteristics of the OTFT


Model Verification: Charge Based


Comparison between simulated (lines) and measured (circles) transfer characteristics of the OTFT in the linear operation region with Vds=-3V (blue line and circles) and saturation operation region with Vds=-30V (red line and circles)

Comparison between simulated (lines) and measured (circles) output characteristics of the OTFT for Vg=-10V, -20V, -30V and -40V.

Model Verification: Temperature Variations


Comparison between simulated (lines) and measured (circles) transfer characteristics of the OTFT in the saturation operation region at different temperatures: T=270K (dark blue) T=280K (light blue) T=300K (green) T=310K (pink) T=330K (red)

Origin of Contact Resistance


Oxide layerContamination layer

Geometric Surface Contamination

Electronic Morphology

Oxide layer will grow on many metals Residues from resist processes etc

Top gate, bottom contact has greater area for injection, thus our preferred architecture

Top Gate

S

G

Bottom Gate

S

G

Voids in OSC to source/drain lead to poor contactCrystal orientation importantS

LUMO

HOMOWF

Electronic barriers to injection into air stable, deep HOMO OSC

Strong Influence of the Series Resistances


-40 -35 -30 -25 -20 -15 -10 -5 -0

-1.4e-03

-1.2e-03

-1.0e-03

-8.0e-04

-6.0e-04

-4.0e-04

-2.0e-04

0.0e+00

2.0e-04

Output Characteristics (no series resistances)

Vd [V]

Id [A

]

Contact Resistance Extraction

)()(

1)( 1

0

GSGSSD

TGSeffGSSDON VSLLVR

VVKW

LLVRR

Channel Length, L

Tota

l Cha

nnel

Resi

stan

ce

Vg = -20V

Vg = -30V

Vg = -40V

Contact resistance at L=0Contact resistance at L=0

VVGS 40VVGS 30

VVGS 20

VVGS 10

500um

Beff

GB

Geff

GB

L

L

L

L

11

11

0


cm

VF

kT

FF

6

00

102

exp)(

Non-Quasi-Static Effects


0GTW

LR

eff

efft

0CTWLC effefft 0

11

DSVGS

DSss

DXSX V

IK

RR

Some Other Open Issues

• Influence of the bias dependent bulk and interface traps:- extended surface potential equation including forward and reverse, (accumulation and depletion) operation mode,- a bias modulated back channel conductivity.

• Leakage currents- bulk and contact limited leakage effect, - temperature dependence.

• Short channel effects: - physics based channel length modulation,- effects of the depletion and strong lateral electric field on the drain side,- space-charge limited transport.

• Gate leakage current model.• The influence of the OSC film thickness on the interface electric field

- layered distribution of the accumulated charge due to the elongated molecular geometry and regular assembly,- floating body potential.

• Aging and hysteresis of the OTFT characteristics within the model and the corresponding circuit design.


Acknowledgement

This work is supported by the UK Technology Strategy Board through the PMOS project TP/J2519J.

We want to thank Prof. Benjamin Iñiguez and his group for valuable recommendations regarding compact

organic TFT modelling.

Documents

Towards Predictable Compact Model Descriptions for Organic Thin-Film Transistors S. Mijalković, D. Green, A. Nejim Silvaco Technology Centre, St Ives,