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Towards Predictable Compact Model Descriptions for Organic Thin-Film Transistors. S. Mijalković , D. Green, A. Nejim Silvaco Technology Centre, St Ives, Cambridgeshire, UK A. Rankov , E. Smith, T. Kugler , C. Newsome, J. Halls Cambridge Display Technologies, Godmanchester , UK. e-nose.

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towards predictable compact model descriptions for organic thin film transistors

Towards Predictable Compact Model Descriptions for Organic Thin-Film Transistors

S. Mijalković, D. Green, A. Nejim

Silvaco Technology Centre, St Ives, Cambridgeshire, UK

A. Rankov, E. Smith, T. Kugler, C. Newsome, J. Halls

Cambridge 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

e-nose

e-skin

Flexibledisplay

Smart fabrics

Organic Electronics:“Harry Potter’’ and “Star Trek” Technologies are Coming your Way!
  • Fast development of organic electronics is supported by applications that require low cost electronic circuits covering mechanically flexible large areas.
  • These include e-skin, e-paper, e-nose, smart-fabrics, flexible displays, printed electronics or radio frequency identification tags (RFID).

Organic Solar Cells

Organic Solar Cells

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

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

organic electronics forecast and opportunities
Organic Electronics: Forecast and Opportunities
  • The organic electronics will be a $30 billion business in 2015 mainly due to logic, displays and lighting.
  • 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.
  • Organic lighting will severely dent sales of both incandescent and fluorescent lighting in the second decade from now.
  • Organic electronics in the form of electronic billboards, posters, signage and electronic books will revolutionize the conventional printing and publishing industry.
  • The future organic market will be newly created without replacing much from the inorganic semiconductors in existing electronics products.

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

organic electronics challenge for electronic design automation eda
Organic Electronics:Challenge for Electronic Design Automation (EDA)
  • Inorganic semiconductor industry relies extensively on EDA software to support the iterative cycles of process, device and circuit technology improvements.
  • To further develop organic electronics industry, equivalent design tools are needed.
  • 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.
  • 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 labto 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
UK Technology Strategy Board (TSB) Project: Physical Modelling of Organic Semiconductors (PMOS)

Project partners

  • 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.

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

ofet architectures and peculiar features
OFET Architectures and Peculiar Features
  • OSC morphology varies from amorphous to (poly)-crystalline.
  • OFETs are commonly realized using an OSC layer without a deliberate doping.
  • The carriers that contribute to the charge distribution and transport in OFETs must be injected from the metallic contacts.
  • 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.
  • 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.
  • Contact resistances often dominate the OFET performance and represent a bottleneck to achieve full potential of the intrinsic transistor effect.
  • OFETs are typically characterized with much lower carrier intrinsic carrier mobility having different bias physical origin then their inorganic counterparts.

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

density of states and carrier concentration
Density of States and Carrier Concentration

E

E

LUMO

CB

TRAPS

VB

HOMO

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.

DOS from localized in-gap trap energy levels.

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

silvaco atlas ofet example
Silvaco Atlas: OFET Example

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

variable range hopping conductivity model percolation theory
Variable Range Hopping Conductivity ModelPercolation Theory

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

sheet channel conductivity
Sheet Channel Conductivity

Exponential DOS distribution

Carrier concentration

Local channel conductivity

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

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

sheet channel conductivity and mobility
Sheet Channel Conductivity (and Mobility)

Exact

Model

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

channel sheet charge surface potential sp model
Channel Sheet Charge: Surface Potential (SP) Model

Exponential DOS carrier concentration gives

SP equation in accumulation operation mode:

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

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

channel sheet charge unified charge control model uccm
Channel Sheet Charge: Unified Charge Control Model (UCCM)

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

intrinsic drain source current
Intrinsic Drain-Source Current

Surface-Potential Based Model

Charge Based Model

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

parameter extraction in utmost iv
Parameter Extraction in UTMOST IV

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

model verification surface potential based
Model Verification: Surface Potential Based

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

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

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

model verification charge based
Model Verification: Charge Based

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

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)

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

model verification temperature variations
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)

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

origin of contact resistance
Origin of Contact Resistance

Surface Contamination

Geometric

  • Top gate, bottom contact has greater area for injection, thus our preferred architecture
  • Oxide layer will grow on many metals
  • Residues from resist processes etc

G

Contamination layer

Oxide layer

S

S

G

Top Gate

Bottom Gate

Morphology

Electronic

  • Electronic barriers to injection into air stable, deep HOMO OSC

LUMO

  • Voids in OSC to source/drain lead to poor contact
  • Crystal orientation important

WF

S

HOMO

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

strong influence of the series resistances
Strong Influence of the Series Resistances

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

contact resistance extraction
Contact Resistance Extraction

Contact resistance at L=0

500um

Vg = -20V

Vg = -30V

Total Channel

Resistance

Vg = -40V

Channel Length, L

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

non quasi static effects
Non-Quasi-Static Effects

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

some other open issues
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.

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

acknowledgement
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.