Thin Film Transistors and Drude Model Quiz

Processable Electronics: Materials Chemistry to Device Applications

• Lecture 9 covers thin-film transistors (TFTs), focusing on organic and small molecule TFTs.
• The lecture also discusses the thin film transistor, MOSFET vs TFT, and transport in organic TFTs.

Thin Film Transistor

• Thin-film transistors (TFTs) are discussed in the context of processable electronics.
• The lecture covers the operation and characteristics of TFTs, particularly focusing on organic/small molecule TFTs.
• TFTs differ from MOSFETs in their operation regions and charge carrier inversion.

MOSFET vs TFT

• MOSFETs typically operate in accumulation, inversion, and depletion regions, whereas TFTs typically operate in accumulation.
• Organic TFTs often exhibit poor inversion regions compared to the broader operation regions of MOSFETs.

Transport in Organic TFTs

• Transport in organic TFTs involves charge carriers, such as holes and electrons.
• The factors influencing the transport are described in the document.

Flexible Thin Film Transistor (TFT)

• A flexible TFT requires each layer to be flexible, stable, and conformable for long-term operation.
• Semiconducting materials, including inorganic (Group III, V, and VI semiconductors), organic (small molecules and polymers), and nanomaterials (e.g., graphene, single-layer MoS₂), are important components in the TFT's design.

Organic Thin Film Transistor

• Organic TFTs differ from inorganic MOSFETs (Si-substrate) in their basic operation of accumulation rather than inversion of charge carriers.
• Organic TFTs rarely demonstrate electron and hole mobility simultaneously.

TFT Design Approaches

• TFT layouts are categorized into staggered and coplanar configurations, further differentiated by bottom-gate and top-gate designs.
• Staggered bottom-gate structures are often used for high-temperature dielectric layers, whilst coplanar top-gate structures are common for high-temperature semiconductors.

The Thin Film Transistor (p-type)

• Charge transport in p-type TFT channels is dependent on the field effect.
• TFTs typically operate in accumulation. Holes are attracted to the semiconductor/oxide interface when a negative gate bias (Vg < 0 V) is applied.

The TFT - Accumulation

• VFB (flat-band voltage) is a crucial factor in TFT operation.
• A negative voltage applied to the gate causes a bending of valence and conduction bands, leading to accumulation of holes at the semiconductor/oxide interface.

TFT Channel Operations

• In TFTs, the metal-semiconductor junctions give rise to Schottky barriers, which play a crucial role in preventing high leakage current during the off-state.
• Applying a voltage between the drain and source contacts creates an off-current by tunneling. The gate electrode controls the potential barrier, thus controlling the drain current.

Schottky Barrier - Poor Blocking

• A Schottky barrier is formed at metal-semiconductor interfaces in TFTs, preventing charge carrier flow.
• The presence of this barrier can yield a leakage current, even in the off state, if this barrier is not high enough.
• The height of the barrier is controlled by factors (e.g. Va applied voltage).

Current in the Linear Region

• When gate voltage is higher than the threshold voltage, it induces a conductive channel in the TFT.
• Assuming μ_d is the mobility, the current is proportional to the product of μ_d, width, gate voltage, and potential difference, divided by the length.

Current in the Saturation Region

• For higher drain voltages, the current becomes constant (saturation).
• The saturation current is also described by mobility but is dependent on the square of the gate voltage.

TFT Operating Regions

• TFTs demonstrate linear and saturation regions of operation, distinct from MOSFETs.
• In the linear region, drain current linearly increases with increasing voltage.
• In saturation, the current levels off for higher drain voltages due to pinch-off.

Example of IGZO-TFT Fabrication (Sol-Gel Method)

• The fabrication of IGZO TFTs by the sol-gel method involves steps to deposit metallic gate electrodes, a dielectric layer (Al₂O₃ or SiO₂), and a channel layer (oxide precursor).
• These steps result in a finished IGZO-TFT.

Towards Flexible Transistors: Organic TFTs

• Conjugated molecules in organic TFTs typically arrange with their long axes parallel to facilitate charge transport.
• The charge density in the accumulation region depends on the stacking of dielectric layers with defined thickness.

Macromolecular Design of Conjugated Polymers

• Factors affecting conjugated polymer design include doping properties, control over bandgap, and electronic transport (conductivity).

Charge Transport in Conducting Polymers

• Describe and discuss types of transport in organic materials (polymers), potentially incorporating various scattering mechanisms (impurities, phonon scattering) and the effect of temperature.

Transport in Conducting Polymers

• Conjugated structures with alternating bonds support intrinsic conductivity in polymers such as polyacetylene.
• Molecular orbital formation directly affects electron density distribution within molecules, influencing electronic transport.

Electrons in Solids

• Wave functions and interfering standing waves can generate new quantum states.
• Multiple wells give rise to quasi-continuous energy levels in the solid.

Transport in Conducting Polymers

• Properties of conjugated materials often involve continuous orbital overlap that facilitates charge transport.
• Lowest energy molecular orbitals represent stable states for molecules.

Band Transport

• Band transport is common in conjugated polymers, often observed in the presence of an applied electric field, and involves scattering processes.
• The conductivity of a polaron is dependent on the mobility and the charge density.

Transport in Conducting Polymers

• Different transport mechanisms exist in conducting polymers. Variable-range hopping is an important consideration in conductivity.
• Conductivity is expressed as a function of temperature, often exhibiting exponential dependence described by the Mott law formula.

Child's Law

• Child's law describes current density in materials with high total charge Q under constant electric field and describes additional factors required to compute total current density.

Traps

• Trap states within a material can store charge and influence the current density.
• Traps affect the mobility within the surrounding space-charge region because of an effective mobility.

Transport in Conducting Polymers

• Band transport and the various potential transport mechanisms (e.g., polaron transport, hopping) in conducting polymers are discussed.

Example of Organic TFTs

• Examples of organic TFT devices include PMMA OTFT and P(VDF-TrFE)/PMMA.
• The characteristics of these devices include mobility, saturation current, and linear current.

Organic small molecules TFT

• Different devices and the properties (mobility) for these different types of conducting organic materials are covered.
• Properties such as mobility and switching behavior of organic small molecules (e.g., pure thiophene) can be modulated or enhanced by blending with certain polymers.

Organic TFTs

• A graph illustrates the increase in electron mobility in various organic TFTs from the 1980s to 2010.

Materials for Electronics

• Various materials for electronic applications, including graphene, single-crystalline silicon, and III-Vs, are discussed in the context of their electron mobility and band gaps.

Test your knowledge on the Drude model and its application to Thin Film Transistors (TFTs). This quiz covers key concepts such as mean free path, electron collisions, and the differences between MOSFETs and TFTs. Discover how material properties influence the performance of TFTs and understand the interactions within semiconductor layers.