Organic Field Effect Transistors (OFET)

Jump to: How Do OFETs Work? | Why Organics in FETs? | OFET Fabrication | OFET Characterization | OFET Limitations | OFET Applications | Flexible Electronics | Biosensors

An organic field effect transistor (OFET) is a device that uses a small gate voltage to control the current flow across an organic semiconductor material. This type of organic thin film transistor (OTFT) serves as an alternative to silicon-based field effect transistors such as MOSFETs. OFET devices consist of drain and source electrodes, a dielectric layer, an organic semiconductor (OSC) and a voltage electrode.

These devices can be used in a range of electronic devices as amplifiers, switches or within digital logic circuits. While their mobility is lower than inorganic FETs, their use of organic materials means that OFETs are lightweight, flexible, and compatible with low-cost solution-based fabrication methods. However, other limitations to overcome include lower mobility, slower switching, and environmental sensitivity.

Having said this, OFETs still remain promising for flexible circuits, biosensors, and even potential uses like artificial skin.

How Does an OFET Work?

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There are three terminals in a field effect transistor (FET): the source, the drain and the gate. The gate voltage acts to control the conductivity of the semiconductor material, tuning the current flow between the source and drain terminals off or on.

In organic FETs, this semiconductor active layer is an organic material. This organic semiconductor is combined with a dielectric layer which provides separation between gate terminal and the source/drain terminals. There are four different OFET device architectures:

• Bottom-gate, bottom-contact (BGBC)
• Bottom-gate top-contact (BGTC)
• Top-gate bottom-contact (TGBC)
• Top-gate top-contact (TGTC)

There are alternative configurations but these four are the most common. There are benefits and trade offs with each type of device configuration. Top contact (TC) devices have a smoother contact interface with the source and drain electrodes. But TC devices are difficult to mass produce due to restricted mask technologies.

BGBC devices are easiest to experiment with, as users can buy substrates with the electrodes and dielectric layers pre-fabricated such as the Ossila OFET Test Chips. Researchers can then simply coat this substrates with their chosen OSC. However in BCBG configurations, the semiconductor is quite exposed which is an issue, as organic components can be sensitive to oxygen and moisture.

TGBC devices have better stabilities as the organic layer is buried beneath other layers providing some encapsulation. However, it can be difficult to deposit the other layers (especially inorganics) on top of organic films without affecting or damaging it.

As previously mentioned the current flowing through the device is partially controlled by the gate voltage. When the gate voltage V_G ≠ 0 in an OFET, excess charge carriers accumulate at the interface between the OSC and dielectric layers. For p-type OFETs, a negative gate voltage (V_G) draws excess holes towards this interface. For n-type OSCs, electrons are drawn to the interface. Either way, this creates a channel for current to flow from the source and drain due the applied voltage between the drain and source (V_DS). P-type OSCs, will facilitate a flow of positive charge carriers (holes) when a negative V_G is applied. With n-type OSCs, a flow of negative charge carriers (electronics) flow when a positive V_G is applied.

OFET performance is defined using many parameters: threshold voltage (V_T), the charge mobility of the semiconductor (µ), the devices on/off current ratio (I_on/I_off), and the subthreshold slope. These values can be extracted from the transfer curve (I_source-drain against V_gate) or the output curve (I_source-drain against V_source-drain).

Transistors have two operating regions: the linear region and saturated region. Individual I-V curves are measured while the OFET is held at various values of V_G.

• When no voltage is applied (i.e the voltage between the drain and source electrodes is zero, V_DS=0), the drain current (i.e. the usable current output, I_D) is also zero.
• Within linear range (where V_DS < V_G-V_T), I_D is proportional to V_DS, and the device acts as a resistor.
• As V_DS increases, a pinch off region forms near drain electrode (where V_DS=V_G-V_T), limiting the movement of charge carriers between the source and drain electrodes. This effectively causes the channel to thin, meaning the channel depth is essentially zero at the drain electrode. The point where this happens is known as the pinch off point.
• Due to this restricted current flow, as V_DS increases, I_D remains constant. This is known as the saturation region.

In the linear region, drain current I_DS can be described about the following equation :

When V_DS<< V_G-V_T, the drain current can be described using the following equation:

The current behaviour past the pinch point is set at a constant:

Why Use Organic Materials in FETs?

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Firstly, OFET materials have to potential to be cheaper and easier to process than inorganic semiconductors. Namely, organic materials can be printed using techniques like spray coating or ink jet printing. Also, the organic semiconductors in OFETs can be specifically designed and optimized to maximize performance. Potential for high efficiency devices with thinner active layers, make these materials suitable flexible or light weight applications such as wearable or bendable electronics. They are also considered to be more environmentally friendly.

OFET are light weight and flexible with low power consumption and potential for easy integration. They can be incredible useful as switches, amplifiers, transducers, drivers, and storage components.

Organic semiconductors do not have delocalized electrons like inorganic semiconductors. Instead, charge transport happens through charge hopping between localized states. Therefore the carrier mobility of OFETs relies on the chemical structure of conjugated frameworks or how the molecules pi-orbitals overlap.

Over time, the electric field mobility of organic semiconductors have improved significantly and now OSC films have less grain boundaries, less defects and trap states. While they cannot compete with inorganic charge mobility, organics have improved to become comparable to polycrystalline silicon FETs.

	Electric Field Mobility
Polycrystalline Si FETs	> 10 cm2 V-1s-1
C10-DNTT	> 10.7 cm2 V-1s-1
Rubrene	> 24.5 cm2 V-1s-1
PDBPyBT	> 2.8-6.3 cm2 V-1s-1

OFET Fabrication

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The best fabrication methods for a given layer depends on the device architecture and materials being used. The easiest architecture to make OFETs in is the BCBG architecture. Here, the gate is deposited first, then the dielectric material, then drain and source can be deposited onto the substrate. Finally, the OSC can be deposited via solution processing methods. The ate is usually a doped silicon wafer, with a silicon oxide dielectric layer, and a metal as the source and drain electrodes. This is the architecture used in prefabricated test chips for OFET research such as these Platinum OFET Test Chips.

Often these layers are deposited with techniques such as thermal evaporation or e-beam deposition. This is where you heat a material under a vacuum until the solid evaporates (often reaching temperatures of >200 °C). You can also deposit organic materials via non-solution based techniques such as thermal evaporation. However, this must be done carefully as polymers can easily decompose under high temperatures.

Having said this, you would ideally like to reduce dependency on high temperature deposition techniques as one of the ideal used of OFETs is to create flexible electronics on plastic substrates. This is where high temperatures (>120 °C) can become an issue.

One of the main benefits of using organic materials is that they can be solution processed. Solution processing techniques can be simple to optimize, easily scaled and don't require extreme temperature processing. However, a lot of conducting polymers have low solubility, requiring volatile solvents to dissolve them. You could attach these polymers to more soluble components to increase their solubility.

A method for creating evaporation-free OFETs is shown below:

• The Ossila pre-patterned ITO substrates (For OFETs and Sensing) come with source-drain contacts ready deposited. The relatively large channel sizes work to minimize contact effects, so it is less important to match the energy levels of the organic semiconductor and contacts.
• Deposit the organic solar cell layer (P3HT in the schematic below) via spin coating, then the insulator or dielectric materials is spun on top
• Finally the gate material can be deposited to finish the device.
• You can spin coat these films then wipe off the areas to create needed pattern. Or use Kapton tape or other masking techniques so only coat a specific area.

OFET Characterization

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To measure an OFET requires three electrode connections: to the source, drain and gate electrodes. The bias beween the source-gate and the source-drain need to be independently controlled. Thin film transistors are often measured using a probe station, as flexible probe placement allows testing of many different device types.

To characterize an OFET device, you can measure its

• Transfer curves (drain current vs gate voltage) at fixed drain voltage
• Output curves (drain current vs drain voltage) at various gate voltages.

From these curves you can extract parameters such as threshold voltage, mobility, on/off ratio, subthreshold slope, etc.

A full characterization system such as the Ossila Electrical Characterization System integrates the probe station with a multichannel SMUs, multiple micromanipulators and all the necessary connectors (the Ossile ECS can excecute measurements between ±10 V.

OFET Limitations

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Some imitations of OFETs include the following:

• Organic semiconductors generally exhibit lower performance and device efficiencies compared to inorganics. For example, their charge carrier mobility is much lower than polycrystalline silicon.
• Lower stability. Lots of organic semiconductors suffer performance degradation with exposure to oxygen and moisture. Secure encapsulation will be needed to avoid this.
• Organic FETs have slower switching speed and lower packing density than inorganic FETs.
• They have a higher operational voltage compared to silicon based transistors or MOSFETs. This can cause issues with device lifetime, and increases the overall power consumption.

OFET Applications

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OFETs act as good sensors as they can have high selectivity and sensitivity. There are therefore plenty of situations where OFET sensors could be beneficial.

Flexible circuitry

OFETs would be great for use in flexible integrated circuits. One example application would involve making operational amplifiers out of OFETs that would be bendable and could be placed directly on skin. These have been show to successfully amplify a heartbeat signal within a noisy EPG. Flexible electronics are really useful for biologic and medical applications, especially when used in patches (people hate injections).

Photonic Sensors (Phototransistors)

Phototransistors combine light sensing of photodetectors and signal amplification of transistors. This creates photo-detecting devices that are more sensitive than photodetectors with lower noise and more tuneability. These can be used in many applications, such as medical imaging, optical communications and biological health monitoring.

Artificial skin

Skin has many integrated sensors. Artificial skin could therefore contain OFETs to potentially detect impedance, pressure, temperature, humidity or strain. To imitate human skin though any materials will need to be:

• Able to stretch (20-30% stretching strain)
• Adhesive
• Work together with other sensors
• Will need to be cheap to produce
• Biocompatible
• Self-powering
• Easily integrated

OFETs for Flexible Electronics

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One of the most promising application of OFET devices is for for use in flexible electronics. The challenge here is balancing electrical performance with the needed mechanical properties and flexibility. For example, polymers with a highly ordered lamellar structure and good crystallinity have increased carrier mobility, but amorphous materials have the best flexibility. Long range order is better for charge mobility, but often leads to a stiffer film.

Some approaches to consider when designing an OFET for flexible electronics include:

1. Enhancing the flexibility of the organic semiconductor materials themselves
2. Designing organic material structures that improve mechanical compliance, for example through backbone modification, interconnections, or framework adjustment

3. Developing elastomeric or flexible substrates to support the device.

ITO is the most commonly used electrode in organic devices. However, its deposition methods are not compatible with many flexible substrates and it is very expensive.

Alternative electrodes include thin metal films (such as Au, Ag, Al). These films should be as thin as possible. Nano-particle and nanowires can lend a good balance of conductivity and flexibility. Including silver nanowires (AgNW) with an organic conductor layer, such as HY E PEDOT:PSS combining Ag nanowires with PEDOT:PSS, can create a great electrode/conductor hybrid. These hybrid layers have high aspect ratio, excellent light transmittance (≈89%), and outstanding strain resistance. However, this will have higher sheet resistance than just silver electrodes alone.

PEDOT:PSS is a commonly used conducting polymer in flexible electronics, owing to mechanical strength, electrical stability and transmittance, good uniformity and bio compatibility. Carbon nanotubes are also excellent electrodes as they exhibit good conductivity, are robust, with high thermal stability and low sheet resistance.

OFETs as Biosensors

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Biosensors are devices that detect and analyze biological or biochemical substances. Usually electrical output of biosensors is modulated by the presence or concentration of the target biological analyte. OFETs function as biosensors when the target analytes attaches to the gate electrode, or interacts with the active layer, affecting device conductivity.

In the latter situation, a biological ligand or receptor on the analyte functionalizes the OSC channel, changing its semiconductor properties. These sensors can identify the presence of analyte as well as measuring its concentration.

Alternatively, the gate electrode in an OFET can be connected to a sensing probe that specifically targets the presence of analyte in an external solution. When the sensing electrode captures analyte targets, this modifies V_G affecting the readout of the OFET.

OFETs could be used as biosensors in a wide range of scenarios such as:

• Glucose sensors - You can functionalize PEDOT:PSS with a Gox enzyme to create a glucose sensitive biosensor, which creates a linear relationship between amount of glucose presence and V_G output. Modified P3HT has been shown to be useful in glucose sensors.
• DNA sensors - OFETs can be used to identify short chains of DNA. Introduce thin maleic anhydride polymer layer to an organic conducting layer, enabling the attachment of peptide nucleic acid.
• Enzyme detection - OFETs can identify specific enzymes through reactions that generate ionic or molecular by-products, which modulate the charge carrier mobility or threshold voltage within the transistor.
• Gas sensors – OFETs can be used as trapped analytes in gases. This relies on gases making covalent pi bonds made with analytes. For example, TIPS-pentacene used for NO₂ sensors.

More Resources

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