Flexible low voltage high frequency organic thin film transistor

Over the past few decades, electronic applications on non-traditional substrates have required low temperature processing methods, which has promoted the development of organic thin film transistors (TFT). Such applications mainly require high-frequency switching (the rate at which electronic switches perform functions) or amplification at low operating voltages. However, most organic TFT technologies exhibit limited dynamic performance unless researchers apply high operating voltages to overcome their high contact resistance and large parasitic capacitance (that is, electronic components or components in circuits or circuits that exist due to proximity to each other Capacitance). In this work, James W. Borchert and a team of interdisciplinary researchers from Germany and Italy in nanoscience, chemistry, quantum science, and solid-state research proposed low-voltage organic TFTs. The device recorded static and dynamic performance, including contact resistance as small as 10Ω-cm, on/off current ratio as large as 10^10, and transit frequency as high as 21MHz. The inverted coplanar TFT structure developed in this work can be easily adapted to industry standard printing technology.

 

 

At present, driven by the latest trend of active matrix organic light-emitting diode (AMOLED) smart phone displays on polyimide substrates, flexible electronics is an industry with an annual output value of US$20 billion. Among the many challenges associated with transformation, scientists must reduce the process of thin film transistor (TFT) technology through low-temperature polysilicon (LTPS) to make it compatible with polyimide substrates while retaining the characteristics of TFTs. In this work, Borchert et al. demonstrated the ability of a previously reported method to develop low-voltage organic TFTs with low contact resistance to improve static and dynamic performance.

They fabricated TFTs and circuits on flexible polynaphthalene ethylene (PEN) wafers, and used high-resolution silicon mesh markers to draw patterns on all device layers. The team combined low contact resistance with a small channel length and small gate-to-contact point overlap to achieve record static and dynamic performance. They used a two-port network analysis (an electrical network with two terminals connected to an external circuit) to measure the dynamic performance of a single TFT operating under saturation. Then, Borchert et al. measured the dependence of the channel length on the transmission frequency and determined the width-normalized contact resistance to be 10±2Ω-cm. These experimental characteristics represent an important proof of concept for the development of low-power flexible circuits based on organic TFTs, which can be used in flexible AMOLED displays.

 

The team designed a small molecule organic semiconductor as the active layer of the TFT on a flexible polymer substrate. The channel length is 8 m, the gate-to-contact overlap is 4 m, and the channel width is 200 m. They determined the transmission and output characteristics of TFTs based on the different semiconductors that make up the device. The experimental results are similar to previous studies, confirming the good reproducibility of the manufacturing process. In the experiment, the scientists used two semiconductor materials abbreviated as DPh-DNTT and C10-DNTT to form a thermally stable thin film transistor (TFT). Then, they used an inverter composed of a TFT based on DPh-DNTT and an 11-stage ring oscillator based on a TFT on C10-DNTT to observe the static and dynamic circuit characteristics. The dimensions of the two circuits are exactly the same and maintain a similar partial load design.

In order to understand the dynamic performance of the inverter, Borchert et al. applied a square wave input signal with a frequency of 2 MHz and an amplitude of 1.5, 2.0, or 2.5 V to analyze the dynamic performance. They detected the minimum time constant (19 and 56 nanoseconds-ns) with a 2.5 V supply voltage of 2.5 V, and then summarized the results of the 11-stage ring oscillator. The team took pictures of the 11-stage ring oscillator circuit with a scanning electron microscope and measured its output signal. At supply voltages less than 50V, the signal propagation delay in this setup is the smallest value reported so far (1.6V supply voltage is 143ns, 4.4V supply voltage is 79ns).

 

The team obtained more detailed information about the dynamic characteristics of a single TFT through dual-port network analysis. Using scattering parameter (S-parameter) measurement, the team studied the high-frequency characteristics of organic TFTs. Based on this method, they conducted a detailed analysis of the dynamic characteristics of thin film transistors and observed that in all measurements, the area normalized gate leakage capacitance was constant with the change of frequency. The scientists determined the transmission frequencies and noticed their correlation with the channel length, thereby extracting the contact resistance and inherent channel mobility.

 

When the semiconductor layer extends beyond the edge of the device, the parasitic edge capacitance effect in the field effect transistor may also appear. As a result, the team reduced the gate-to-source overlap, while keeping the total gate-to-contact overlap and channel length constant to achieve a smaller total gate capacitance and higher transmission frequency. By optimizing the size of the TFT, the scientists obtained a transmission frequency of 21 MHz, which is the highest value reported so far for organic transistors on flexible substrates. The research results show that organic TFTs with static and dynamic properties can be used for high-frequency mobile electronic applications on flexible substrates. The result of this work is close to the industry standard low-temperature polysilicon TFT, while using the TFT structure in line with the existing industry standard manufacturing process.

 

Over the past few decades, electronic applications on non-traditional substrates have required low temperature processing methods, which has promoted the development of organic thin film transistors (TFT). Such applications mainly require high-frequency switching (the rate at which electronic switches perform functions) or amplification at low operating voltages. However, most organic TFT technologies exhibit limited dynamic performance unless researchers apply high operating voltages to overcome their high contact resistance and large parasitic capacitance (that is, electronic components or components in circuits or circuits that exist due to proximity to each other Capacitance). In this work, James W. Borchert and a team of interdisciplinary researchers from Germany and Italy in nanoscience, chemistry, quantum science, and solid-state research proposed low-voltage organic TFTs. The device recorded static and dynamic performance, including contact resistance as small as 10Ω-cm, on/off current ratio as large as 10^10, and transit frequency as high as 21MHz. The inverted coplanar TFT structure developed in this work can be easily adapted to industry standard printing technology.

 

At present, driven by the latest trend of active matrix organic light-emitting diode (AMOLED) smart phone displays on polyimide substrates, flexible electronics is an industry with an annual output value of US$20 billion. Among the many challenges associated with transformation, scientists must reduce the process of thin film transistor (TFT) technology through low-temperature polysilicon (LTPS) to make it compatible with polyimide substrates while retaining the characteristics of TFTs. In this work, Borchert et al. demonstrated the ability of a previously reported method to develop low-voltage organic TFTs with low contact resistance to improve static and dynamic performance.

 


Post time: Dec-30-2020

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