Integrated circuits (ICs) based on organic transistors have many valuable applications, for instance, in the fabrication of paper-like displays or other large-area electronic components. Over the past few decades, electronics engineers worldwide have developed a variety of these transistors.
A promising alternative to these transistors are vertical-channel dual-gate organic thin-film transistors. These transistors have several advantageous properties, such as short channel lengths and tuneable threshold voltages (VTH). Despite these advantages, due to a lack appropriate p- and n-type devices, developing complementary inverter circuits for these transistors has so far proved challenging.
Researchers at Technische Universitat Dresden, Helmholtz-Zentrum Dresden Rossendorf (HZDR) and Northwestern Polytechnical University have recently developed vertical organic permeable dual-base transistors that could be integrated in logic circuits. They evaluated the potential use of these transistors in complex integrated circuits.
“The dual-gate Transistor we developed as part of our previous research consists of a single vertical-channel thin-film Transistor with an additional second gate and second dielectric, which can be used to tune its threshold voltage,” Erjuan Guo, one of the researchers who carried out the study, told. “In our new study, we studied the function and benefit of the vertical-channel dual-gate transistors in more complicated integrated circuits, for example, organic complementary inverters and ring oscillators, further.
Guo and her colleagues created integrated complementary inverters by connecting vertical n-channel organic permeable dual-base transistors (OPDBTs), and vertical p-channel organic permeable base transistors (OPBTs). Notably, the second gate in the OPDBTs can control the on and off-states of the transistors, thus influencing the states of the inverters.
“Based on the measurements we collected, we find that dual-base transistors enable a wide range of switching voltage controllability of a complementary inverter over 0.8 V, at an input voltage of <2.0 V, in a deterministic manner,” Guo said. “We hence realized a switching voltage-tuneable inverter circuit using a VTH-tuneable n-type OPDBT and a p-type OPBT.”
Based on dynamic response characteristics, the inverters developed by Guo and her colleagues can maintain high/low output signals even at 10 MHz of the input signal. In addition, they can also reach very short rise and fall time constants of 5 ns and 6 ns.
In addition to realizing a new organic complementary inverter, Guo and her colleagues fabricated seven-stage complementary organic ring oscillators, integrating 7 inverters. These inverters allowed them to demonstrate the advantages of vertical organic transistors for dynamic performance.
“At a supply voltage of 4.0 V, the measured signal propagation delay is 11 ns per stage for the ring oscillator is in a similar range as the rise and fall time of the single inverter,” Guo said. “These signal delays are short in comparison to those reported so far for organic ring oscillators on any substrate at supply voltages of less than 10 V.”
Guo and her colleagues were the first to use vertical-channel dual-base organic thin-film transistors to fabricate integrated complementary inverters. Their work could inspire other teams to create similar inverters, thus paving the way towards the creation of new electronic components.
The study confirms the potential of using vertical organic transistors to fabricate high-frequency logic circuits. The transistors are not currently fast enough to be implemented on a large scale, so the team plans to conduct further studies aimed at increasing the speed and reducing the size of vertical-channel dual-base organic transistors.
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