Title: Tunable Threshold Transistor for improving the reliability of Large Area Electronics
Name: PIYUSH RANJAN
Date/Time: Mar 17th / 15:00:00
Location: SVN Auditorium IAP
Research Supervisor: Sanjiv Sambandan
Abstract:
Recent years have seen significant advancements in large-area electronics. Owing to their low manufacturing cost, compatibility with ambient-temperature processing, and the flexibility of the underlying substrates, these technologies have enabled a wide spectrum of applications, including wearable sensors, low-power energy-harvesting systems, modern display platforms, biodegradable electronic devices, and pressure-sensing systems designed for extreme environments such as space. However, the transition from concept to reliable circuit implementation remains challenging due to several mechanical and electrical reliability issues in thin-film transistors (TFTs), which serve as the fundamental building blocks of large-area electronics. One of the most critical concerns is the shift in threshold voltage, which can arise from prolonged electrical biasing, mechanical deformation during bending or stretching, and variations introduced during fabrication. These effects collectively pose major challenges for analog integrated-circuit designers and limit the stability and predictability of circuit performance. Several strategies have been explored to address threshold-voltage instability in thin-film transistors, including material, structural, and circuit-level approaches. Material methods use passivation layers or nanolaminates to suppress trap states, while structural techniques protect the device by positioning the active region near the neutral plane or embedding it beneath stiff layers. Circuit-based solutions introduce voltage, current, or feedback compensation to counter threshold drift. Although these approaches effectively manage instability, they often require additional processing, modifications to device structure, or application-specific design. Moreover, a common limitation is that they only compensate for threshold shifts but do not provide flexibility to set user defined threshold voltage.
This thesis presents the concept, design, and implementation of a novel three-TFT subcircuit, termed the Tunable Threshold Transistor (TTT), which operates like a conventional TFT but includes an additional control pin that allows the threshold voltage to be set by the user. The TFTs in the sub-circuit are scaled based on parameters derived from the density of defect states, resulting in a design where a user-defined voltage 𝑉𝑃 produces a threshold voltage of approximately 2𝑉𝑃 . This behaviour was confirmed through AIM-SPICE simulations and further validated experimentally by fabricating TTTs on polycarbonate substrates with silver electrodes, Parylene-C dielectrics, and TIPS-pentacene semiconducting channels. The measured threshold voltages closely followed the predicted 2𝑉𝑃 relation, with minor deviations attributed to several factors that are analysed and explained in detail using a mathematical framework. Moving forward, this thesis investigates a key application of the Tunable Threshold Transistor (TTT) in the form of a self-balancing inverter designed to address the inherently asymmetric voltage transfer characteristic (VTC) of conventional non-complementary inverters. This asymmetry arises from the absence of complementary devices, leading to a shifted switching threshold . In the proposed architecture, the conventional driver TFT is replaced by a TTT, enabling controlled tuning of the effective threshold voltage through the control terminal 𝑉𝑃 . By appropriately adjusting 𝑉𝑃 , the VTC symmetry is significantly improved. This improvement is first demonstrated through AIM-SPICE simulations and subsequently validated experimentally using fabricated circuits processed with the methodology described earlier. Overall, this work establishes the Tunable Threshold Transistor as a flexible and robust circuit-level solution for addressing threshold-voltage variability in large-area electronics. By enabling post-fabrication threshold control and demonstrating its impact at both device and circuit levels, the proposed approach offers a practical pathway toward more stable and reliable flexible electronic systems.