MgZnO-based high voltage thin-film transistors
Creators: name: Li, Yuxuan
Publisher: name: No Publisher Supplied
Year: 2022

description: Thin-film transistors (TFTs) have been widely used in large-area electronic systems such as pixel switching elements for flat panel displays because they can be fabricated at low temperatures with cost-effective processes. Such an operation requires fast driving speed and high drive currents but low voltages. Recently, building-integrated photovoltaics (BIPV) have been emerging, where a complete PV system is integrated into building envelopes to harvest solar energy effectively and aesthetically with a low cost. In BIPV, high voltage thin-film transistors (HVTFT) are critical devices serving as micro-inverters which are indispensable for energy conversion in such systems. The conventional high voltage transistors using wide-bandgap compound semiconductors like SiC and GaN are excluded from these applications because they require high-temperature epitaxial growth on the lattice-matched single crystalline substrates. On the other hand, Si-based high voltage transistors are opaque, thus incompatible with the transparent PV system on a glass window.This dissertation focuses on study and demonstration of the novel MgₓZn₁₋ₓO (MZO)-based HVTFT technology. MZO material retains ZnO’s excellent properties, such as a wide energy bandgap benefiting high voltage performance; it also improves TFTs' thermal and bias stabilities by suppressing high defect density in the pure ZnO. The novel MZO HVTFT on glass is designed and fabricated. The high-resistive ungated offset region in the channel is optimized to achieve a balance between the requirements of blocking voltage and on-resistance. Furthermore, the stacking gate dielectrics consisting of high-permittivity (high-κ) Al₂O₃ and high-quality SiO₂ ensure the high blocking voltage and decent on-current. We have demonstrated an unprecedented high blocking voltage over 900V while retaining an on/off current ratio larger than 10⁹. The MZO HVTFT on glass shows a promising potential application for the micro-inverters required in the emerging BIPV system.In addition to the PV on opaque components such as walls and roofs, the transparent photovoltaic (TPV) directly built on glass is complementary to fully utilize the PV energy in BIPV. The research is extended to the MZO-based high voltage transparent thin-film transistor (HVTTFT) on glass. For the HVTTFT, the metal electrodes are replaced by the transparent conductive oxide (TCO), aluminum-doped ZnO (AZO). Thus, the HVTTFT uses ZnO-based materials with different functions for two roles: a semiconductor Mg₀.₀₁Zn₀.₉₉O (MZO) for the channel and a transparent conductive AZO for the electrodes. This unique feature significantly reduces cross-contamination and materials, processes, and equipment costs. The HVTTFT demonstrates high voltage blocking capability with mean values over 800 V from room temperature to 60 °C and 691 V at elevated temperatures up to 100 °C, respectively. The average transmittance of the HVTTFT on glass reaches 81% over the visible spectrum. The HVTTFT on glass is promising for the TPV windows in BIPV and other glass-integrated distributed TPV systems.We have also conducted feasibility studies to build the MZO-based HVTFT on flexible substrates. The flexible HVTFT (f-HVTFT) is an ideal option as the power management component for triboelectric nanogenerator (TENG) in a self-powered wearable system. A centrosymmetric circular structure in the f-HVTFT enables stable and consistent electrical performance under the bending along different directions. Additionally, a low-temperature process is developed to reduce the overall thermal budget compatible with the flexible plastic substrates. The deposition temperature of the gate dielectric is ~100 °C, while the channel and electrodes are deposited at room temperature. The f-HVTFT is tested under flat and bent positions. The preliminary results show that the on/off ratio stays over 10⁹, and the blocking voltage remains over 125 V while the bending radius is larger than 10 mm. The f-HVTFT shows promising potential for TENG and power management in integrated self-powered wearable systems.The MZO-based HVTFT technology demonstrates excellent high voltage blocking capability and transfer characteristics. It presents the unique advantages of integrating multifunctional oxide materials on glass and plastic substrates. The device is promising for BIPV by turning the vulnerable energy loss building components like windows into active electricity-generating elements; concomitantly, the original aesthetic design is not compromised. Furthermore, with transparent materials and a low-temperature process, the MZO-based HVTFT is promising for applications in self-powered transparent electronics and flexible electronics for wearable systems.