Thin-film transistors (TFTs) are a critical component in modern display technologies and flexible electronics, enabling high-resolution screens and adaptable electronic systems. Unlike conventional metal-oxide-semiconductor field-effect transistors (MOSFETs), which are fabricated on bulk silicon wafers, TFTs are built on insulating substrates such as glass or plastic, using thin layers of semiconductor, dielectric, and conductive materials. This architecture makes them ideal for applications where lightweight, transparency, or mechanical flexibility are essential. The three most widely used semiconductor materials for TFTs are amorphous silicon (a-Si), polycrystalline silicon (poly-Si), and metal oxides such as indium gallium zinc oxide (IGZO). Each material offers distinct advantages and challenges in terms of performance, fabrication complexity, and stability.

Amorphous silicon TFTs were the first to be commercialized and remain prevalent in liquid crystal displays (LCDs). The disordered atomic structure of a-Si results in low carrier mobility, typically in the range of 0.5 to 1 cm²/Vs, limiting their use to low-frequency applications. However, a-Si TFTs are relatively easy to fabricate using plasma-enhanced chemical vapor deposition (PECVD) at temperatures below 300°C, making them compatible with large-area glass substrates. The primary drawback of a-Si is its susceptibility to threshold voltage shifts under prolonged gate bias stress, a phenomenon known as the Staebler-Wronski effect. This instability necessitates compensation circuits in display backplanes, adding complexity to the design.

Polycrystalline silicon TFTs address the mobility limitations of a-Si by offering significantly higher carrier mobility, ranging from 50 to 200 cm²/Vs. This improvement stems from the crystalline grain structure, which facilitates more efficient charge transport. Poly-Si TFTs are commonly used in active-matrix organic light-emitting diode (AMOLED) displays, where higher current drive is required for pixel illumination. The fabrication of poly-Si involves laser crystallization of a-Si, a process called excimer laser annealing (ELA), which locally melts and recrystallizes the silicon film. While ELA enables high-performance devices, it introduces non-uniform grain boundaries that can lead to variability in transistor characteristics. Additionally, the high processing temperatures, often exceeding 600°C, restrict the use of poly-Si to rigid substrates such as glass.

Metal oxide TFTs, particularly those based on IGZO, have emerged as a leading alternative due to their superior mobility, transparency, and stability. IGZO TFTs typically exhibit mobilities between 10 and 50 cm²/Vs, outperforming a-Si while maintaining low off-currents and excellent uniformity. The amorphous nature of IGZO allows for deposition at low temperatures, often below 300°C, enabling compatibility with flexible plastic substrates. Furthermore, IGZO TFTs demonstrate remarkable stability under bias stress, attributed to the low density of defect states in the bandgap. These advantages have led to widespread adoption in high-resolution LCDs and OLED displays, where low power consumption and high refresh rates are critical. However, the reliance on scarce elements like indium raises concerns about long-term material sustainability, prompting research into alternative oxide semiconductors.

The fabrication processes for TFTs vary depending on the semiconductor material and target application. For a-Si TFTs, the process begins with the deposition of a gate electrode, typically made of chromium or molybdenum, followed by a silicon nitride gate dielectric via PECVD. The a-Si active layer is then deposited and patterned, followed by the formation of source/drain contacts using doped silicon or metal layers. Poly-Si TFT fabrication involves additional steps such as ELA to convert a-Si into poly-Si, along with ion implantation to dope the source/drain regions. IGZO TFTs employ sputtering or solution-based methods to deposit the oxide semiconductor, often requiring careful control of oxygen partial pressure to optimize performance. Passivation layers, such as silicon oxide or aluminum oxide, are critical for protecting the semiconductor from environmental degradation.

Performance metrics for TFTs include mobility, threshold voltage, subthreshold swing, and on/off current ratio. Mobility determines the speed at which charges move through the semiconductor, directly impacting switching speed and current drive. Threshold voltage defines the gate voltage required to turn the transistor on, with stability being crucial for long-term operation. Subthreshold swing measures the sharpness of the transition between off and on states, influencing power efficiency. The on/off current ratio, typically exceeding 10⁶ for display applications, ensures minimal leakage in the off state. These parameters are influenced by material properties, interface quality, and fabrication conditions.

Challenges in TFT technology include achieving high mobility without compromising stability or uniformity. For a-Si, the trade-off between low mobility and bias stress instability remains a fundamental limitation. Poly-Si TFTs suffer from grain boundary effects, leading to device-to-device variability that complicates large-area manufacturing. IGZO TFTs, while promising, face challenges related to oxygen vacancy control and environmental sensitivity. Additionally, the integration of TFTs into flexible electronics introduces mechanical stress considerations, as repeated bending can degrade performance or cause delamination.

Compared to conventional MOSFETs, TFTs differ in several key aspects. MOSFETs are fabricated on single-crystal silicon wafers, offering superior mobility and reliability but lacking flexibility and transparency. TFTs, on the other hand, are designed for large-area applications where cost, weight, and form factor are prioritized over performance. The absence of a crystalline substrate in TFTs necessitates alternative approaches to defect management and interface engineering. Despite these differences, both device types share similar operating principles, relying on gate-controlled modulation of a conductive channel.

The future of TFT technology lies in advancing materials and processes to meet the demands of next-generation displays and flexible electronics. Research efforts focus on improving oxide semiconductor compositions, developing low-temperature poly-Si techniques, and exploring novel dielectric materials to enhance stability. Innovations in fabrication, such as roll-to-roll processing and inkjet printing, aim to reduce costs and enable scalable production. As the requirements for higher resolution, lower power consumption, and greater flexibility intensify, TFTs will continue to evolve, solidifying their role in the electronics industry.