A recent study published in Advanced Electronic Materials introduces a self-aligned gate architecture for carbon nanotube (CNT)-based phototransistors designed for shortwave infrared (SWIR) detection.
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The devices, which operate on a heterojunction-gated (HG) structure, demonstrate high responsivity and detectivity while maintaining a simplified fabrication process.
This approach avoids the complexity and cost of traditional techniques such as electron-beam lithography, offering a scalable pathway for high-performance, low-light infrared sensing applications.
Background: CNTs and Phototransistor Design
Heterojunction-gated phototransistors benefit from internal gain mechanisms that enhance sensitivity to weak light signals. CNTs are well suited for this application due to their high carrier mobility and compatibility with silicon-based fabrication workflows. Despite these advantages, challenges remain in achieving high yield and uniform performance at scale.
This work builds on existing research by combining CNTs with a zinc oxide (ZnO) film and lead sulfide (PbS) colloidal quantum dots. The self-aligned gate structure allows for complete coverage of the CNT channel, improving optical and electrical coupling without the alignment precision constraints of conventional fabrication methods.
Device Fabrication and Structure
The fabrication process begins with the deposition of a hafnium oxide layer, which serves as the back-gate dielectric. A polymer-sorted CNT solution with over 99.9 % purity is then applied, forming a randomly oriented network with a density of approximately 20 to 25 tubes per micrometer. The device is patterned using photolithography, and source and drain electrodes are defined via metal evaporation.
To form the self-aligned gate structure, an additional hafnium oxide dielectric layer is deposited, followed by a 40 nm n-doped ZnO layer applied through sputtering. This ZnO film fully encapsulates the CNT channel, enabling optimized electrostatic control and enhanced optical absorption. Structural and material quality were verified using scanning electron microscopy (SEM) and Raman spectroscopy.
Optical and Electrical Performance
Under 1300 nm illumination, the self-aligned CNT phototransistor achieved a responsivity of 2.9 × 105 A W-1 and a detectivity of 9.6 × 1013 Jones. The devices detected incident light as low as 0.8 nW cm-2, making them suitable for ultra-low-light applications such as starlight imaging.
Despite its simplified fabrication, the self-aligned architecture maintained performance levels comparable to devices manufactured using electron-beam lithography. Optimizing the gate-to-channel length ratio improved electrostatic control, contributing to higher gain and device stability.
The phototransistor exhibited response times in the hundred-microsecond range, addressing common limitations seen in gain-speed trade-offs. The self-aligned gate also reduced variability associated with manual alignment, supporting consistent device behavior. When benchmarked against other infrared photodetectors, the CNT-based phototransistor demonstrated competitive advantages in both low-light sensitivity and temporal response, highlighting its suitability for rapid, low-intensity light detection.
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Conclusion
This work demonstrates a high-performance CNT-based SWIR phototransistor incorporating a self-aligned heterojunction gate. The architecture enables full channel coverage, improves gate control, and allows for efficient light absorption and signal amplification. The device shows strong potential in applications requiring sensitive infrared detection, such as night vision, biological imaging, and remote sensing.
Importantly, the fabrication process remains compatible with scalable manufacturing techniques, making it a practical candidate for future image sensor arrays. The combination of performance, sensitivity to ultra-weak light, and simplified processing positions this design as a promising direction for further development in CNT-based optoelectronics.
Journal Reference
Ge J., et al. (2025). Self-aligned heterojunction gate carbon nanotube phototransistors for highly sensitive infrared detection. Advanced Electronic Materials, 2400966. DOI: 10.1002/aelm.202400966, https://advanced.onlinelibrary.wiley.com/doi/10.1002/aelm.202400966