Millimeter-wave detection and imaging is a rapidly developing field with both commercial and national security applications. This research is focused on the design and demonstration of high performance backward diode detectors and imaging arrays. The heterostructure backwards diode has been shown to outperform existing detectors at room temperature. The devices are promising for applications in security, avionics and signal detection, where they can image through smoke, haze, and opaque materials such as fabric for remote chemical and explosion detection. In addition to imaging, the sensors are part of systems for electronic data collection, in detection, categorization and analysis of signals. The detectors do not require cooling to obtain major sensitivity enhancements, which reduces cost and increases efficiency. Improvements in detectors and arrays come through an interdisciplinary program of device design and numerical simulation, coupled with fabrication and measurement-based models for the devices. The project engages undergraduate and graduate students and will generate a series of case students for curricular enrichment. The continued development of semiconductor materials for electronic applications benefits a broad spectrum of disciplines and applications. Spin-off applications of this sensor technology include communication, high-performance wireless networking, medical imaging, astronomy and general metrology.
The primary focus of this project was on the development and demonstration of improved millimeter-wave sensors, based on heterostructure backward tunnel diodes. Imaging based on millimeter-wave sensing has potential applications in avionics (e.g. brown-out landing guidance) due to the transparency of atmospheric propagation even in the presence of smoke, dust, sand, and fog, as well as security screening since many common materials such as cloth and light building materials can be penetrated my millimeter-wave signals. High-sensitivity detectors in the millimeter-wave range would also be useful for applications such as remote chemical detection. A key impediment to realizing the potential of these systems has historically been the performance of the detectors; improvements in the sensitivity and noise performance of the detector elements would enable more cost-effective implementations of millimeter-wave based systems. The technical challenges that were addressed in the project included detailed physical study of the underlying physics within heterostructure backward diodes and developing device models to support the design of more advanced devices, design and fabrication of heterostructure backward diodes to validate the models and to experimentally confirm the detection limits possible with this technology, and characterization of the devices for evaluation of their performance and to permit projection of the performance possible in imaging and detection systems. Under this project, we achieved record experimental performance for millimeter-wave detectors, with measured unmatched sensitivities of 4700 V/W, impedance-matched sensitivities of just under 50,000 V/W, and measured detector noise equivalent power as low as 0.18 pW/Hz1/2. This reflects considerable improvements in both sensitivity (by approximately 50%) and noise (by approximately 10 times). An unexpected result was that the devices developed had greatly increased frequency response compared to prior demonstrations. Intrinsic cutoff frequencies for this class of device have now been demonstrated as high as 8 THz (the previous limit was approximately 800 GHz, a 10x improvement). The modeling and design effort has also illuminated promising directions for future improvement of detector performance, and suggests that significant improvements in noise (reduction by approximately 2x from the current state of the art) and increases in sensitivity (increases of approximately 25-30% from the current best devices) are possible with more highly-optimized device heterostructure designs. In addition to these improvements in the device figures of merit, the device modeling effort also contributed to the understanding of interband tunneling in broken-gap semiconductor systems (such as the InAs/AlSb/GaSb material system explored here), with the development of a mathematical framework for accurately predicting the performance of these devices from nominal device structures. This predictive ability has been leveraged in recent work on tunneling field-effect transistors for beyond-CMOS computation and logic, and thus has reached beyond the realm of millimeter-wave detection and imaging. The broader impacts of this project include educational initiatives, training of several researchers, and advancing the technology of millimeter-wave detection to enable cost-effective deployment in a range of applications. The educational initiatives included using this project as the basis for several undergraduate-level research theses, and including results and case studies from the project in course material taught at the University of Notre Dame. Two students received their PhD degrees with support from this project, and a third student has been supported for part of her PhD. Several REU students were also supported by this project. The project resulted in technologies for which patents are being pursued, and the technology has been licensed to a private company for commercialization. If the detector technologies developed can be successfully commercialized, it will likely have a significant impact on the availability and ubiquity of millimeter-wave passive imaging systems for a range of societal benefits such as non-invasive security screening and avionics.
