A proposed improved method of externally controlled time multiplexing of the readouts of focal plane arrays of pairs of stacked quantum well infrared photodetectors (QWIPs) that operate in different wavelength bands is based on a dual detector common plane circuit configuration. The method would be implemented in a QWIP integrated-circuit chip hybridized with a readout integrated circuit (ROIC) chip.
There are alternative methods of multicolor readout, but they involve more on-chip circuitry and the attendant disadvantages of greater size, power dissipation, number of control signals, and complexity of circuitry on the QWIP and ROIC chips. To minimize size of, and power dissipation in, a multicolor pixel, one must minimize the number of transistors and control signals; this can be achieved only by time multiplexing of colors in each pixel. The time multiplexing can be controlled by signals from external or internal circuitry. The proposed reliance on external control for time multiplexing would make it possible to use a commercial single-color ROIC with only minor modifications in the form of extra metallization for an additional detector common plane and for clocking bias potentials. (If, instead, one were to rely on internally controlled multiplexing, it would be necessary to radically redesign the ROIC, at considerably greater development cost.) Other advantages of the external control approach over the internal-control approach are greater flexibility and the possibility of using a technique, known as “skimming,” for subtracting unwanted dark-, background-, and noise-current contributions from readout signals
The figure schematically depicts the circuitry in one pixel according to an internally controlled multiplexing scheme and according to the proposed externally controlled multiplexing scheme. In both schemes, the time multiplexing would be accomplished by switching (clocking or ramping) the biases applied to the QWIPs via detector common planes. In the internal-control case, a bias signal would be applied via a single detector common plane and two internal electronic switches. In the proposed external-control case, bias signals would be applied via two detector common planes, without internal electronic switches. A previously unmentioned advantage of the external-control scheme shown in the figure is the need for only one indium bump (instead of two) in each pixel.
This work was done by Sir B. Rafol, Sarath Gunapala, Sumith Bandara, John Liu, and Jason Mumolo of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) ee on-line at www.techbriefs.com/tsp under the Semiconductors & ICs category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-30523, volume and number of this NASA Tech Briefs issue, and the page number
This Brief includes a Technical Support Package (TSP).
Dual Common Planes for Time Multiplexing of Dual-Color QWIPs
(reference NPO-30523) is currently available for download from the TSP library.
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Overview
The document titled "Technical Support Package for Dual Common Planes for Time Multiplexing of Dual-Color QWIPs" from NASA's Jet Propulsion Laboratory discusses advancements in the technology of Quantum Well Infrared Photodetectors (QWIPs) and their application in dual-color imaging systems. It focuses on the internal multiplexing of dual-color QWIPs, which allows for efficient signal processing and reduced component count in readout integrated circuits (ROICs).
One of the primary advantages of this multiplexing approach is the significant reduction in the number of MOSFETs and control signals required. The system can operate with just five MOSFETs, one capacitor, and seven control signals, which simplifies the design and potentially lowers costs. The document outlines four operational modes for the QWIPs: QWIP_1 alone, QWIP_2 alone, alternating frames between QWIP_1 and QWIP_2, and summing the signals from both detectors. This flexibility allows for tailored applications depending on the specific imaging requirements.
However, the document also highlights several disadvantages associated with this technology. For instance, each pixel requires two indium bumps, which complicates the manufacturing process. Additionally, the system cannot implement skimming, and the switch impedance must be significantly higher than the detector impedance, necessitating careful design considerations. Furthermore, a redesign of the ROIC is required to accommodate the internal switches.
The document also discusses the configuration for color multiplexing, which involves using four indium bumps on the ROIC for a four-color system. This setup allows for spatially co-located colors that are read at different times, requiring additional control signals for operation. The document emphasizes that different biasing is needed for each amplifier, which adds complexity to the system.
Overall, the document provides a comprehensive overview of the technical aspects and operational modes of dual-color QWIPs, detailing both the advantages and challenges of implementing this technology in aerospace applications. It serves as a valuable resource for understanding the intricacies of time multiplexing in dual-color imaging systems and the potential for future advancements in this field.
