Although single-channel quantum-well infrared photodetector arrays have arrived on the commercial market, multiwavelength arrays have not left the lab. Among impediments to commercialization are the cost and complexity of integrating discrete readout circuits for each quantum well, designing gratings for pixels with disparate wavelengths and achieving good fill factor. Researchers have found solutions for the first two problems, but they have done so at a cost to the third. Stacking N- and P-type quantum wells, an approach that was developed at the Institute for Microstructural Sciences, may help to clear the last hurdle for multiwavelength arrays. Under the direction of the National Research Council, the institute demonstrated such a device based on epitaxial integration of N- and P-type GaAs/AlGaAs quantum wells separated by an LED to enable pixelless imaging. The device locates the N-layer -- detecting around 9 µm -- at the bottom of the stack. Next comes the LED layer, which is capped with a P-type quantum well detecting near 5 µm. At operating temperatures of 70 K, the device can toggle between these bands by alternating bias to 10 and 15 V, respectively. Scanning between the bands sequentially forms an image. The LED serves to up-convert the mid-infrared light to the near-infrared, making it accessible to common CCD imagers. "What we wanted to demonstrate was a single-pixel element that showed good switching and that we know how to make an imaging device out of that," explained Emmanuel Dupont, who led the research. "CCD technology is much more mature and simpler to design in than integrating individual readout circuits on the array." The institute has yet to demonstrate an array, but its device potentially reduces the complexity of designing readout circuits for two-dimensional arrays. Moreover, its configuration simplifies grating design: While the N-type well still requires a grating, the P-type does not. "It's easy to have a grating working at one wavelength," Dupont said. "At two you have trade-offs." The absorption per well is a bit small, leading to 0.12-A/W responsivity at 9.2 µm and only 0.01 at 5.1 µm. However, Dupont said that this was intentional for this stage of the research. The next step is to optimize the absorption of the N- and P-type layers and, depending on the results of that, possibly to develop a two-color array.