I. Introduction
Infrared focal plane arrays (FPAs) are useful for thermal imaging, night vision, satellite imaging, distance ranging, and improvised explosive device detection in both military and commercial applications [1]–[4]. There are more established technologies in both HgCdTe [5] and band-gap-engineered quantum well (QW) infrared photodetectors (QWIPs) that have produced FPAs capable of sensing and measurement across most of the infrared spectrum from midwave to very long wave with low noise [1], [6]–[8]. Many of these devices are well characterized and have demonstrated adequate performance in the applications above. However, adding spectral tunability to these sensors expands their potential applications and suitability for each application. Several papers have reported on the characterization of a hybrid device between QWIPs and the quantum dot (QD) infrared protectors (QDIPs), which is called the dot-in-a-well (DWELL) [9]–[14] device that exhibits this tunability. Although not as well developed as the established QWIP devices, these DWELL structure include advantages such as multispectral response with a bias-dependent spectral tunability and reproducible control of the operating wavelength like a QWIP and the low dark current and normal incidence operation of a QDIP [11]. The multispectral response is a result of multiple transition energies (dot to dot, dot to well, or dot to continuum), and the spectral tunability is a result from band bending with applied bias voltage changing the transition energies [9]–[11]. More recently, the DWELL structure has been modified by embedding QDs in a QW structure and then embedding this hybrid structure within another QW, which is called a double DWELL or DDWELL [12] in this paper. This new structure has the advantage of lower strain in the heterostructure, which leads to higher temperature operation while maintaining low dark current. The remainder of this paper compares the original DWELL FPA and two versions of the new DDWELL FPAs to a commercially available QWIP FPA. A previous work on characterization of the DDWELL was from an intermediate structure only. This paper elaborates with further characterization of that intermediate DDWELL, expands with characterization of the newer complete DDWELL, and compares them to the original DWELL device and the well-established QWIP [14]. Although the QWIP device demonstrates the best performance and lowest noise, the second generation of DDWELL detectors have dramatically increased performance over the first generation and have the added benefit of spectral tunability [11], which many established infrared (IR) FPA technologies lack.