Signal Processing In The Element Research Articles

Resolution-equivalent D* for SPRITE detectors

It is desirable for design purposes to model a signal-processing-in-the-element (SPRITE) detector simply as a discrete-element detector with an integration-enhanced D*. We present a method for normalization of measured D* for SPRITE detectors to yield an equivalent-discrete D*. The multiplicative factor is the square root of the ratio of two noise-equivalent bandwidths: one is that of the SPRITE detector with no boost filter, and the other is that of the SPRITE detector with a boost filter that approximately compensates for carrier diffusion, yielding a spatial resolution that approaches that of a discrete detector the same size as the readout. This approach allows a resolution-equivalent D* comparison of SPRITE detectors with discrete-element detectors and facilitates such comparisons among SPRITE detectors. We find that, to obtain the D* of an equivalent-discrete detector, a measured SPRITE D* should typically be multiplied by a factor ranging from 0.85 to 0.57 for 8- to 12-µm SPRITE detectors and by a factor ranging from 0.50 to 0.23 for 3- to 5-µm SPRITE detectors.

Modal analysis of transport processes in SPRITE detectors

Carrier transport in signal-processing-in-the-element (SPRITE) detectors is an important phenomenon because it determines properties such as the responsivity and the modulation transfer function (MTF). The previous literature has presented approximate solutions to the transport problem that neglect boundary effects, which have long been thought to play a major role in SPRITE behavior. We present a new solution to the problem through the use of modal analysis. This method intrinsically includes boundary conditions and thus is more complete than the previous analysis. Furthermore we use this solution to derive expressions for the MTF. The effects of the boundary conditions on the MTF are studied to determine their optimum values.

Modulation-transfer function measurement of SPRITE detectors: sine-wave response

A method is presented for measuring the modulation transfer function of signal processing in the element (SPRITE) detectors with a HgCdTe composition optimized for the 3-5-microm band. This method incorporates a 3.39-microm He-Ne laser to generate Young's fringes of varying spatial frequency, which are scanned across the detector elements. The results are consistent with theoretical models for these devices and indicate a limited resolution capability for SPRITE detectors used for the 3-5-microm band.

Electrical network model for SPRITE detectors

Impedance measurements were made on SPRITE (signal processing in the element) detectors. The resistive contributions were found to contain a contact resistance term, along with a term proportional to the semiconductor material length between terminals. The capacitive contributions were strongly frequency dependent and were largely independent of material length. This behavior would appear to be caused by nonohmic contacts at the metal-semiconductor interfaces where the SPRITE is bonded to its external electrical connections. When the SPRITEs were illuminated, the resistance decreased as expected, but the capacitance showed a sizable increase as well. A <i>T</i>-network model is developed that is consistent with the set of two-terminal impedance measurements. A model of this type should provide a useful starting point for electronics optimizations in SPRITE systems, including the design of the bias source, the readout mechanism, and the preamplifier.

Spatial filtering by a line-scanned nonrectangular detector: application to SPRITE readout MTF

Any finite-sized photodetector has an effect on the spatial frequency content of the detected image. An expression for the modulation transfer function (MTF) of a nonrectangular detector in the along-scan direction is obtained. A comparison of our theoretical prediction is made with published experimental and numerical values for the MTF of a photosite having an exponentially tapered shape. Structures of this form are used as the readout region in SPRITE (signal processing in the element) detectors.

Modulation transfer function and number of equivalent elements for SPRITE detectors

SPRITE (signal processing in the element) detectors are three-contact photoconductive structures made of HgCdTe, in which a time-delay-and-integration function is performed in the detector element itself without the need for external circuitry. Spatial frequency-dependent expressions are developed for the modulation transfer function and the number of equivalent elements N(eq) of the SPRITE. The development is based on a Green's function method, which accounts for carrier generation, recombination, and diffusion processes. The usual low-frequency approach of defining a square resolution element on the SPRITE is avoided. The resulting expressions are functions of spatial frequency and are also dependent on physical variables such as the length of the SPRITE element, carrier lifetime, carrier mobility, and operating voltage. These expressions are then applied to the design of a SPRITE element, optimized for operation over a particular range of spatial frequencies.

Recent developments in cadmium mercury telluride infrared detectors

A II–VI compound, cadmium mercury telluride, has dominated recent advances in the detection of infrared radiation. Although the main application is in thermal imaging, other applications include instrumentation and guidance. In this paper we describe the history of the development of cadmium mercury telluride as a detector material, with emphasis on the importance of material parameters and the role of the material scientist. High purity material, free from defects and of good crystal quality, is needed in order to ensure good minority carrier lifetime. Because of the segregation of impurities during solidification, material produced by the Bridgman technique offers considerable advantages over material produced by cast recrystallize techniques. Detectors based on a principle established by C.T. Elliott of the RSRE, Malvern, have led to families of thermal imagers that produce near perfect imagery. These detectors incorporate signal processing in the element by providing the time delay and integration functions that are normally performed off the focal plane in conventional serial scanned systems. A particular requirement of these SPRITE detectors is for long minority carrier lifetime. This has called for improvements in both material and fabrication technology which have led to an advanced technology that has benefited all aspects of the manufacture of cadmium mercury telluride detectors. Near background limited performance will be described in both the 8–14 and the 3–5 μm atmospheric transmission bands. There is, however, more image blurring in the 3–5 μm band than occurs in the 8–14 μm band. The direction of present work is towards detectors combining cadmium mercury telluride elements with advanced integrated circuits to provide more complex signal processing in the focal plane. This has been driven by the need to improve the sensitivity of thermal imaging systems where the scan rates are too low to allow useful time delay and integration in SPRITE detectors and by future needs for simplified missile guidance without mechanical scanning. Although the approaches have included extrinsic silicon, IR sensitive Schottky barriers and silicon hybridised with a variety of IR sensitive materials, the main thrust of present work in Europe and the United States is towards cadmium mercury telluride-silicon hybrids and intrinsic cadmium mercury telluride CCD focal planes. In the United Kingdom, the emphasis has been on two dimensional staring arrays of cadmium mercury telluride photodiodes combined with silicon substrates that incorporate either CCD circuits or MOSFET switch addressing. The switched approach has led to a novel line scanned array (LSA) that has enabled the first demonstrations of staring array imagery in the 8–14 μm band. Recent work will be described both on LSA and on CCD focal planes.

An integrating detector for serial scan thermal imaging

The operation and characteristics are described for a new type of infra-red detector which is known by the acronym SPRITE (signal processing in the element). In its simplest form the device is a three lead structure in n-type cadmium-mercury-telluride which performs the same function in a thermal imaging system as an array of serial scanned elements together with the associated preamplifiers and time-delay-integration circuits. The operating principle and basic theory of the device are described and compared with experimental data. The emphasis is on devices operating at 77K in the 8–14 μm band though some information on 3–5 μm band operation at temperatures in the region of 200K is also presented. The principal application of the devices at present is in 8–14 μm band systems where they provide equivalent performance to arrays of approximately 100 BLIP elements but with considerable simplification of the system electronics and detector fabrication.

The practical realisation and performance of sprite detectors

This paper details the practical realisation of a novel device for use in thermal imaging systems—the SPRITE detector. This type of detector incorporates signal processing in the element by providing the time delay and integration functions, normally performed off the focal plane in conventional serial scanned systems. The detector has particular significance for high performance thermal imagers, allowing a more compact design with simplified electronics. The development of the fabrication technology for SPRITE is described together with the performance of devices for operation in both the 8–14 and the 3–5 μm bands.