Germanium photoconductors offer excellent sensitivity in the 50-140μm spectral range. Coupled with their modest
cooling requirements and their compatibility with the silicon cryo-CMOS readout technology, these detectors are the
most attractive candidates for far IR astronomy in this wavelength range. Over the years we have been pursuing the
advancement of this technology and our initial effort has produced a 2x16 Ge:Sb array with an NEP in the low 10-18
W/√Hz range, rivaling the best far IR arrays currently available. Further work has resulted in design and fabrication of
a low noise, 2-side buttable 32x32 (64x64 mosaic) CTIA readout, the first 1k-pixel Ge:Sb fully assembled focal-plane
array, a new hybrid design better suited for far IR photoconductors, and the preliminary design of a 2-side buttable
64x64 (128x128 mosaic) CTIA readout. Our developmental work continues and we believe that sensitivity levels
below 10-18 W/√Hz are within reach. This paper presents an overview of our progress so far and outlines our roadmap
for further work.
The SB349 is a 32x32 readout multiplexer specifically designed for far IR photodetectors and is capable of operating at
cryogenic temperatures at least as low as 1.8K. This readout is a capacitive-transimpedance amplifier multiplexed to
eight outputs and is buttable on two sides to form a 64x64 mosaic array. It features eight selectable gain settings, auto
zero for better input uniformity, sample-and-hold circuitry, and provisions to block the readout glow. A special, 2-micron cryo-CMOS process was adopted to prevent freeze out and ensure low noise and proper operation at deep
cryogenic temperatures. An overview of the design and the results of the tests performed on this device are reported in
Development of large format, far infrared focal-plane arrays has been identified as a pressing need for future
astronomical instruments. In particular, array sizes as large as 128x128 with sensitivities equal to or better than 10-18
W/√Hz are the goals to be achieved within the next fifteen years. As part of our continuing effort to further this
technology, we are developing a 32x32 Ge:Sb photoconductor FPA with a CTIA cryogenic readout multiplexer. A
new, layered-hybrid architecture is employed to block the readout glow, improve heat dissipation and temperature
uniformity across the array, and alleviate the potential problems associated with the large CTE mismatch between the
Ge detector and the Si readout. This is the first 1k-pixel photoconductor FPA of its kind and is meant to be a pathfinder
for future large format FPAs. Based on the test results of a prototype 2x16 Ge:Sb array of similar design, we expect the
sensitivity of this FPA to be as low as 10-18 W/√Hz. This paper presents the design, characteristics, and the expected
performance of this array.
Development of two-dimensional cryogenic readouts suitable for far infrared and submillimeter detectors is a key step
in fabrication of large format far infrared focal-plane arrays. In collaboration with Raytheon Vision Systems, we have
designed and fabricated the first 32x32 readout multiplexer, SB349, capable of operating at cryogenic temperatures as
low as 1.7K. This readout is a capacitive-transimpedance amplifier multiplexed to eight outputs and is buttable on two
sides to form a 64x64 mosaic array. It features eight selectable gain settings, AC coupling (auto zero) for better input
uniformity, sample-and-hold circuitry, and provisions to block the readout glow. A special, 2-micron cryo-CMOS
process has been adopted to prevent freeze out and ensure low noise and proper operation at deep cryogenic
temperatures. An overview of the design and the results of the initial functionality tests on this device are reported in
The full potential of far infrared and submillimeter detectors, operating at deep cryogenic temperatures (<4.2K), is only realized if large, two-dimensional arrays of these detectors are developed. The technology for fabricating suitable readouts for such detectors has been one of the main impediments in achieving this objective. In this paper, we present the design parameters of the first 2-side buttable, 32x32 (64x64 mosaic) readout multiplexer, specifically designed for direct-hybrid far IR detector arrays. The readout employs a high open-loop gain, capacitive transimpedance unit-cell design with eight outputs. It features eight selectable gain settings, AC coupling (auto zero) for better input uniformity, sample-and-hold circuitry, and provision to limit the readout glow. A special, 2-micron cryo-CMOS process has been adopted to prevent freeze out and ensure low noise and proper operation at deep cryogenic temperatures. Based on the performance of its predecessors, CRC696 and SBRC190, this device is expected to have CDS read noise of better than 100e- at 2K.
Germanium detectors are extensively used in astronomical instruments for far infrared observations. To meet the science objectives of future space projects, large-format far IR detectors are needed. As a first step toward this goal, we have fabricated a 2x16 Ge:Sb array with the 1x32, SB-190 CTIA cryogenic readout. The detector design as well as the preliminary results of our parametric tests are presented here. The array exhibits very good noise performance with an NEP as low as 4.0E-18 W/√Hz, and confirms the viability of the design for large format arrays.
Fabrication of direct hybrid far infrared photoconductor arrays, especially for low background astronomy, is particularly challenging due to arrays' relatively large pixel size, susceptibility to stray radiation even from the faintest IR source, and the requirement for low bias levels. In fact, these problems have hindered the development of far IR direct hybrid arrays, which has been the standard practice for the near and mid IR arrays. In this paper, a new and novel design is presented that addresses the complications afflicting the direct hybrid approach and paves the way for the development of large-format far IR arrays. In particular, the readout glow, detector heating, and thermal mismatch between the readout and the detector array are addressed. The use of a capacitive transimpedance amplifier effectively eliminates the detector debiasing.
We have observed, for the first time, microwave-assisted photoconductivity in high purity GaAs. The enhancement of response appears to be dictated by two distinct mechanisms. First, a broadband enhancement which is believed to be due to detrapping of the free carriers and, therefore, increased photoconductive gain. Secondly, microwave-ionization of the excited states. We expect that both of these mechanisms contribute very little, if any, to the detector noise and, therefore, improve the detector's NEP. In this paper, we report the results of our preliminary tests showing broadband enhancement in response and an indication of enhancement of the excited-state response. Further investigation is currently underway.
Testing of a 40 to 125 μm Ge:Sb photoconductor array for AIRES (Airborne Infra-Red Echelle Spectrometer) is described. The prototype array is a 2×24 module which can be close-stacked with other modules to provide larger two-dimensional formats. Collecting cones on a 0.08 inch pitch concentrate incident radiation into integrating cavities containing the detectors. The array is read out by two Raytheon SBRC 190 cryogenic multiplexers that also provide a CTIA (capacitive transimpedance amplifier) unit cell for each detector. We have conducted a series of tests to evaluate the array dark current, responsivity and detective quantum efficiency.
The SBRC 190 cryogenic readouts were developed for use in far-infrared arrays of Ge:Sb and Ge:Ga photoconductor detectors. The SBRC 190 provides an AC-coupled CTIA (capacitive trans-impedance amplifier) unit cell for each detector and multiplexes up to 32 detectors. This paper presents our test results characterizing and optimizing the performance of these novel devices. We discuss their basic behavior and investigate their performance in different clocking schemes.
SBRC-190 readout multiplexer is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) especially suitable for use with infrared detector arrays requiring low-bias levels--such as Ge:Ga far infrared detector arrays. The unit-cell design employs a feedback loop which keeps the bias across the detector constant and, therefore, prevents debiasing--a critical requirement for far infrared detectors. We have tested a number of these multiplexers at various cryogenic temperatures, down to 1.7K. In this presentation we will report the results of our tests and will discuss gain, uniformity, and read noise of the bare mux under correlated-double sampling at 4.2K.
We have constructed a far infrared detector array consisting of a 1×16 Ge:Ga and a 1×16 Ge:Sb placed side-by-side to form a linear, 1×32 array. An SBRC-190 readout multiplexer, which is a 1×32, multi-gain, capacitive transimpedance amplifier (CTIA) manufactured by Raytheon Infrared Operation, is wire bonded to the array. The array has been tested in the temperature range of 3.2K to 2.6K under various infrared radiation levels. In this presentation we will discuss the design and will report on the results of the preliminary tests conducted on this array at 3.0K. This developmental effort is intended to test the viability of the SBRC-190 unit-cell design for far infrared detector arrays and help foster improvement and further development of readout electronics based on the CTIA design. In addition, we hope to extend the detector design and develop a two-dimensional, monolithic array.
Theoretical analysis indicates that resonant infrared photoconductors can achieve unit quantum efficiency at resonant frequencies. This concept, which is based on establishing a resonant absorption cavity internal to the detector element, offers numerous other enhancements and advantages over conventional detectors. Here, we present an overview of the operation of the device and outline the fabrication process of a Ge:Ga far-IR photoconductor. The preliminary test results performed on a prototype Ge:Ga detector show the expected resonant fringes with enhanced response. The summary of the results and the status of the project will be discussed.
In this paper we report the results of an extensive study on the far-infrared photoconductivity of high purity n-type GaAs. The crystal, which was grown at Max Plank Institute using liquid- phase epitaxy, exhibited the fine structures of the excited state transitions of the residual shallow level impurities. The major peak in the spectral response belongs to the 1s-2p transition, with its responsivity about thirty five times higher than the continuum. At 3.4K detector temperature, 625 mV bias, and 100 Hz chopping frequency the detector responsivity at 35.4 cm-1 (279 micrometers ) was measured to be 0.017 A/W. Under these same conditions, the NEP was 5.9 X 10-14 W/(root)Hz. The dark current at 25 mV bias was 5.6 X 10-14 A.
In this presentation we introduce a new concept for an infrared photoconductor and demonstrate that such a detector can, theoretically, exhibit unit quantum efficiency at selected frequencies. The idea is based on establishing a relatively high finesse absorption-cavity internal to the detector element and tuning the front surface reflectivity and the dopant concentration of the detector to achieve improved performance. A theoretical analysis demonstrates this concept and provides the relevant design parameters. This approach offers many other advantages over conventional photoconductors as well as impurity-band-conduction approach. Among those are enhanced photoconductive gain, improved noise performance, and better immunity against ionizing radiation.
High purity GaAs exhibits excited state far-infrared photoconductivity in the temperature range from 2K to 4.2K. The response is characterized by an exceptionally sharp peak which is magnetically tunable over a broad range. This dominant peak, at 35.8 cm-1 (279 μm), belongs to the 1s-2p transition of the residual shallow level impurities, and its response is over an order of magnitude above the continuum. The preliminary results of the measurements of responsivity, dark current, and NEP of this device are reported.
The performance of a multielement Ge:Ga linear array under low-background conditions is investigated. On-focal plane switching is accomplished by MOSFET switches and the integrated charge is made available through MOSFET source followers. The tests were conducted at 106 microns and the radiation on the detectors was confined to a spectral window 1.25 microns wide using a stack of cold filters. At 4.2 K, the responsivity was measured to be nominally 584 A/W, and the NEP was 1.0 x 10 exp -16 W/sq rt Hz. A detailed description of the test setup and the procedure is presented.