In this review article a comprehensive analysis of the developments in ultraviolet (UV) detector technology is described. At the beginning, the classification of UV detectors and general requirements imposed on these detectors are presented. Further considerations are restricted to modern semiconductor UV detectors, so the basic theory of photoconductive and photovoltaic detectors is presented in a uniform way convenient for various detector materials. Next, the current state of the art of different types of semiconductor UV detectors is presented. Hitherto, the semiconductor UV detectors have been mainly fabricated using Si. Industries such as the aerospace, automotive, petroleum, and others have continuously provided the impetus pushing the development of fringe technologies which are tolerant of increasingly high temperatures and hostile environments. As a result, the main efforts are currently directed to a new generation of UV detectors fabricated from wide band‐gap semiconductors the most promising ...

Semiconductor ultraviolet detectors

New concept ultraviolet photodetectors

Charge-coupled devices (CCDs) are presently the technology of choice for most imaging applications. In the 23 years since their invention in 1970, they have evolved to a sophisticated level of performance. However, as with all technologies, we can be certain that they will be supplanted someday. In this paper, the Active Pixel Sensor (APS) technology is explored as a possible successor to the CCD. An active pixel is defined as a detector array technology that has at least one active transistor within the pixel unit cell. The APS eliminates the need for nearly perfect charge transfer--the Achilles' heel of CCDs. This perfect charge transfer makes CCD's radiation 'soft,' difficult to use under low light conditions, difficult to manufacture in large array sizes, difficult to integrate with on-chip electronics, difficult to use at low temperatures, difficult to use at high frame rates, and difficult to manufacture in non-silicon materials that extend wavelength response. With the active pixel, the signal is driven from the pixel over metallic wires rather than being physically transported in the semiconductor. This paper makes a case for the development of APS technology. The state of the art is reviewed and the application of APS technology to future space-based scientific sensor systems is addressed.

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Active Pixel Sensors: Are CCD's Dinosaurs?

Substrate selection presents particular challenges for the production of high‐quality high‐temperature superconducting (HTS) thin films suitable for applications. Because the substrate is generally a passive component, it is often ignored and assumed to have a negligible effect on the structure residing on top of it. There is also a technological motivation to use substrates that conventional wisdom would argue are unlikely to support high‐quality HTS films. These facts have led to rediscovery of many of the fundamental issues governing the role of the substrate in determining the properties of the thin film(s) it supports. For this reason, the study of issues in substrate selection for HTS materials presents a microcosm for substrate selection more generally. We consider the major issues governing the role of the substrate in HTS thin‐film technology and discuss many of the material classes and specific materials that have been studied for their suitability as substrates for HTS films.

Substrate selection for high‐temperature superconducting thin films

CMOS based imagers are beginning to compete against CCDs in many areas of the consumer market because of their system-on-a-chip capability. Sensitivity, however, is a main weakness of CMOS imagers and enhancements and deviations from standard CMOS processes are necessary to keep up sensitivity with downscaled process generations. In the introductory section several definitions for the sensitivity of image sensors are reviewed with regard to their potential to allow meaningful comparison of different detector structures. In the main section, the standard CMOS sensor architecture is compared to detector structures designed to improve the sensitivity, namely the photogate (PG), the pinned photodiode (PPD) and the thin film on ASIC (TFA) approach. The latter uses a vertical integration of the photodiode on top of the pixel transistors. A careful analysis of the relevant electrical, optical and technological parameters and many previously published experimental data for different imagers reveals that only the PPD and the TFA enhancements provide satisfactory sensitivity and withstand scaling down to 0.18 /spl mu/ processes. Due to the higher fill factor and the higher quantum efficiency TFA provides significantly better values than PPD. The radiometric sensitivity of a 5 /spl mu/m/spl times/5 /spl mu/m TFA pixel is found to amount to 11.9 V/(/spl mu//cm/sup 2/) for a 0.25 /spl mu/m process and 27.5 V/(/spl mu/J/cm/sup 2/) for a 0.18 /spl mu/m process.

Sensitivity of CMOS based imagers and scaling perspectives

The authors report the UV photoresponse of an a-SiC/a-Si heterojunction p-i-n diode with the structure of glass/TCO (transparent conducting oxide, SnO/sub 2/:F)/p-a-SiC:H/i-a-Si:H/n-a-Si:H/Al. The diode has been designed for a high-sensitivity and low-noise UV detector. The diode has its peak responsivity (0.254 A/W) and quantum efficiency (81.5%) at 385 nm. This structure possesses (1) the window effect by using the wide-bandgap a-SiC:H as the front layer (p-layer) and (2) the carrier confinement effect at the p-SiC:H/i-a-Si:H interface. Enhancements are proposed to raise UV response and suppress long-wave responsivity. The diode was designed to be operated under zero external bias to suppress the dark-current-induced noise. Results show a 200% higher UV sensitivity than a GaAsP Schottky photodiode in the 200-400-nm wavelength region. >

An amorphous SiC/Si heterojunction p-i-n diode for low-noise and high-sensitivity UV detector

A glass/ITO/a-Si:H/Cr photodiode with a voltage selectable spectral response was fabricated. The diode consists of two back-to-back Schottky junctions. As the bias at the Cr side changes from 1.5 to -1.5 V, three color peak responses can be found, i.e., 480 nm (blue) at V=0.5 V, 530 nm (green) at 1.5 V, and 620 nm (red) at -1.5 V. The basic idea consists of using the different bias voltages to change the location and depth of the photon collection region in which photons with different wavelengths are mainly collected. The photoresponse time was measured under a red LED light injection with an intensity of 150 mu W. The typical photoresponse time for red is 60 ns under -1.5-V bias. These basic studies show that the vertical-type Cr/a-Si:H/ITO back-to-back Schottky diode is a good candidate for a high-speed colour image sensor. >

A vertical-type a-Si:H back-to-back Schottky diode for high-speed colour image sensor

The effects of material and structure parameters on an amorphous-silicon-germanium alloy Schottky barrier diode's responsivity have been investigated in detail. The effects contradict each other. A compromise is made in selecting parameters for construction of a Si/sub 1-x/Ge/sub x/:H Schottky barrier for an IR detector. The optimized amorphous-silicon-germanium alloy Schottky diode using x=0.43 alloy with a 900-nm-thick i-layer was found to have a peak at 850 nm with a responsivity of 0.6 A/W under -2-V bias. The diode also has a response time of 33 mu s and slight photodegradation. Thus, the diode becomes a candidate for manufacturing the infrared OEIC (optoelectronic integrated circuit) on glass substrate in applications which require size, reproducibility, and low cost more than sensitivity. >

A metal-amorphous silicon-germanium alloy Schottky barrier for infrared optoelectronic IC on glass substrate application

The design and fabrication of a high-gain amorphous silicon/amorphous silicon germanium (a-Si:H/a-Si,Ge:H) bulk barrier phototransistor for infrared light detection applications are reported. The a-Si,Ge:H material featured a lower energy gap and is suitable for the absorption of longer wave light, but it also leads to a low breakdown voltage and high dark current. An additional a-SiC:H thin-film layer was used at the collector/base interface in the conventional amorphous bulk barrier phototransistor to enhance the function of the bulk barrier and obtain high optical gain. >

S.-B. Hwang

Papers

An amorphous SiC/Si heterojunction p-i-n diode for low-noise and high-sensitivity UV detector

A vertical-type a-Si:H back-to-back Schottky diode for high-speed colour image sensor

A metal-amorphous silicon-germanium alloy Schottky barrier for infrared optoelectronic IC on glass substrate application

An a-Si:H/a-Si,Ge:H bulk barrier phototransistor with a-SiC:H barrier enhancement layer for high-gain IR optical detector

Deposition of high Tc superconductor thin films on silicon substrates by rapid thermal annealing and with a TiN/Ti/Ag buffer layer