Solid-state sensors detectors

The most recent generation of NDIR analyzers have evolved to satisfy the frequently harsh industrial environments encountered. These analyzers utilize solid-state sensors for the detection of infrared radialion. Most frequently used sensors are lead selenide (PhSc). thermopiles, or pyroelectric detectors. The gas analyzers generally are configured as single-path instruments, dual-beam with a reference palh. or dual-channel with a reference filter. 

Modern sensors and electronic systems depend heavily on fast solid state sensors and solid state electronic components. Thus the majority of detector systems commercially available are sufficiently fast for the vast majority of chromatography applications. In general, the overall time constant of the detecting system should be less than 50 milliseconds. For special applications involving very fast separations, this value may need to be reduced to around 15 milliseconds. Sensors and electronics, with very small time constants, unfortunately, will also readily respond to high frequency noise. 

Zampolli and coworkers have developed a selective hybrid microsystem based on GC to enhance indoor air quality monitoring applications such as CO and NO2 [41]. In the miniaturized GC, a solid-state sensor acts as the detector along with a silicon micromachined packed GC column, a zero grade air unit, a minipump, and a minivalve. It was possible to detect benzene, toluene, and m-xylene in both synthetic and real indoor air down to 5 ppb. 

Recently, interest in using a mass spectrometry detector as an e-nose sensor array has grown. The MS sensor approach offers important advantages over current solid-state sensors, including no problem with water, alcohols, or poisoning a linear response to vapor concentrations much less drift and significantly improved reproducibility. 

Wood et al. (1992) Integrated Uncooled Infrared Detector Imaging Arrays by R. A. Wood, C. J. Han, and P. W. Kruse, Proc. IEEE Solid State Sensor and Actuator Workshop, Hilton Head Island, S. C. pp. 132-135. 

P. G. LeComber and W. E. Spear, The Development of the a-Si H Field-Effect Transistor and Its Possible Applications D. G. Ast, a-Si H FET-Addressed LCD Panel S. Kaneko, Solid-State Image Sensor M. Matsumura, Charge-Coupled Devices M. A. Bosch, Optical Recording A. D Amico and G. Fortmato, Ambient Sensors H. Kukimoto, Amorphous Light-Emitting Devices R. J. Phelan, Jr., Fast Detectors and Modulators J. I. Pankove, Hybrid Structures... 

The light detector is usually made of an extremely thin optical fiber, whose tip is much smaller than the diameter of a human hair. The light diffused by the sample is collected by the tip of the needle and next conduced to an electronic detector. The detector converts the incoming light intensity into an electrical pulse feeding the computer. In some cases, instead of the optical fiber, the sensor extremity can be just a simple very small solid-state detector directly converting directly the light into an electric pulse. 

Figure 6.4. Schematic of a solid-state detector. (Courtesy of International Sensor Technology, Irvine, CA.)...

A UV detector having a 90° cone of vision. The UV generated in that viewing area will cause the UV detector to send a voltage pulse to the signaling process section of the electronic ampli-fier/controller. Once UV from the exploding fireball enters the sensor s 90° cone of vision, the solid-state switch will close in less than 10 milliseconds, which will actuate the extinguishing system. 

An example of a solid state detector is the Reticon self-scanning photodiode array, which was specifically designed for spectroscopic applications [104]. These diode arrays contain 512 or 1024 silicon diode sensor elements on 25 jum centers corresponding to a density of 40 diodes mm" . Each diode is 2.5 mm high giving each element a slit-like geometry with a 100 1 aspect ratio. Beam registration problems do not apply to the diode array since the channel dimensions are defined by a photomask and hence the detector element size and position are completely reproducible. Solid state detectors do not suffer from lag [105] and althou they will bloom, the effect is much less severe than in a vidicon. 

Both thick- and thin-film versions of a solid state, resistive hydrogen sensor were designed and fabricated at ORNL [69, 70], Both versions of the sensors (25 mm x 25 mm x 0.6 mm) are small enough to be incorporated into hand-held leak detectors or distributed sensor systems for safety monitoring throughout large areas. 

All of the sensors described above are chemical sensors utilizing the chemical interaction between COg and a sensing material. In point of conventional analytical equipments, a extremely miniaturized IR sensor for COg was recently developed by Shibata et.al[71,72] of Sanyo Electric, Japan. Miniaturization was achieved by using a new type of psn-oelectric IR detector composed of LiTaOg, a solid-state chopper and an optical filter in a same... 

While much of the surveyed research exhibits promising vapor-phase sensing performance, many of the technologies remain experimental and bound to a laboratory setting. Most of the commercial gas sensors available today utilize older, more mature technologies such as electrochemical cells, catalytic beads, photoionization detectors (PID), SAW, metal oxide semiconductors (MOS), and QCM. The dearth of viable organic solid-state vapor-phase chemosensors indicates that there is much work still to be done (in terms of material stability, selectivity, etc.) before commercialization becomes commonplace for organic sensors. 

Ion-Selective Field Effect Transistors. Ion-selective field effect transistors (ISFETs) are semiconductor devices related to the solid-state detectors used in spectroscopy (discussed in Chapter 5). In this case, the surface of the transistor is covered with silicon nitride, which adsorbs H ions from the sample solution. The degree of adsorption is a function of the pH of the sample solution and the adsorption of H" " ions results in a change in the conductivity of the ISEET channel. The cell requires an external reference electrode. ISEET pH sensors can be made extremely small (about 2 mm ) and are extremely rugged, unlike the fragile glass bulb pH electrode. They have rapid response times and can operate in corrosive samples, slurries, and even wet solids such as food products. The sensor can be scrubbed clean with a toothbrush, stored in a dry condition, and does not require hydrating... 

In biomedical sensing, some of the solid-state devices based on thermal sensing cannot be used effectively. The reason is that the sensor itself has to be heated or is heated quite hot by catalytic surface reactions. Thus pellistors (oxides with catalytic surfaces and embedded platinum wire thermometer), chemiresistors, and Figaro sensor smoke detectors have not found many biologic applications. 

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