Device Sensitivity

Each device is challenged with concentrations of the CWA/compound vapor to determine its threshold sensitivity using a vapor generated under controlled atmospheric conditions. The threshold concentration is defined as the concentration of the substance at which an alarm within 2 min of exposnre time or a definitive 

The determined threshold concentration level or minimum detectable level (i.e., MDL) is used in subsequent tests at various humidity and temperature conditions. MDL is determined based on detection occurrence with a minimal target substance dosage. For example, a longer exposure time is permissible if the device can demonstrate capacity to detect the target substance at a lower concentration to produce the lowest concentration/time relationship (Ct). 

Testing the device with higher vapor concentration levels produces information on its abihty to quickly provide the necessary warning to minimize exposure dosage and to clear out vapor residue after contamination. Detectors are tested at low, medium, and high concentration levels to generate response curves. 

Source. U.S. Department of the Army (see p. 42). JCAD Study Plan (draft 5/11/94). 

Agent Threshold Exposure Concentration (mg/m ) Threshold Exposure Response Time Maximum(s) Relative Humidity (% RH) Range Temperature Range (°C) 

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An essential stage in the production of an effective sensor is an understanding of the device sensitivity with a view to its optimisation. 

While planar optical sensors exist in various forms, the focus of this chapter has been on planar waveguide-based platforms that employ evanescent wave effects as the basis for sensing. The advantages of evanescent wave interrogation of thin film optical sensors have been discussed for both optical absorption and fluorescence-based sensors. These include the ability to increase device sensitivity without adversely affecting response time in the case of absorption-based platforms and the surface-specific excitation of fluorescence for optical biosensors, the latter being made possible by the tuneable nature of the evanescent field penetration depth. 

In this chapter, we will numerically analyze the effects of the HRI overlay on the cladding modes distribution with particular attention to its influence on the device sensitivity to the SRI changes and to the overlay refractive index changes. The structure to which we refer is depicted in Fig. 3.2. 

The sensitivity in each of four cases is expressed as multiplication of device sensitivity and waveguide sensitivity... 

Based on the definition, device sensitivity is related to the variation of transducing optical parameters, and thus depends on the transducing method. For the resonant wavelength shift scheme, it is expressed as... 

Using the method described in Sect. 8.3.3.1 to obtain the electric field in a microtube resonator, the device sensitivity can be calculated with the perturbation theory for different modes. This provides more systematic insight into the operation of microtube resonators. First, bulk index sensing is considered. The sensitivity is proportional to the integrated optical field inside the liquid core region over that of the entire space, and can be expressed as72 ... 

Fig. 16.6 Estimates of device performance in response to nucleic acid binding, (a) FDTD simulation showing the mass sensitivity of the device plotted as a function of the number of functionalized holes. The circles indicate the sensitivity values calculated from the simulations. The solid curve shows a least squares fit using an analytical model for the device sensitivity, which is described below, (b) Plot illustrating the dependence of the shift in resonant wavelength of a resonator on the number of functionalized holes. The blue circles indicate the data obtained from 3D FDTD simulations. The solid curve is a best fit curve of the form a( 1 eTbN) where a and b are arbitrary constants. The values of a and b used here are 6.159 nm and 0.4273, respectively. Reprinted from Ref. 37 with permission. 2008 Optical Society of America...

Figure 15. Effect of different I-layer and C-layer on the device sensitivity and response time to a) 1000 PPM H2 in air at 150°C. The device is a Pd/Top I-layer (Si02. AI2, O3, Ta2 O3, Si3 N4)/ Si02/p-Si capacitor and the initial ambient is air (after Ref. 16, 1984 IEEE) and b) 7 PPM, 25 PPM, and 70 PPM NH3 in air at 150 C, device used here is an ultra thin Pt-layer/Pd/Si02/p-Si capacitor (after Ref. 9, with permission).

If a noise current of 100 mA at 1 MHz is to find its way into the ground wire between the two devices, the noise voltage must be 2 V, which is enough to cause the devices to lose communication and perhaps even sustain damage, depending on the device sensitivity. This example is a simple situation consisting of only two devices however, hundreds and perhaps thousands of such devices or circuits might be present in an actual computer or communication data center. All these devices require a common reference from which to operate. This is accomplished by the use of the SRG. 

To produce electromagnetic interference, three components must exist (1) a source of interference, (2) a victim susceptible to EMI, and (3) a medium for the coupling of EMI between the source and the victim, which is any device sensitive to the interference. The coupling medium could be inductive or capacitive, radiated through space or transmitted over wires, or a combination of these. Identification of the three elements of EMI as shown in Figure 7.11 allows the EMI to be treated in one of three ways ... 

The data show that although the AId due to the admission of H2 is not large, it is possible to measure the H2 concentration. Improvement in the structure should be continued to enhance the device sensitivity. 

For each of these, displacement maxima occur at the crystal faces, making the device sensitive to surface perturbations. The perturbations to be considered in this section include surface loading by (1) an infinitesimally thick mass layer, (2) a contacting Newtonian fluid, and (3) a viscoelastic layer of finite thickness. 

The gravimetric sensitivity, S , of each of our four main sensors can be expressed in a simple form that permits easy comparison of device sensitivity. The key is to focus on the dimensions of the active region of each device where wave energy is present. 

AW device sensitivity to viscoelastic parameters and electrical pnqieities can be used to advantage in some film characterization techniques. In these situations, a comparison of the AW device response to a model of the AW/thin film interaction is often crucial to the effective evaluation of thin film parameters. These additional interaction mechanisms typically involve changes in both the wave velocity and the wave attenuation for SAW, APM and FPW devices, and changes in both resonant frequency and admittance magnitude in TSM devices. In contrast, mass loading does not contribute to wave attenuation or decreases in admittance since moving mass involves no power dissipation (see Chapter 3). 

Semiconductor devices sensitive to infrared radiation in either of the two atmospheric infrared windows can be built, in large numbers, on the surface of a chip. An infrared image focused on the surface of sudi an array can be read off electronically as image information, and this information used to construct a visible-light image on a screen. 

Filippini, D. Fraigi, L. Aragbn, R. Weimar, U., Thick film Au-gate field-effect devices sensitive to N02, Sens. Actuators. B 2002, 81(2-3), 296-300... 

Numerical simulations of this model were carried out to test expectations. The results are shown in Figs. 8 and 9. The figure 8 shows the amplitude of the detected signal at the third harmonic of the modulation frequency when the operating point corresponds to the second node, i.e., n = 2, and the Bo is selected for the detection of radiation in the vicinity of 400 GHz, which lies near the lower edge of the Terahertz band, where the device sensitivity to such radiation has been confirmed by our experiments, see Fig. 3, for example. As confirmed by the simulations, the 3 harmonic sensing concept yields indeed a narrow band detector, with sensitivity between roughly 200 and 800 GHz, as... 

As we expand on below there are two methods by which we can increase this detection sensitivity. Figure 3 shows how by reducing the number of functionalized holes in the resonator the total amount of mass will increase. As can be seen a device sensitivity of 3.5 nm/fg is possible with the first two holes being functionalized. Assuming the same detectable linewidth as the above, an absolute LOD on the order of 3 ag can be achieved. One method by which this could be done is by exploiting some of the recent work done on improving the Q-factor of such 1-D resonators [6, 7]. The combination of these techniques will help pave the way towards the detection of sub-attogram amounts of bound mass. 

The quartz crystal resonator is a useful device for the study of thin-layer and interfacial phenomena. The crystals commonly employed have a fundamental resonance frequency of 5 -10 MHz and a resolution of the order of 0.1 -0.5 Hz. This high resolution makes the device sensitive to a myriad of physical phenomena, some of which are interrelated and some quite independent of each other. It cannot be overemphasized that the quartz crystal resonator acts as a true microbalance (more appropriately a nanobalance) only if in the course of the process being studied, the nature of the interface (its roughness, sHp-page, the density and viscosity of the solution adjacent to it, and the structure of the solvent in contact with it) is maintained constant. 

Besides the obvious advantages, there are also some limitations the most important one is the fact that a customary SAW is very sensitive towards high viscosity and loading. This makes it virtually impossible to operate it in liquid phase, as the surface wave is completely damped by the sample matrix. The second consequence is that sensitive layers for SAW coating usually are thinner than for QCM, however, the highly increased device sensitivity by far overcompensates the lower amount of interaction sites. Additionally, thin layers lead to very favourable sensor response times. 

Finally, we carried out measurements on complete functional devices. We measured the incident photon-to-current conversion efficiency (IPCE) spectra for devices sensitized with AR25 using as electrolyte a solution containing the redox couple iodine/iodide (see Experimental Section). Figure 4 illustrates the IPCE spectra for an AR25/DSSC. 

In contrast the use of the antibodies for the detection of antigens has not an amplification stage involved and then the affinity reaction should be amplified in order to have a clear transduction. We have two possibilities, one is the use of a bioconjugate involving a bound enzyme, like in the classical ELISA test the second is the inherent amplification given by the mass of the biological element involved, a piezoelectric device (sensitive to mass) can detect minute amount of large proteins (like antibodies) if they are attracted on the surface of the sensor. 

Ultraviolet radiation is one of the primary causes of aging in organic materials. Measuring the ultraviolet radiation thus tells us much more about the stress on the material than measuring the total radiation. The irradiance in the ultraviolet range, l v. usually measured with a device equipped with a photocell receiver and a broadband ultraviolet filter. This combination makes the measuring device sensitive to the spectrum between 300 and 400 nm. A diffusing cap ensures that the evaluation of the radiation takes proper... 

Similarly, devices sensitive to NO2 or CO2 can be fabricated by attaching NaN02 (Na+ ionic conductor) or Li2C03-based composite salt (Li+ ionic conductor) to the gate, respectively. The response mechanisms involved can be understood in the same way. In the NO2 device, for example, the half cell reaction is expressed as follows ... 

Recently, the concept has also been utilized for the development of field effect devices sensitive to carbon dioxide (CO2) and nitrogen dioxide (NO2). By the introduction of a solid electrolyte for which, instead of oxygen anions, carbonates or nitrites/nitrates (N02 /N03 ) are incorporated in the... 

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