Diode type sensors

A conclusion has been drawn based on the results of the studies that a most convenient source of RGMAs suitable for sensor measurements is represented by discharge sources of the diode type, sources that operate in the d.c. discharge, glowing cathode mode. 

In this review the basis for the chemical sensitivity of these devices will be explored and the various device structures used for these sensors will be discussed. A survey of the performance of the diode-type and capacitor-type structures will be presented and a comparison of characteristics of these two classes of solid state gas sensors will be given. 

Comparison of Diode-type and Capacitor-type Sensors... 

As was shown in Figure 3.159, cryogenic temperatures can be detected by integrated circuit diodes types K, T, and E thermocouples (TCs) class A and B resistance temperature detectors (RTDs) acoustic and ultrasonic thermometers germanium and carbon resistors and paramagnetic salts. As TCs and RTDs will be discussed in separate subsections, here the focus will be on the other sensors. 

There are two basic types of chiral detectors for LC, those that measure optical rotation and those that measure circular dichroism. At the time of writing this book, the only commercially available chiral detectors are those that measure optical rotation. Nevertheless, a detector that measures circular dichroism and utilizes a diode array sensor system is thought to be in the design stage and will be briefly described later. 

In this entry, we focus on the discussion of the platform technology for electrochemical sensors, metal oxide semiconductive (MOS) sensors, and piezoelectric based quartz crystal microbalance (QCM) sensors. There are other types of chemical sensors, such as optical sensors, Schottky diode based sensors, calorimetric sensors, field-effect transistor (FET) based sensors, surface acoustic wave sensors, etc. Information of these specific sensors can be found elsewhere and in current journals on sensor technologies. Because of the increasing importance of microfabricated sensors, a brief discussion of microsensors is also given. 

Shimizu Y etal 1989 The sensing mechanism in a semiconducting humidity sensor with platinum electrodes J. Electrochem. Soc. 136 3868-71 Lukaszewicz J P 1991 Diode-type humidity sensor using perovskite-type oxides operable at room temperature Sensors Actuators B 4 227-32 Anderson RC etal 1990 Investigations of porous silicon for vapor sensing Sensors Actuators A 23 835-9... 

Electrical conduction is also very important for many devices that exploit the huge area of surface or interface per unit volume in zero-dimensional nanostructured materials such as nanoporous materials, granular materials, nanocomposites, and nanoparticle assemblies. Examples of such devices are chemiresistor-type sensors, solar cells, light-emitting diodes, and energy-storage cells. From the point of view of electron... 

Among the main advantages of colorimetric sensing is the easy and widely available instrumentation. The oldest and still perfectly viable method of colorimetric sensing is an old-fashioned naked-eye observation. There are hundreds of examples of dipstick -type sensors, vide infra. The most widely used method for data collection is UV-vis absorption spectroscopy. Similarly, some multiwell plate readers comprise a polychromatic light source and a CCD camera similar to common diode-array spectrophotometers and are also capable of recording the whole spectra from each well. One assumes that the above methods of data acquisition need not be discussed in detail. 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

Figure 7. Exemplary Raman sensor layouts a reflection-type probe with single excitation fibre and collection fibre bundle b angular probe with internal laser diode c sensing fibre probe.

As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

An NIR biosensor coupled with an NIR fluorescent sandwich immunoassay has been developed. 109 The capture antibody was immobilized on the distal end of an optical fiber sensor. The probe was incubated in the corresponding antigen with consecutive incubation in an NIR-labeled sandwich antibody. The resulting NIR-labeled antibody sandwich was excited with the NIR beam of a laser diode, and a fluorescent signal that was directly proportional to the bound antigen was emitted. The sensitivity of the technique increased with increasing amounts of immobilized receptor. There are several factors involved in the preparation of the sandwich type biosensor. A schematic preparation of the sandwich optical fiber is shown in Figure 7.14. 

Kim et al. have compared the hydrogen and methane sensitivity at 400-600°C of Pt and Pd-SiC Schottky diodes fabricated on n-type 6H-SiC. The Pd or Pt (<80 nm) was sputter deposited in 1-mm dots [8]. The sensitivity was measured as the change in current at a constant forward bias of 3V. The Pd-SiC Schottky diodes showed a higher sensitivity, as well as faster speed of response to both hydrogen and methane. The stability of the hydrogen response was tested for 30 days at 500°C and showed excellent results for both types of sensors. 

Figure 2.16 Sensor signals, the voltage over the diodes at 0.6 mA, for (a) p-type and (b) n-type Schottky diodes while exposed to alternating 2% Oj/Nj or 2% H2/N2 ambient at 300°C.

The SiC Schottky diodes and capacitors that have been processed by the authors were processed on either 6H or 4H substrates (n-type, about 1 x 10 cm ) with a 5-10- m n-type epilayer (2-6 x lO cm" ) [123, 124]. A thermal oxide was grown and holes were etched for the metal contacts. In the case of the Schottky sensors, the SiC surface was exposed to ozone for 10 minutes before deposition of the contact metal. This ozone treatment produces a native silicon dioxide of 10 1 A, as measured by ellipsometry [74, 75]. The MISiC-FET sensors (Figure 2.9) were processed on 4H-SiC, as previously described [125]. The catalytic metal contacts consisted of 10-nm TaSiyiOO-nm Pt, porous Pt, or porous Ir deposited by sputtering or by e-gun. 

General Behavior. For the same reasons discussed in the diode performance section, most of the work on capacitor-type C-I-S sensors has focused on structures for detecting hydrogen or hydrogen-bearing gases. Consequently, this section will mainly examine the performance of C-I-S hydrogen sensors however, detection of CO will be discussed. 

---