Sensors Schottky barrier

The contact leakage current for an ideal Schottky barrier is the saturation current J, which depends on the barrier height according to Eq. (9.14). Examples of the forward J-V characteristics of some p-4-n sensors are shown in Fig. 10.6 and compared with a palladium Schottky barrier sensor. The ideality factor of the p-i-n devices is... 

Lechuga LM, Calle A, Golmayo D, Briones F (1992) Different catalytic metals (Pt, Pd and Ir) for GaAs Schottky barrier sensors. Sens Actuators B Chem 7 614-618... 

Schottky diode sensors based on other wide bandgap materials have also been investigated, as previously mentioned. GaN Schottky diodes processed on either the Ga or N face have been examined by Schalwig et al. [11,21]. A Pt/GaN Schottky diode with a barrier height of 1-eV has been shown to reversibly transform into an ohmic contact through exposure to [94]. Kokobun et al. have also investigated Pt-GaN Schottky diodes as hydrogen sensors up to 600°C [15]. 

There have been several proposed mechanisms for the operation of these sensors (Gopel, 1985 Franke et al., 2006). They all seem to converge on the existence and modulation of the Schottky barrier heterojunctions formed between the grains of the polycrystalline layer. They are equivalent to a chain of resistive elements connected in series. The density of surface states affects the depth of the Schottky barrier and depends on the interaction with the adsorbate (Fig. 8.8). The size of the grains apparently plays a major role. As the diameter of the grains decreases to below 5 nm, the space charge is smeared and the relative response of the sensor increases (Fig. 8.9). 

Another possible effect of PdAu deposits on PdAu/SnOx sensors is through the formation of a Schottky barrier between PdAu and SnOx, as in the case of the Pd/CdS hydrogen sensor. If such a barrier is formed, then a depletion layer is created inside the semiconductor tin oxide. Since the Pd work function can be reduced by hydrogen absorption through dipole or hydride formation (14,15), the width of the depletion layer in tin oxide may be reduced. The reduction of the depletion layer width causes the sample resistance to decrease. Such a possibility was checked and was ruled out, because a good ohmic contact was obtained between Pd (-50 nm thick) and SnOx- It is also commonly known that gold forms an ohmic contact with tin oxide. 

Schottky-Barrier Diode and Metal-Oxide-Semiconductor Capacitor Gas Sensors Comparison and Performance... 

Schottky-barrier diode and metal-oxide-semiconductor (MOS) capacitor gas sensors have established themselves as extremely sensitive, versatile solid state sensors. 

Schierbaum, K.D., U.K. Kirner, J.F. Geiger and W. Gopel (1991). Schottky-barrier and conductivity gas sensors based upon Pd/Sn02 and Pt/Ti02. Sensors and Actuators B-Chemical, 4(1-2), 87-94. 

Light sensors made from a-Si H are either p-i-n or Schottky barrier structures. Unlike crystalline silicon, a p-n jimction is ineffective without the undoped layer, because of the high defect density in doped a-Si H. Illumination creates photoexcited carriers which move to the junction by diffusion or drift in the built-in potential of the depletion layer and are collected by the junction. A photovoltaic sensor (solar cell) operates without an externally applied voltage and collection of the carriers results from the internal field of the junction. When the sensor is operated with a reverse bias, the charge collection generally increases and the main role of the doped layers is to suppress the dark current. A Schottky device replaces the p-type layer with a metal which provides the built-in potential. 

Fig. 10.6. Examples of the forward bias current-voltage characteristics of p-i-n sensors, compared to a Schottky barrier device.

Pt-doped Ti02 sensors showed low operation temperature (350-800°C), improved gas sensitivity and short response time (<0.1 s) [300, 301]. The oxygen-sensing mechanism is a combination of F JTi02 interfaces in a Schottky-barrier mechanism and an oxygen vacancy bulk effect mechanism [302]. It has been demonstrated that at high temperatures. 

Abstract This chapter focuses on the performances of gas sensors based on single-walled carbon nanotubes (SWCNTs).The chapter first reviews chemiresistor and field-effect transistor sensor architectures. Theoretical models based on doping effect and Schottky barrier modulation are correlated to the percolation of metallic carbon nanotubes. The functionalisation strategies of carbon nanotubes, used to acquire a gas selectivity, are discussed and offer promising application for the monitoring of air quality, or military and medical applications. 

The commonly accepted physico-chemical gas sensing mechanisms of SWCNT-based gas sensors are discussed in detail by several authors (Bradley 2003 Pengetfli,2009 Battie eta/.,2012b).These mechanisms,which will be described in Section 10.3, are the charge transfer between adsorbed gas molecules and SWCNTs, and the modulation of the Schottky barrier established between semiconducting SWCNTs and metallic electrodes. 

Gas sensing mechanism must be highlighted to improve the sensor performances. Two sensing mechanisms are considered in this section the charge transfer between adsorbed molecules and SWCNTs, and the Schottky barrier modulation at the contacts. 

Hie Hiales group has patented an original sensor architecture based on an array of CNTFETs with various metals for the electrodes (BondavaUi et al., 2008, 2010). As described in Section 10.3.3, the sensing mechanism of CNTFETs is mainly attributed to the modulation of the Schottky barrier built at the contacts between metallic electrodes and semiconducting SWCNTs. Assuming that the adsorption of gas at the contacts depends on its chemical affinity with the electrodes, each gas-metal couple should induce different modulation of the Schottky barrier. 

Specific applications Schottky barriers and FET gas sensors electronic nose... 

Korotcenkov s research results are well known in the study of Schottky barriers, MOS structures, native oxides, and photoreceivers based on III-V compounds. His current research interests, starting from 1995, include material sciences and surface science, focused on metal oxides and solid state gas sensor design. 

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