Ž. Sensors and Actuators B 60 1999 71–77 www.elsevier.nlrlocatersensorb Influence of binders on the sensing and electrical characteristics of WO -based gas sensors 3 Jong-In Yang a , H. Lim a,) , Sang-Do Han b a Department of Electronic Engineering, Ajou UniÕersity, Suwon 442-749, South Korea b Korea Institute of Energy Research, Taejon 305-600, South Korea Received 27 April 1999; received in revised form 3 May 1999; accepted 7 May 1999 Abstract Ž. The effects of various binders added in the solidification process, such as polyvinyl alcohol PVA , silica sol and Al O , on the 23 sensing and electrical characteristics of WO -based n-type semiconductor gas sensors are investigated. Grain sizes show a slight variation 3 according to the employed binders. In the case of WO films fabricated with silica sol or Al O binder, some residue of binders at grain 323 boundary and agglomerates of WO grains are observed. The electron concentration of WO film around room temperature and the 3 3 temperature dependence of film resistance in normal air do not show any systematic dependence on the employed binders. In NO gas, x however, the optimum operation temperature and the sensitivity of WO sensors at that temperature are observed to depend greatly on the 3 employed binders. The resistance of the WO films shows an exponential temperature dependence in NO gas in the temperature range of 3 x 110–3758C, and the increase of film resistance in NO gas is observed to depend greatly on the binders added in WO films. Sensitivity x 3 to various ambient gases does not show any systematic variation. All these results mean that the binders affect the sensing characteristics of WO -based gas sensors mainly through the modification of chemical conditions at grain boundary rather than the modification of grain 3 size and electron concentration. q 1999 Elsevier Science S.A. All rights reserved. Keywords: WO NO gas sensor; Grain boundary barrier height; Binder effects 3 x 1. Introduction The semiconductor gas sensing devices are based on the conductivity change of the semiconductor material due to its interaction with gas. When gas molecules are adsorbed on the surface of a semiconductor, electron transfer occurs between the semiconductor and the adsorbates. If the electron affinity of the adsorbates is larger than the work function of the n-type semiconductor, the adsorbates ac- cept electrons from the semiconductor. In the opposite case, the semiconductor accepts electrons from the adsor- bates. This electron transfer continues until the Fermi level of the gas-adsorbed semiconductor surface becomes equal to that of the bulk. As a result of this electron transfer, a depletion or accumulation of charges occurs near the semi- conductor surface. Then the accompanying variation of surface potential barrier induces a change in the electrical wx conductivity or resistivity 1 . Therefore, under the ambi- ) Corresponding author ence of oxidizing gas such as NO , electrical resistivity of x a polycrystalline n-type semiconductor film is increased due to the increase of the potential barrier height at the Ž. surface andror grain boundary GB of the polycrystal. In the ambience of reducing gas such as CO and H , electri- 2 cal resistivity decreases. WO is an n-type semiconductor whose electron con- 3 centration is determined mainly by the concentration of stoichiometric defects such as oxygen vacancy like any other metal oxide semiconductors. The first work on the feasibility of WO films as a gas sensor was reported by 3 wx Shaver 2 , who observed that the conductivity of WO 3 films changed greatly upon the exposure to the H ambi- 2 ent. Following this pioneering work, many works have been performed on the structural and electrical properties and sensing characteristics of WO films made by various 3 wx methods such as sol–gel coating 3 , magnetron sputtering wx wx wx 4 , thermal evaporation 5 , chemical vapor deposition 6 . Recently, WO -based gas sensor has exhibited an excellent 3 sensing characteristic for NO gas irrespective of the WO x 3 0925-4005r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. Ž. PII: S0925-4005 99 00248-8 () J I. Yang et al.r Sensors and Actuators B 60 1999 71–7772 wx structure whether it is amorphous or polycrystalline 3–8 . It has also demonstrated a good selective detectability for the NO gas with an excellent cross-sensitivity for the x wx interference gases such as CO and CH 5 . In general, the 4 sensing characteristics, such as sensitivity and selectivity, of the polycrystalline sensor depend greatly on the mi- crostructural properties of the polycrystal and the metal wx impurities doped as catalysts 8,9 . In the case of sol–gel coated WO films, the binders are commonly added to 3 enhance the solidification. The binders also influence the wx sensor characteristics 3,10 , since the microstructure of wx sol–gel coated WO can be affected by the binders 3 . For 3 example, the grain size of WO film decreases as the 3 wx amount of SiO binder is increased 3 . Thus the addition 2 of a suitable amount of SiO in WO sol–gel is believed 23 to increase the porosity of the composite film and in turn wx the sensitivity of the sensor 3 . To our knowledge, how- ever, the origin of the binder effects on the sensing charac- teristics of WO films has not been clarified, since most of 3 the studies are concerned with the sensing characteristics related with fabrication methodology andror microstruc- ture of the polycrystals. In this paper, we have investigated the effects of binders on the sensing characteristics and the structural–electrical properties of WO films. The physical properties of WO 33 films, such as grain size and the electron concentration around room temperature, show little dependence on the employed binders. But the sensing characteristics and the electrical resistance in NO gas depend greatly on the x binders. It is concluded, from these facts, that the property mostly affected by the binders is GB barrier heights in the ambience of gas and this phenomenon is related to the chemical nature at GB. It is also discussed that optimum operation temperature of the sensors for NO gas is simply x determined by the ratio of electrical resistances in NO gas x to that in normal air rather than the temperature depen- dence of adsorptionrdesorption kinetics of NO gas. x 2. Grain boundary potential barrier and sensitivity The operation of a semiconductor gas sensor is realized through the change of the surface andror interface poten- tial barrier due to the adsorbed radicals. Although an actual WO polycrystal has a random GB potential barrier net- 3 work, suppose that it has a planar type interface for the simplicity of discussion. As a result of gas adsorption at GB, electrons are depleted from the GB region, and the potential barrier height V at GB is given by i qN W 2 d V s 1 Ž. i 2 e under the depletion approximation. Here, q is the elec- tronic charge, N the donor concentration, W the depletion d region width, and e is the permittivity of the semiconduc- tor. From the charge neutrality condition between the interface charge Q and the space charge in depletion i regions, we obtain Q sy2qN W.2 Ž. id In actual polycrystalline films with rather small grain size like ours, the extent of the space-charge layer depends on the Debye length given by 1r2 e kT L s 3 Ž. D 2 ž/ qn wx and the contacting shape of GB between grains 1,11 . Here, k, T, and n are the Boltzman constant, absolute temperature, and the electron concentration in the grain, respectively. Even in this case, the charge neutrality condi- tion should always be satisfied, and the electrons are filled wx up to the so-called neutral surface Fermi level 12 . Ac- 3 Ž cording to Tersoff, the sp hybrid energy the dangling . bond energy plays the role of neutrality level for the wx intimate contact between covalent semiconductors 13 . The interface formed on an etched semiconductor surface andror on the GB of a polycrystal is not an intimate contact, and the neutral surface Fermi level of this non-in- timate contact varies according to the species and the wx amount of adsorbed atoms 14 . This fact means that the potential barrier height of the GB of a given material may depend on the material processing or the chemicals used. It also indicates that the gas molecules adsorbed at the GB can change the interface charge Q and the potential i barrier height V by changing the distribution of interface i states and the electron occupation on them. In some cases, even the concentration of the adsorbed gas ions can be wx measured from the change of potential barrier 15 . The potential barrier at the GB of the polycrystal im- pedes the flow of the electrons between the grain. Then, due to the potential barrier at GB, the conductance of a wx polycrystal has an activation form as 1 yqV i G sG exp , 4 Ž. gb o ž/ kT where G is the conductance of the grain itself. When an o oxidizing gas like NO is diffused into the GBs of WO x 3 film, the absolute value of interface charge Q increases i and thus the GB barrier height changes from V to V q DV . ii i Therefore, the resistance between two electrodes in NO x gas is given by qVqDV Ž. ii R s R exp , 5 Ž. gas o kT where R is the resistance when the GB potential barrier is o absent. Therefore, when the sensitivity S is defined as the () J I. Yang et al.r Sensors and Actuators B 60 1999 71–77 73 ratio of the resistance in NO gas to that in normal air, S is x given by RqDV gas i Sssexp . 6 Ž. ž/ RkT air Thus, the influence of binders on the sensitivity of the WO gas sensors can be studied by observing DV in a 3i given gas ambient for each of the employed binders. 3. Experiments In this study, WO powder mixed with 4 wt.% TiO 32 were prepared by the sol precipitation method from the appropriate mixture of TiCl and WCl . The film with this 46 composition has been observed to show the best sensitivity to NO gas in various WO films doped with different x 3 wx species and amount of catalysts 10,16 . The powder mixed with methanol was then ball-milled for 30 min without Ž. binder or with the binder of polyvinyl alcohol PVA , silica sol, and Al O , respectively. The concentration of 23 the added binders was 5 or 10 wt.%. Finally the sensor material paste was screen-printed on the alumina substrate and then annealed at 8008C for 2 h in an air flow condi- tion. For the electrodes of the sensor, interdigitized Au contacts were formed on the front surface of the alumina substrate before the paste printing. Pt was also deposited on the back surface of the substrate as a heating resistor for the sensor. Fig. 1a shows the cross-sectional view of the sensor used in this work. The microstructural analysis of the samples was per- formed using a Philips 515 scanning electron microscope Ž. Ž. Fig. 1. a Cross-sectional view of the employed film gas sensors and b the measurement circuit. Ž. SEM and an H-7100 transmission electron microscope Ž. TEM . Source NO gas was obtained by mixing NO gas x and NO gas with the volume ratio of nine to one. The 2 desired NO gas ambience was provided by injecting the x prescribed amount of NO gas to a chamber with the x dimension of 40= 25=18 cm 3 and then stirring the gas mixture with a small fan for 3 s. The sensing character- istics of the samples were measured in this closed chamber using the circuit shown in Fig. 1b. The sample resistance was measured with the control voltage of V s 10 V, and C the sensitivity S of the sample was defined as the ratio of the resistance in gas to that in normal air, R rR . gas air Sensor temperature was controlled by varying the heater voltage V in Fig. 1b and was measured using a Minolta H IR 0506C spot thermometer. The temperature dependence of the sample resistance, in NO gas or in normal air, was x measured by an HP 4194A RLC meter with the probing signal of 100 kHz and 20 mV. Before the measurement of the temperature-dependent sample resistance in NO gas, x the samples were maintained long enough time in NO x ambience at the predetermined optimum operation temper- Ž. ature see Fig. 3 for the stabilization of gas adsorption. Then the resistance was measured raising the sample tem- perature after the samples had been cooled down to room temperature. 4. Results and discussion From the Hall measurements performed in normal air, the room-temperature electron concentration of all the samples was found to be about 1= 10 17 cm y3 with a slight sample-to-sample variation. However, we could not find any systematic influence of employed binders on the carrier concentration. Electron concentration was observed to increase slightly in the temperature range of 80–2308C after a rather fast increase between 30 and 808C. Electron concentration is thus believed to be mainly determined by the concentration of oxygen vacancies like any other metal oxide semiconductors. The rather fast increase of electron concentration around 808C observed in some samples might be due to the electron emission from the interface traps andror some deep defects. The average grain size, deter- mined by the TEM observation on the sintered samples, was 61 nm for the samples fabricated without binder and 56, 58, and 61 nm for the samples fabricated with 5 wt.% binders of PVA, silica sol and Al O , respectively. The 23 SEM observation revealed that the grain sizes were dis- tributed more homogeneously in the samples fabricated without binder or with a PVA binder compared with those fabricated with silica sol or Al O binder. We also ob- 23 served from the energy dispersive X-ray spectroscopy that about 3% of SiO and 3.5% of Al O remained at the 223 grain edges of the samples fabricated with silica sol and Al O binders, respectively. We could not find any PVA 23 () J I. Yang et al.r Sensors and Actuators B 60 1999 71–7774 residue in the samples fabricated with PVA binder which had the smallest grain size. The added PVA is thus be- lieved to have almost completely evaporated in the sinter- ing process at 8008C. Typical sensing characteristics are shown in Fig. 2 for the samples fabricated without binder and with 5 wt.% PVA binder. One can note that the sensor fabricated with PVA binder has better response to NO gas in sensitivity x and response time compared with the one fabricated with- out binder. One can also notice that the WO film resis- 3 tance is decreased in CO gas since it provides a reducing gas ambience. However, the response of WO films to CO 3 Ž gas is very small compared with that to NO gas Note the x differences in ambient gas concentration and output volt- . ages . It is therefore clear that these sensors, especially the one fabricated with PVA binder, can selectively detect NO gas from the mixture of NO and CO. xx Fig. 3 shows the temperature dependence of the sensi- tivity in 15 ppm NO gas for the samples that are different x in species and amount of binders. As can be seen in this figure, the sensitivity S and the optimum operation temper- ature T depend on the binders used. For the employed o binders, the samples fabricated with 5 wt.% binders show Ž better sensing characteristics i.e., lower T and higher S o . value compared with the samples with 10 wt.% binders except for the case of the sensitivity of the silica sol added film. The sample fabricated with 5 wt.% PVA binder shows best sensing characteristics, i.e., it shows highest S Ž. value at lowest TSs375 at T s 1808C . But adding oo silica sol or Al O binder reduces the sensitivity of 23 WO :TiO film to NO gas. Table 1 shows the response 32 x time and the recovery time of the samples in the same NO x gas concentration. One can note that the samples fabricated with binders respond faster to the NO gas than the one x Fig. 2. Typical output voltage responses of the sensors at various concen- trations of NO and CO gases. x Fig. 3. Operation-temperature dependence of the sensitivity in 15 ppm NO gas for the samples fabricated with different species and amount of x binders. The WO :TiO film with 5 wt.% PVA binder shows best 32 sensing characteristics. fabricated without binder. But the samples fabricated with PVA binder, which show highest sensitivity, recover most slowly in the normal air. Hereafter, we consider the WO films fabricated with 5 3 wt.% binders only, since they show better sensing charac- teristics than the ones fabricated with 10 wt.% binders. The dependence of sensitivity on NO concentration is x shown in Fig. 4. As can be seen in this figure, the sensor fabricated with the PVA binder shows the highest sensitiv- ity consistently for all the employed NO concentration. x One can also note that the sensitivity of the samples fabricated with binders increases steeply up to 30 ppm of NO and then increases much slowly, while that of the x sample without binder increases rather steadily. Sensitivity is related to the variation of GB barrier height according to Ž. Eq. 6 . Therefore, the saturation of sensitivity around 30 ppm of NO , observed especially in the samples with x binders, means that the GB barrier height does not increase further above this NO concentration presumably due to x the consumption of available sites at GB for the NO x molecule adsorption. In this context, a slow increase of the Table 1 Response and recovery times of the studied sensors in 30 ppm NO gas at x each T o Ž. Binders wt.% None PVA Silica sol Al O 23 5 10 5 10 5 10 Ž. Response time min 3.5 2.3 2 2 1 1 1 Ž. Recovery time min 5 6 6 6 4.5 4.5 4.5 () J I. Yang et al.r Sensors and Actuators B 60 1999 71–77 75 Fig. 4. Dependence of sensitivity of the WO :TiO films with different 32 binders on the NO concentration at optimum operation temperatures. x sensitivity of the samples with SiO binder and without 2 binder for the NO concentration higher than 30 ppm x might be due to a further penetration of NO gas deep x from GB under the ambience of high NO concentration. x As described in Section 2, sensitivity is mostly related to the difference in the effective GB potential barrier in gas and in normal air due to the adsorptionrdesorption of gas molecules at GB interface. The effective potential barrier height hindering electron transport is affected by wx the grain size and the Debye length 9 . In our case, the electron concentration does not show a large sample-to- sample dependence. Thus the influence of Debye length on the binder-dependent sensing characteristics should be marginal. One may imagine that the grain size affects some sensing characteristics of our samples since the NO x sensitivity of WO film with small grain size can be larger 3 wx than that with large grain size 8 . For example, the samples fabricated with PVA binder show highest sensitiv- ity and longest recovery time. Since these samples have the smallest grain size in our samples, the ratio of surface area to volume should be largest. Moreover, small pores would be created as the PVA binder evaporates in the annealing process at 8008C. Thus these samples may show a higher sensitivity compared with any other samples. NO x gas would be desorbed in normal air rather slowly after being adsorbed at the GB andror small pores that lie deep from the sample surface. A similar reasoning might also explain why the sample with PVA binder responds to the NO gas more slowly compared with the samples with x silica sol and Al O binders. However, the dependences of 23 optimum operation temperature and GB barrier heights in Ž. NO gas on the employed binders Figs. 3 and 5 cannot x be explained by the grain size effects. The influence of binders on the sensor characteristics is investigated more systematically by directly measuring the temperature dependence of sensor resistance as described in Section 3. Fig. 5 shows the temperature dependence of the samples fabricated without binder or with binders in normal air with 50% relative humidity and in 30 ppm NO x gas, respectively. As can be seen in this figure, all the samples show similar temperature dependence in normal air, and the film resistance shows an exponential tempera- ture dependence only in a narrow temperature range. The samples have a rather high electron concentration of the order of 10 17 cm y3 . Electrons can then easily tunnel through the small and narrow GB barriers. Thus the scat- tering mechanism other than GB scattering can be impor- tant especially at high temperatures. The fast variation of electron concentration at about 758C is believed to affect the variation of resistance in this temperature range. Be- cause of these phenomena, the film resistance would show an exponential temperature dependence only in a narrow temperature range. The GB barrier height in normal air is estimated to be about 0.17–0.25 eV from the resistance values in the temperature range of 100–2008C. In 30 ppm NO gas, the sample resistances increase by more than one x order of magnitude. They also show exponential tempera- ture dependences in a wider temperature range above 1108C. These facts clearly indicate that the GB barrier heights are increased due to the adsorption of oxidizing NO gas, i.e., the anionic adsorption of NO molecules, xx Fig. 5. Variation of resistance as the sample temperature is varied in normal air condition and in the ambience of 30 ppm NO . In the case of x normal air, the relative humidity was 50% at room temperature. The film resistance is increased and shows an exponential temperature dependence above 1108C in 30 ppm NO gas. x () J I. Yang et al.r Sensors and Actuators B 60 1999 71–7776 and maintained up to a rather higher temperature. The non-exponential temperature dependence below 1108Cin NO gas might also be related to a rather fast variation of x electron concentration in this temperature range. If the decrease of sensitivity above the optimum operation tem- Ž. perature T see Fig. 2 is due to the desorption of NO o x gas, then the resistance should decrease with the increase of temperature faster than the exponential one since the desorption effect adds on the exponential temperature de- Ž. pendence given by Eq. 5 . But the decrease of film resistance is faster than the exponential one only above 3758C in some samples. Therefore, the decrease of sensi- tivity in the temperature range of 200–3508C observed in Fig. 3 is not due to the desorption of NO gas. The x decrease of sensitivity due to the desorption of NO gas, if x it exists, should appear only above 3758C where the resis- tance decreases faster than the exponential one in some samples. One can also notice in Fig. 5 that the increase of resistance in NO gas is different for the samples fabri- x cated with different binders. For example, the samples Ž. with PVA silica sol binder that shows the smallest Ž. largest resistance values in normal air show largest Ž. smallest resistance in NO gas. The values of GB poten- x tial barriers deduced from the exponential temperature dependence of resistance in the temperature range of 110– 3758C are given in Table 2 for the ambience of 30 ppm and 120 ppm NO gases. One can see that the GB barrier x heights in NO gas depends quite on the choice of binders. x As mentioned earlier, the grain size of WO :TiO poly- 32 crystals has little dependence on the choice of binders. Moreover, Debye length is comparable in all samples. Thus the origin of the observed difference in GB barrier heights in NO gas should be due to the dependence of x anionic adsorption at GB on the employed binders. An- ionic adsorption can occur differently due to the different chemical compositions at GB when different binders are employed. This is quite possible since some binders can remain at GB as observed in the samples fabricated with silica sol or Al O binder, and then the different choice of 23 binders would result in the different oxygen deficiency at GB. For example, the sample with the PVA binder may have a larger oxygen deficiency at GB compared with the samples with Al O or SiO binder and thus can provide 23 2 more sites for the NO adsorption. Since the composition x at GB depends on the binders, the distribution of interface states that affect the GB barrier height would do so, too. We can thus safely say that the binders influence the Table 2 Ž. Typical values of GB potential barriers in meV in the ambience of 30 and 120 ppm NO gases x Binders None PVA Silica sol Al O 23 30 ppm NO 410 460 410 350 x 120 ppm NO 590 660 520 430 x Fig. 6. Histogram for the sensitivities of the WO :TiO films with 32 different binders in various ambient gas conditions. The sensitivity scale is shown in left and right axes for NO gas and other gases, respectively. x Sensors fabricated with binders show better selectivity to NO gas. x sensing characteristics of the polycrystalline WO sensors 3 mainly through the chemical nature of GB and the distribu- tion of interface states. Fig. 6 shows the variation of sensitivity according to the various binders and ambient gases employed. For the gases other than NO , a high gas concentration had to be main- x tained to see a slight difference in sensitivity. Each sensi- tivity value is measured at the relevant optimum operation temperature for each sample and ambient gas. Noting the difference in the sensitivity scales for NO gas and others, x we can see that the WO films, especially the ones fabri- 3 cated with binders, show a really excellent selectivity to NO gas. One can also notice that the sensors do not show x any systematic trend in sensitivity except that the sensors fabricated with binders have a lower sensitivity than the one fabricated without binder for the ambient gases except NO . If the binders affect the sensitivity through the x physical properties of the polycrystal, such as grain size andror electron concentration of the grain, the sensitivity is expected to show a general trend to the binders, since the above physical properties would systematically affect the electrical resistance. Thus the results in Fig. 6 is another evidence showing that the binders affect the sens- ing characteristics of WO film mostly through the chemi- 3 cal nature of GB rather than the modification of physical properties of polycrystals. 5. Conclusion The origin of binder effects on the sensing character- istics of WO gas sensors is investigated for the PVA, 3 silica sol, and Al O binders. The binders are observed to 23 () J I. Yang et al.r Sensors and Actuators B 60 1999 71–77 77 influence little the physical properties of sintered WO 3 films such as grain size and electron concentration around room temperature. They are, however, observed to affect greatly the sensing characteristics of films for various ambient gas and the electrical resistance in NO gas. It is x discussed, from these facts, that the binders affect the gas sensing characteristics of WO film through the modifica- 3 tion of the chemical nature of the GB rather than the modification of physical properties of the polycrystal. It is also discussed that the optimum operation temperature of WO gas sensor to NO gas is not determined by the 3 x adsorptionrdesorption kinetics of NO gas but by the x simple temperature dependence of R rR ratio. gas air Acknowledgements Parts of this work was supported by the BSRI program Ž. 98-015-D00056 of the Korean Ministry of Education. References wx 1 P. Romppainen, V. Lantto, The effect of microstructure on the height of potential energy barriers in porous tin dioxide gas sensors, Ž. J. Appl. Phys. 63 1988 5159–5165. wx 2 P.J. Shaver, Activated tungsten oxide gas detectors, Appl. Phys. Ž. Lett. 11 1967 255–257. wx 3 X. Wang, G. Sakai, K. Shimanoe, N. Miura, N. 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State Phys. 14 1981 L191–L194. wx 15 C.G. Scott, C.E. Reed, Surface Physics of Phosphors and Semicon- ductors, Academic Press, London, 1975, pp. 230–232. wx 16 D.S. Lee, S.D. Han, Y.M. Son, D.D. Lee, Fabrication and NO x sensing characteristics of WO based thick film devices doped with 3 Ž. TiO and noble metals, J. Korean Sens. Soc. 6 1997 274–279. 2 Jong-In Yang received the B.E. degree from Kangnung National Univer- sity and M.E. degree from Ajou University in electronic engineering in 1997 and 1999, respectively. He is now working as a development engineer in Korea LPE Products. His current research activities are on the development of III–V LEDs and sensors.Han-jo Lim was born in Kyung- buk, Korea in 1947 and obtained M.S. degree in physics from Seoul National University, Korea in 1974. He received the PhD degree in solid state physics from the University of Montpellier II, France in 1982. He was appointed as an assistant professor in the electronic engineering department of Ajou University, Korea in the same year. His research field includes semiconductor physics, semiconductor devices and material characterization, and electron device reliability. He has published about 80 papers on the international journals and 30 papers in the domestic journals.Sang-Do Han received his PhD degree in solid state chemistry, University of Bordeaux, France in 1994. He worked at LG semiconductor for 1978–1980, and is currently working at Korea Institute of Energy Research since 1980. His main research interests are the electronic materials, chemical sensors and their applications. He serves as the editor of Journal of the Korean Sensors Society. . the depletion approximation. Here, q is the elec- tronic charge, N the donor concentration, W the depletion d region width, and e is the permittivity of. binders affect the gas sensing characteristics of WO film through the modifica- 3 tion of the chemical nature of the GB rather than the modification of physical