br Various modeling and simulation routes In Fig the various

Various modeling and simulation routes In Fig. 1 the various routes for modeling/simulation of a gas sensor are identified. Computational software program such as Matlab/Mathematica can be used for simulating chemisorption induced surface bending, surface potential and theoretical verification of current through sensor. For analyzing the field effect based sensors response in steady state, Technology Computer Aided Design (TCAD) software such as SILVACO or Sentaurus can be used. Whereas, for the transient analysis of these field effect based sensors, one can use simulation packages such as COMSOL and Matlab in conjunction. The various method of simulating a gas sensor can be classified into three paths, and each path includes the device physics involved when a sensor is exposed to a gas. Exposure of semiconductor surface to target gas will induce chemisorption effect, which leads to the change in device characteristics such as channel/carrier concentration modulation, Schottky barrier formation or contact transformation. As shown in Fig. 1, path one involves, the sensor’s governing equations which are often solved simultaneously to analyze the device response to the targeted ambient. These coupled equations can be solved iteratively using mathematical packages such as Mathematica or Matlab. The second path identifies an another possible approach, in which, the influence of ambient gas on the semiconductor surface can be analyzed in the steady or transient state. The second path defines the method to simulate the sensor characteristics by combining finite gpr119 agonist analysis (FEA) and/or finite difference method (FDM)/finite element method (FEM) simulations. The surface bending and chemisorption effect is captured using mathematical packages such as Matlab/Mathematica. And the device equations can be handled with a FEA simulators such as COMSOL [9]. By using both the computational packages in conjunction, where the surface bending results from the mathematical package will be called to the FEA simulator for obtaining the sensor response. Therefore, this path can lead to the extraction of both steady state and transient response. In the third path, the gas sensing response can be obtained by defining the sensor structure and material properties in a TCAD simulator and considering the steady state surface bending which may lead to the sensor’s steady state response.
Device structure The device structure investigated in this section is a back gated field effect device reported in [8] (Fig. 2(a)), an n-type Zinc Oxide (ZnO) nanowire as the sensing layer. A heavily doped p-substrate act as gate, silicon dioxide () as gate oxide and Ni/Au metal is used for source and drain contacts. The work function difference between the contact metal and the ZnO leads to the formation of non-ohmic contacts with depletion widths ( and ). The depletion width due to the oxide-semiconductor work-function difference is depicted as in Fig. 2(b).
Result and discussion By following the third route which is depicted in Fig. 1, the structure described in [8] is simulated in a TCAD software [10] for its change in the threshold voltage with exposure to target gas. When the ZnO nanowire surface is exposed to the oxidizing gas ( in this case) the surface depletion width increases and the effective channel thickness reduces and therefore the voltage required to deplete the channel reduces. Figs. 3 and 4 shows the simulated transfer () and output () characteristics respectively. As can be seen from the figure, upon an exposure to 5ppm of , a 2.5V shift in the threshold voltage can be observed in simulated results. The device parameters used for the simulation study are depicted in Table 1.
Conclusion
Conflict of interest
Acknowledgments This work was carried using funds from Global Research Collaboration (GRC) Project, CSR Grant Application No. G11019, by Semiconductor Research Corporation, NC. Authors are thankful to the Head, Centre for Nanotechnology, IIT Guwahati for extending the facilities to carry out the simulation work.