Numerical simulation of solid-state optical and chemical sensors

This activity started in 1989 with the implementation of the optical generation effect in the device simulator HFIELDS. The impinging electromagnetic radiation is described as a superposition of plane monochromatic waves of given polarization and intensity. A black-body radiation may be added to them by defining the black-body temperature and area. The waves impinge on the device through illumination windows of any orientation with respect to the device. Before reaching the semiconductor, the radiation crosses a layer of dielectric or absorbing materials with given thickness and electromagnetic properties. The electromagnetic radiation generates in the semiconductor electron-hole pairs as a function of frequency and intensity. The introduction of the optical generation effect in HFIELDS allows one to simulate optical sensors with complex geometries. This is possible thanks to the flexibility of definition of the device geometries and of the illumination windows in the simulator. In the last year the code has been improved in order to simulate solid-state chemical sensors. More specifically, models which describe the effect of the surface charge originated by the oxide-electrolyte interaction have been added. On these effects are based the Ion Sensitive FETs (ISFETs). In parallel, the optical-generation effect has been generalized to the case of an optical signal modulated by a sinusoid. The new models related to chemical effects together with the small-signal optical generation allow for the simulation of ISFET sensors driven by optical signals (LAPS). Among the microsensors of ion species in solution, LAPS (Light-Addressable Potentiometric Sensor) play a relevant role in the environmental and biochemical applications. In a LAPS, which is basically an EIS capacitor, an optical radiation modulated in intensity, impinges on the semiconductor producing electron-hole pairs, with a time-dependent rate. The optically-generated pairs produce a current which can be measured by an external circuit. Besides the intensity and frequency of the optical radiation, this current depends on the electric potential at the interface between insulator an semiconductor which, in turn, is a function of the H+ ion concentration in the solution. Therefore, measuring the alternate current in the EIS capacitor as a function of the applied voltage, one can obtain a measure of the Ph of the solution.

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