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.