Abstract
A light-addressable potentiometric sensor (LAPS) is a chemical sensor with a field-effect structure based on semiconductor. Its response to the analyte concentration is read out in the form of a photocurrent generated by illuminating the semiconductor with a modulated light beam. As stated in its name, a LAPS is capable of spatially resolved measurement using a scanning light beam. Recently, it has been pointed out that a part of the signal current is lost by the return current due to capacitive coupling between the solution and the semiconductor, which may seriously affect the sensor performance such as the signal-to-noise ratio, the spatial resolution, and the sensitivity. In this study, a circuit model for the return current is proposed to study its dependence on various parameters such as the diameter of contact area, the modulation frequency, the specific conductivity of the solution, and the series resistance of the circuit. It is suggested that minimization of the series resistance of the circuit is of utmost importance in order to avoid the influence of the return current. The results of calculation based on this model are compared with experimental results, and its applicability and limitation are discussed.
Highlights
IntroductionA dc voltage is applied to induce a depletion layer, the thickness of which varies due to the field effect in response to the analyte concentration on the sensing surface
Sensor including its spatial resolution and frequency characteristics, Poghossian et al [14] pointed pointed out the the influence influence of of capacitive capacitive coupling coupling between between the the solution solution and and the the semiconductor semiconductor substrate substrate in in the the out non-illuminated region, which was not included in existing models
In chemical chemical imaging imaging based based on on aa light-addressable potentiometric sensor (LAPS), LAPS, aa focused focused light light beam beam scans scans the the semiconductor semiconductor substrate, substrate, In and the signal current is recorded at each pixel
Summary
A dc voltage is applied to induce a depletion layer, the thickness of which varies due to the field effect in response to the analyte concentration on the sensing surface. A photocurrent generated by illuminating the semiconductor is measured to detect the variation of the capacitance of the depletion layer and to determine the analyte concentration. [6,7,8].InIn this model, separation of electrons byelectric the electric conventionally this model, separation of electrons and and holesholes by the field field inside the depletion layer is represented by an internal ac current source. Which is divided by inside the depletion layer is represented by an internal ac current source I , which is divided by the the capacitance of the depletion layer that theinsulating insulatinglayer layerCCi connected connected to to the the series d and capacitance of the depletion layer
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