AlGaN-based pH-sensors : impedance characterisation, optimisation and application for foetal blood sampling

In this thesis the application of AlGAN-based pH-sensors for fetal blood sampling is investigated. To gain insight into these sensors' electrical and sensory properties, AlGaN/GaN-heterostructures with metal- and electrolyte-contacts are characterised using impedance spectroscopy. An equivalent circuit employing constant phase elements is derived to account for the two-dimensional depletion characteristic of these heterostructures. Their pH-sensitivity is determined, various drift phenomena are characterised and optimisation steps aimed at improving sensor stability and measurement accuracy and precision are investigated. It is shown that an accuracy of ±0.03pH can be achieved for the determination of pH-value in aqueous solutions as well as umbilical cord blood. The results of this work are utilised for developing a micro-blood-pH analyser with a minimal sample volume of only 10 μl, which satisfies the accuracy and precision requirements of the German Medical Association guidelines.