The passivation/modification of AlGaAs/GaAs surfaces by amorphous TiO2 for the bio-sensing use in electrolytes

In order to apply two-dimensional electron-gas field effect transistors (2DEG-FETs) for bio-sensing devices operating in electrolytes, the chemical/electrical properties of TiO2 thin film is investigated as a material for the gate oxide of FETs. TiO2 films of thickness 13–15 nm are prepared with sol–gel technique on the gate surfaces of AlGaAs/GaAs 2DEG-FETs, followed by heat treatment at 450 °C. The TiO2 surface has nanometer-scale roughness with a peak-to-peak height difference of approximately 10 nm. The TiO2/2DEG-FETs exhibit a good performance for operation in electrolytes in the dark, with a transconductance of 0.20 mA/V. Large and continuous threshold shifts to the negative side of Vgs are resulted when the TiO2/2DEG-FETs are operated under light at negative gate biases (Vgs < 0 V). This reveals that photoexcited holes are accumulated at the interface of TiO2/GaAs by the negative gate biases. The sensitivity for pH and H2O2 is examined for the TiO2/2DEG-FETs, where a sensitivity of 71 mV/pH and a finite response for H2O2 are obtained. Cell culture on TiO2 surfaces is successfully demonstrated, showing a high potential of the TiO2/2DEG-FETs for cell activity sensors.