We have demonstrated a selective area AgCl anodization process, which can be integrated with the fabrication of AlGaN/GaN high electron mobility transistors (HEMTs) for chloride ion detection. A limit of detection of chloride ion concentration achieved was 1x10(-8) M using a 20x50 mu m anodized Ag/AgCl layer on the HEMT gate sensing area. Unlike the conventional open-circuit potential measurement used for the electrochemical measurement, the drain current of the HEMT was monitored as the output signal in our sensor. The effects of anodization bias voltage and time on the AgCl film properties were investigated. A continuous anodized AgCl film was achieved with the bias voltage of 0.5 or 1 V. However, AgCl films anodized at higher biases (5 V) were not continuous and a larger grain size was obtained. Energy-dispersive X-ray analysis was used to analyze the composition of the anodized AgCl and showed that slightly chloride deficient films (around Ag:Cl=57:43) were obtained. (C) 2008 The Electrochemical Society.