Electrochemistry and biochemistry have always been ideal applications for diamond due to its chemical inertness and stability, sensitivity and biocompatibility. Several diamond ChemFET concepts have been proposed to date, however further improvements are still needed to obtain functional devices that can be operated efficiently beyond the reach of the well established silicon ISFET technology [P. Bergveld, Sensor and Actuators B. Chem. 88 (2003). pp. 1]. In this paper we describe a novel ISFET structure in which a boron doped diamond electrochemical gate electrode is combined and monolithically integrated with an InAlN/GaN HEMT structure. The new device merges the high chemical stability of diamond with the high transconductance and low pinch-off voltage of InAlN/GaN heterostructure FETs, resulting in a highly stable ISFET with high sensitivity. First devices have been fabricated and electrochemically characterized, expressing high current levels, a pH sensitivity of about 50 mV/pH, complete current modulation when operated within the electrochemical window of the electrode in the range of pH 1 to pH 13 and high stability upon pH cycling and the application of high anodic overpotentials. (C) 2009 Elsevier B.V. All rights reserved.