Field effect transistors based on AlGaN/GaN heterostructures are of great interest for chemical and biochemical sensor applications. In addition to their high temperature stability, which has been exploited in the realization of gas sensitive devices for the detection of hydrogen and hydrocarbons up to temperatures of 600C [1,2], AlGaN alloys are chemically inert in aqueous solutions and non-toxic to living cells, which are basic requirements for the application as a substrate material for biosensors [3].
Recently, the deposition of highly mobile lipid membranes on hydrophilized III-nitride surfaces by vesicle fusion was demonstrated [3]. This allows the design of biosensors based on ultrathin lipid membranes deposited directly on the nonmetallized gate regions of AlGaN/GaN transistors. These devices allow the electronic detection of ligand binding to specific receptors in the lipid membrane [4] as well as electrochemical detection of transmembrane transport. A different approach for the realization of biosensors is the cultivation of living cells directly on the gate area and the measurement of their ionic response to chemical or physical stimuli [5].
We have investigated the response of AlGaN/GaN transistors and transistor arrays to changes in the concentration of specific ions in the ambient electrolyte. The potential drop at the interface between the gate surface and the electrolyte solution was measured and used as a sensor signal. A linear sensor response of approximately 56 mV/pH was found for variations in the H+-concentration between 10^-1 and 10^-13 mol/l, which is close to the theoretical Nernst limit, determined by the native oxide layer present on the surface [3,6]. The sensitivity towards Na⁺, K⁺, Ca^´{2+} and Cl^- ions, which give the main contribution to cell action potential and long term stability of the transistor devices in aqueous solutions were also investigated. As the limiting factors for the detection of relatively small cell sig
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