Liquid-phase sensors using open-gate AlGaN∕GaN high electron mobility transistor structure

Abstract

Liquid-phase sensing characteristics of open-gate AlGaN / GaN high electron mobility transistor ͑HEMT͒ structures were investigated in aqueous solutions and polar liquids. In de-ionized water, the open-gate HEMT clearly showed good drain I-V characteristics with current saturation and pinch-off behavior, very similar to I-V characteristics of typical Schottky-gate HEMTs. We observed a fine parallel shift in the transfer curves according to change in the pH value in a solution, indicating the corresponding potential change at the AlGaN surface. The sensitivity for the potential change was 57.5 mV/ pH, very close to the theoretical value of 58.9 mV/ pH at 24°C for the Nernstian response to H + ions. In the low drain bias region, the drain current linearly decreased with the pH value. This also indicated a systematic potential change at the AlGaN surface due to pH change. The present open-gate device showed a fast response to the pH change and a stable operation at fixed pH values. A possible mechanism for the pH response of the AlGaN surface is discussed in terms of equilibrium reactions of hydroxyls at the AlGaN surface with H + in a solution. It was also found that the device was quite sensitive to changes in the electrostatic boundary conditions of the open-gate area by exposure to polar liquids. The drain current linearly decreased with increasing normalized liquid dipole moment.

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Fabrication and Characterization of GaN-Based Two Terminal Devices for Liquid Sensing

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