Abstract
The sensing responses in aqueous solution of an open-gated pH sensor fabricated on an AlGaN/GaN high-electron-mobility-transistor (HEMT) structure are investigated. Under air-exposed ambient conditions, the open-gated undoped AlGaN/GaN HEMT only shows the presence of a linear current region. This seems to show that very low Fermi level pinning by surface states exists in the undoped AlGaN/GaN sample. In aqueous solution, typical current-voltage (I-V) characteristics with reasonably good gate controllability are observed, showing that the potential of the AlGaN surface at the open-gated area is effectively controlled via aqueous solution by the Ag/AgCl gate electrode. The open-gated undoped AlGaN/GaN HEMT structure is capable of distinguishing pH level in aqueous electrolytes and exhibits linear sensitivity, where high sensitivity of 1.9 mA/pH or 3.88 mA/mm/pH at drain-source voltage, VDS = 5 V is obtained. Due to the large leakage current where it increases with the negative gate voltage, Nernstian like sensitivity cannot be determined as commonly reported in the literature. This large leakage current may be caused by the technical factors rather than any characteristics of the devices. Surprisingly, although there are some imperfections in the device preparation and measurement, the fabricated devices work very well in distinguishing the pH levels. Suppression of current leakage by improving the device preparation is likely needed to improve the device performance. The fabricated device is expected to be suitable for pH sensing applications.
Highlights
Many semiconductor materials have been tested for their suitability as ion sensors; in particular there is an emerging interest in the use of wide band gap semiconductors as sensitive chemical sensors.Gallium nitrides (GaN) are chemically stable semiconductors with high internal spontaneous and piezoelectric polarization, which make them very suitable materials to create very sensitive but robust sensors for the detection of ions, gases and polar liquids, at high temperatures and in harsh environments [1,2]
There are some major works that study the effect of oxide layers on open-gated AlGaN/GaN HEMT devices [9,10]
It can be said that the undoped AlGaN/GaN structure may produce very low Fermi level pinning by surface states
Summary
Many semiconductor materials have been tested for their suitability as ion sensors; in particular there is an emerging interest in the use of wide band gap semiconductors as sensitive chemical sensors. We have reported the sensing responses of both bulk n-GaN and undoped AlGaN/GaN based two-terminal devices in aqueous solution with various pH levels and in polar liquids [8]. The expected advantages of using undoped-AlGaN/GaN as compared with doped structures are lower gate leakage current, lower pinch-off voltage and less noise due to the absence of any donor in the AlGaN. These are the reasons why many groups prefer non-modulation doped gallium nitride HEMT structures for electronic device applications [11,12].
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