Sains Malaysiana 42(2)(2013): 197–203

Surface Reaction of Undoped AlGaN/GaN HEMT Based Two Terminal Device

in H+ and OH- Ion-contained Aqueous Solution

(Tindak Balas Permukaan Peranti Dua Terminal HEMT AlGaN/GaN di dalam

Larutan Akuas Mengandungi Ion H+ dan OH-)

Mastura Shafinaz Zainal Abidin

Faculty of Electrical Engineering, Universiti Teknologi Malaysia, 81310 Skudai,

Johor, Malaysia

Shahjahan & Abdul Manaf Hashim*

Malaysia-Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia International Campus, Jalan Semarak 54100 Kuala Lumpur, Malaysia

Received: 7 January 2012/Accepted: 21 May 2012

ABSTRACT

Gallium nitride is considered as the most promising material for liquid-phase sensor applications due to its chemical stability and high internal piezoelectric polarization. In this work, the sensing responses of undoped-AlGaN/GaN two terminal devices upon exposure to various pH levels in aqueous solution (a mixture of HCl and NaOH) as well as their possible sensing mechanism have been investigated. No reference voltage or gate voltage is applied. The changes in drain-source current, IDS as a function of pH level were evaluated. In the acidic region, there was an almost linear change in IDS where IDS decreased with the increase in pH level. Hence, the translated channel resistance increases with the pH level. High H+ ion concentration at low pH level which corresponds to the large net positive potential on the surface leads to the enhancement of the flow of electrons in 2DEG channel. As the pH level was increased towards neutral point in the acidic region which corresponds to the increase of OH- ion concentration, the net surface potential on the surface starts to be dominated by negative potential. As a result, the 2DEG channel starts to deplete which resulted in the increase of channel resistance. The estimated current and resistance change for sensing area of 1 mm2 and drain-source voltage, VDS of 1- 6 V are in the range of 2.16-80.1 mA/pH and 154.6-500.5 kΩ/pH, respectively. However, the linear decreases of IDS were not continuously observed in the basic region where OH- ions were dominant. The IDS levels were high, showing that the flows of carriers in 2DEG channel were enhanced again. The resistance was low and almost constant in the basic region. It seems to be resulted by the formation of thin Ga(x)O(y) layer on the AlGaN surface contributed by the interaction of OH- with the Ga-face surface. Hence, the net potential on the AlGaN surface seems to be dominated again by the net positive surface potential.

Keywords: Gallium nitride; liquid sensor; two terminal devices

ABSTRAK

Galium nitrida (GaN) adalah bahan yang menjanjikan bagi aplikasi penderia cecair disebabkan ciri kestabilan kimianya dan juga kualiti dalaman polarisasi piezoelektrik. Di dalam kajian ini, tindak balas pengesanan peranti dua terminal struktur HEMT AlGaN/GaN terhadap pelbagai nilai aras pH dalam larutan akuas (campuran HCl dan NaOH) serta mekanisme penderiaannya dikaji. Tiada voltan rujukan atau voltan get digunakan. Perubahan arus salir-punca, IDS sebagai fungsi aras pH dinilai. Di dalam rantau asid, perubahan IDS hampir linear dan IDS berkurang dengan kenaikan nilai aras pH.  Oleh yang demikian, rintangan saluran yang ditaksirkan meningkat dengan kenaikan nilai aras pH. Kepekatan ion H+ yang tinggi pada aras pH rendah merujuk kepada keupayaan positif yang tinggi pada permukaan, mendorong kepada peningkatan aliran elektron dalam salur 2DEG. Semakin tinggi aras pH ke arah titik neutral di dalam rantau asid yang merujuk kepada peningkatan kepekatan ion OH-, keupayaan bersih permukaan mula didominasi dengan keupayaan negatif. Kesannya, salur 2DEG mula merosot dan mengakibatkan peningkatan rintangan salur. Anggaran perubahan arus dan rintangan bagi kawasan penderiaan bersaiz 1 mm2 dan voltan salir-punca, VDS bernilai 1- 6 V adalah dalam julat 2.16-80.1 mA/pH dan 154.6-500.5 kΩ/pH. Walau bagaimanapun, penurunan linear IDS diperhatikan tidak berterusan di rantau bes dengan ion OH- adalah dominan. Aras IDS yang tinggi menunjukkan aliran pembawa di dalam salur 2DEG meningkat kembali. Rintangan adalah rendah dan hampir tidak berubah di dalam rantau bes. Hal ini mungkin berpunca daripada pembentukan lapisan tipis Ga(x)O(y) di atas permukaan AlGaN yang berpunca daripada interaksi OH- dengan permukaan muka-Ga. Seterusnya, keupayaan bersih di atas permukaan AlGaN kelihatan didominasi semula oleh keupayaan permukaan positif.

Kata kunci: Galium nitrida; penderia cecair; peranti dua terminal

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*Corresponding author; email: manaf@fke.utm.my