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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
Diserahkan: 7
Januari 2012/Diterima: 21 Mei 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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*Pengarang untuk surat-menyurat; email: manaf@fke.utm.my
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