High sensitivity AlGaN/GaN High
Electron Mobility Transistor based
sensors for pH and ion measurements
Yiannis Alifragis, A. Georgakilas, G. Konstantinidis,
E. Iliopoulos, A.Kostopoulos, T.McMullen,
A.Volosirakis, N.A. Chaniotakis
Purpose of this work
The development of novel chemical and biochemical sensors
based on the unique chemical and physical properties of
Gallium Nitride (GaN) crystal
™ Laboratory of Analytical Chemistry,
Department of Chemistry, University of Crete, Greece
™ Microelectronics Research Group (MRG), IESL, FORTH,
Iraklion-Crete, Greece
GaN crystal structure
™ Hexagonal wurtzite structure
™ On the surface of the (0001)
GaN wurtzite crystal, each gallium
atom has three complete bonds to
the underlying nitrogen atomic
plane
™ These gallium atoms are
relatively electropositive, due to
the induced polarity of the Ga to
the N bond
Ga
Ga
Ga
N
Ga
+
+ - +
+
™ Thus the gallium atoms of
GaN are expected to interact
with the anions of the solution
Ga
Gaa
G
Ga
N
N
N
N
Ga
Ga
N
N
*
™ In such a case, the formation of a
double layer at the GaN/solution
interface can be measured, and thus
can be the basis for the development
of a Chemical sensor
Ga
Ga
Ga
Ga
Ga
Ga
Ga
N
Gallium Nitride surface
™ GaN films with atomically flat surfaces were grown on
Al2O3(0001) substrates by nitrogen RF plasma source
Molecular Beam Epitaxy (RF-MBE)
High resolution AFM image of the GaN surface morphology
Cl
Cl
Cl
N
Ga
Ga
N
N
Ga
Ga
N
N
Ga
* O. Ambacher J. Phys. D: Appl. Phys. 31 (1998) 2653
Ga
Ga
N
Cl
Cl
Cl
N
N
N
N
Ga
N
Ga
N
Cl
Cl
Cl
N
N
pH response of GaN electrode
™ The potentiometric response is obtained upon additions
of HCl in 0.1mole/lt Tris solution
™ The sensor showed considerable sensitivity to pH
changes.
Y. Alifragis et.al Electroanalysis 17, 527 (2005)
1
Potential (mV)
Anion Response
-500
F-
-525
Cl-
-550
NO 3 -
-575
Epitaxial HEMT Structures
™ AlGaN/GaN High Electron Mobility Transistor (HEMT)
structures were grown by RF-MBE
I-
-600
-625
20 Å
150 – 400Å
2D Electron
Gas (2DEG)
-650
-675
2-4µm
-700
-725
-6
-5
-
-4
log[X ] mole/lt
-3
-2
™ The sensitivity of the sensor is depended on the surface
potential generated due to the specific interaction of anions
with the surface of GaN.
N. A. Chaniotakis et.al, Analytical Chemistry 76, 5552 (2004)
™ Extremely high 2DEG densities exceeding 1013 cm-2 can be
realized and are used as conductive channel of a HEMT.
™ MBE growth was optimized for high crystal purity,
atomically abrupt interfaces
HEMT based on 2DEG channel
HEMT with a metal gate
5000
V
IDS(µA)
4000
IDS
=0V
GS
3000
2000
V =-1V
GS
1000
V
™ The 2DEG density is modulated by voltage applied to the gate
™ Small voltage changes on the gate are amplified into much
larger changes in an external circuit connected to the source and
drain
™ Devices without gate metallization could sense charges
adsorbed onto the exposed gate area
Masks for sensor development
V
0
0
15
VDS(Volt)
20
25
Fabrication of Sensors
100µm
Hall
10
™ Lg= 36 µm and Wg= 100 µm
™ Low drain-source saturation voltage (VDSsat) of ~2.5 V at VGS=
0V and IDSsat≈ 45 mA/mm and high transconductance (gm) in the
current saturation region (gmsat) of approximately 30 mS/mm
80µm
ChemHEMTs
5
=-2V
GS
=-3V
GS
20µm
100µm
Meander
ChemHEMTs
Schottky
Gated HEMTs
™ Gateless and conventional (metal gate) HEMTs with
different layouts and gate dimensions were designed
2
Electrochemical measurements
Packaging for Electrochemical
measurements
Drain
Source
Ohmic
metal Gate area
exposed
Design and fabrication
of glass carrier with
interconnection lines
mesa
Electrical
contacts
pH response of Gateless HEMTs
Electrolyte Gate HEMT IDS-VGS
3000
1100
1100
1600
1000
900
IDS (µA)
VGS= -400V
VGS= -600V
1200
800
VGS= -800V
VGS= -900V
800
700
VDS=0.6V
2500
VDS=0.5V
2000
VDS=0.4V
600
500
VDS=0.3V
400
0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0
900
1000
200
100
-1000
-0.2 0.0 0.2
-600
-400
-200
VGS (mV)
0
200
™Average response was ∂VGS/ ∂ pH=50 mV/pH
600
VDS=0.4V
0.4
0.6 0.8
VDS=0.3V
400
VGS=0V
0
-800
700
VDS=0.5V
500
VDS=0.2V
VDS (V)
VDS=0.6V
800
500
300
400
1500
1000
Air
pH=3.35
pH=6.84
pH=12.45
IDS (µA)
VGS= 0V
VGS= -200V
IDS (µA)
pH=6.02
IDS (µA)
2000
VDS=0.2V
300
1.0 1.2 1.4
1.6
2
4
6
8
VDS (V)
pH
10
12
14
™ Lg= 80 µm and Wg= 100µm
™ Current variation was ∂IDS/∂ pH =41 µA/pH at VDS= 1.0 V
Response of Gateless HEMTs to Anions
Response to different anions
4000
3600
3500
KCl
KBr
KNO3
3400
2500
KSCN
3200
2000
10-4M KCl
10-22M KCl
1500
1000
IDS (µA)
I
(µA)
DS
3000
-1
10 M KCl
1M KCl
500
3000
2800
0
0.0
0.5
1.0
1.5
™ Lg= 80 µm and Wg= 100µm
2.0
2.5
3.0
3.5
4.0
V (V)
DS
2600
-4
-3
-2
-1
0
log C (mol/L)
™ The average response in the concentration range of KCl
between 10-4M and 1M was -159 µA/decade, at VDS=2.5V
3
ChemHEMT with potassium membrane
10000
10-4M KCl
10-3M KCl
10-2M KCl
8000
VDS=2.5V
7000
with potassium selective membrane
6500
4000
IDS (µΑ)
IDS (µΑ)
6000
7500
2000
3500
without potassium selective membrane
0
3000
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
VDS (Volt)
™ A polymeric potassium-selective membrane (thickness
~50µm) has been deposited on the area of the gate
-4
-3
-2
-1
0
log [KCl] (mol/lt)
™ The average response in the concentration range between
10-4M and 10-1M was +255.2 µA/decade for KCl, at VDS=2.5V
Conclusions
™ GaN crystal grown on sapphire substrate is shown to be
an excellent anion selective sensing element
™ It has been shown that the Ga atoms of the outer
surface of that crystal coordinate selectively and
reversibly with the anions in solution
™ The response of AlGaN/GaN ChemHEMT devices to
anions in aqueous solution, should be the basis for
developing a novel family of Chemical and Biochemical
sensors
Acknowledgments
This work has been supported financially by the European
Community through the project NMP4-CT-2003-505641
“GANANO”
and
GSRT, Hellenic Ministry of Development through the project
PENED 01ED 583 “PRONITRO”.
Project “PRONITRO” is also supported from INTRACOM S.A.
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