AbstractCharacterization and optimization for biosensor implementation with open gate AlGaN/GaN transistors is described. Probe-DNA was immobilized on the gate. As target, complementary DNA at 10-12 - 10-7mol/L was added. To investigate the impedimetric properties of the sensing area, electrochemical impedance spectroscopy was used. For very low frequencies, the bio-functionalization layer was modeled as a membrane with a charge transfer resistor in series with a Warburg element. This component presents impedance to diffusion of electrolyte ions. Its behavior is intermediate between a capacitor and a resistor (membrane impedance). After probe-target matching, the charge transfer resistance and Warburg impedance were increased (lower flow of electrolyte ions through the membrane). Using this working principle, a dynamic detection of targets is proposed

Ambacher, O.

Cimalla, V.

Espinosa, N.

Podolska, A.

Schwarz, S.U.

Characterization and optimization for biosensor implementation with open gate AlGaN/GaN transistors is described. Probe-DNA was immobilized on the gate. As target, complementary DNA at 10-12-10-7mol/L was added. To investigate the impedimetric properties of the sensing area, electrochemical impedance spectroscopy was used. For very low frequencies, the bio-functionalization layer was modeled as a membrane with a charge transfer resistor in series with a Warburg element. This component presents impedance to diffusion of electrolyte ions. Its behavior is intermediate between a capacitor and a resistor (membrane impedance). After probe-target matching, the charge transfer resistance and Warburg impedance were increased (lower flow of electrolyte ions through the membrane). Using this working principle, a dynamic detection of targets is proposed

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Impedance characterization of DNA-functionalization layers on AlGaN/GaN high electron mobility transistors

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Impedance Characterization of DNA-functionalization Layers on AlGaN/GaN High Electron Mobility Transistors 

 Procedia Engineering  120 ( 2015 )  912 – 915 
1877-7058 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
doi: 10.1016/j.proeng.2015.08.790 
ScienceDirect
Available online at www.sciencedirect.com
 
EUROSENSORS 2015 
Impedance characterization of DNA-functionalization layers on 
AlGaN/GaN high electron mobility transistors 
N. Espinosaa,b*, S. U. Schwarza,b, V. Cimallab, A. Podolskac and O. Ambachera,b  
aDepartment of Microsystems Engineering,University of Freiburg, Georges-Köhler-Allee 106, 79110 Freiburg,Germany 
bFraunhofer Institute for Applied Solid State Physics, Tullastr. 72, 79108 Freiburg, Germany                                               
cAustralian Resources Research Centre, Curtin University, 26 Dick Perry Avenue, Kensington 6151,Western Australia   
Abstract 
Characterization and optimization for biosensor implementation with open gate AlGaN/GaN transistors is described. Probe-DNA 
was immobilized on the gate. As target, complementary DNA at 10-12 - 10-7 mol/L was added. To investigate the impedimetric 
properties of the sensing area, electrochemical impedance spectroscopy was used. For very low frequencies, the bio-
functionalization layer was modeled as a membrane with a charge transfer resistor in series with a Warburg element. This 
component presents impedance to diffusion of electrolyte ions. Its behavior is intermediate between a capacitor and a resistor 
(membrane impedance). After probe-target matching, the charge transfer resistance and Warburg impedance were increased 
(lower flow of electrolyte ions through the membrane). Using this working principle, a dynamic detection of targets is proposed. 
© 2015 The Authors. Published by Elsevier Ltd. 
Peer-review under responsibility of the organizing committee of EUROSENSORS 2015. 
Keywords: GaN/AlGaN HEMT; DNA layer characterization; charge transfer resistance; Warburg element; dynamic detection 
1. Introduction 
Nowadays, new biosensor technologies offer highly sensitive, cheap and miniaturized chips. A particular case is 
the application of open gate AlGaN/GaN transistors [1]. Although these devices are highly sensitive, chemically 
stable and biocompatible, their potential for biosening is usually not exploited using conventional methods. For 
example, very low change of the drain-source current is shown after addition of target molecules due to screening 
lengths of few nanometers [2, 3]. For improvement, deeper understanding regarding the insulator/biolayer interface 
                                                          
* Nayeli Espinosa. Tel.: +49-761-51-259; fax: +49-761-515-971-259. 
E-mail address: nayeli.espinosa@iaf-extern.fraunhofer.de 
© 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 
(http://creativecommons.org/licenses/by-nc-nd/4.0/).
Peer-review under responsibility of the organizing committee of EUROSENSORS 2015
913 N. Espinosa et al. /  Procedia Engineering  120 ( 2015 )  912 – 915 
 
is necessary, however, only few papers have modeled the sensing principle of DNA sensors with open gate silicon 
transistors [4-6]. They consider the DNA-layer as a perfect capacitor, which is uncertain for the following reasons: 
1) Biolayers are non-uniform and should be modeled as porous membrane, where ions can penetrate; 2) Probe- and 
target-biomolecules have intrinsic charge (negative for DNA) and do not behave as a perfect dielectrics; and 3) Bio-
functionalization layers are physical barriers with a resistance to ionic transfer and diffusion, especially at very low 
frequencies. The goal of this work is to clarify these mechanisms by electrochemical impedance spectroscopy (EIS), 
in order to take advantage of these properties. EIS is a powerful tool to investigate electrical and electrochemical 
properties of electrolyte/electrode interfaces. Here, the characterized sample acts as working electrode. A low 
amplitude AC voltage with a wide range of frequencies is applied as input, UAC = A0sin(Ȧt) with a typical amplitude 
of 5 mV. The input signal is the voltage between the working and reference electrodes. As output, the working 
electrode current is measured, Iout = I0sin(Ȧt+ĳ). The presented experiments were performed without offset voltage. 
A linear relation between the reference electrode voltage and current from drain and source terminals was assumed. 
 
Nomenclature 
R  gas constant 
T  temperature in Kelvin degree 
F  Faraday constant 
k  ion transfer rate constant 
D  ion transfer coefficient 
c+, c-  average concentration of positive or negative ions in the DNA layer  
D+, D-  diffusion constant for positive or negative ions through the biolayer 
2. Biosensor implementation 
Transistors were fabricated on a heterostructure (120 nm AlN, 1800 nm GaN, 15 nm of Al0.25Ga0.85N, 6 nm GaN) 
grown by metal organic chemical vapor deposition on a SiC substrate and have a threshold voltage of -0.5 V. The 
gate surface was 400 x 200 μm². As bio-functionalization, probe-DNA was immobilized by a photochemical method 
[2]. Average probe density was 1.5x10-13 cm-2. As probe- and target-DNA, 5’-GCTT-ATCG-AGCT-TTCG-3’and 
5’-CGAA-AGCT-CGAT-AAGC-3’ were used. As electrolyte, 1 mmol/L phosphate buffered saline was added.  
3. Results and interpretation 
EIS demonstrates the insulator/electrolyte interface without DNA behaves close to a perfect capacitor (Figure 
1a). Nonetheless, the insulator/DNA/electrolyte interface is modeled by a Randles circuit (Figure 1b), on which two 
types of currents flow (Ic and If). At high frequencies, Ic dominates and charges an imperfect double layer capacitor 
formed by an electrolyte diffuse layer and DNA molecules. For low and very low frequencies, a transport of ions 
through the DNA membrane occurs (If). This current is defined by a charge transfer resistor and Warburg element 
(membrane impedance to diffusion of ions). Since both elements presented a regular behavior at different target 
concentration, this work is focused in them. Re is low and is neglected in both kinds of measurements. The charge 
transfer resistance depends on the ion concentration inside the membrane, for a monovalent electrolyte [7]: 
ൌ

ிమ௞ሺశሻןሺషሻభషഀ
   (1) 
The Warburg element considers accumulation of ions in a porous DNA layer. Its 45° phase represents an 
intermediate behavior between a resistor and a capacitor (Figure 1b). The resistive component is the electrolyte 
resistivity inside the biolayer cavities, whereas DNA molecules and their counterions are capacitive.  
914   N. Espinosa et al. /  Procedia Engineering  120 ( 2015 )  912 – 915 
 
 
Fig. 1. Scheme of the insulator/electrolyte interface and Nyquist plot (a) prior to and (b) after functionalization. After functionalization, few 
pinholes appeared (Rph) without affecting sensor performance. At intermediate frequencies, Rct and Cdl (double layer capacitor) in parallel are 
dominant (semicircle). Cins  is insulator capacitance. 
In electrochemistry, the impedance of a porous material is usually compared with a RC transmission line [7, 8]. 
The product of the resistivity and capacitance per unit length provides D. The Warburg impedance depends directly 
on D+, D- and frequency. For a monovalent electrolyte and a diffusion length shorter than the membrane thickness, 
this impedance is expressed as [7]: 
ൌ

	ʹξʹ
൤ ͳ
௖షඥ஽ష
൅ ͳ
శඥ஽శ
൨ ൤ ͳ
ඥɘ
൨ൌ൤ ͳ
ඥɘ
൨   (2) 
For very high frequencies, ZW tends to be zero. Rct and W increased with target-DNA concentration (Figure 2). As 
probe-target is matched, the flow of electrolyte ions is reduced. 
4. Dynamic detection 
For very low frequencies, the circuit is approximated by Rct + ZW. To take benefit from the membrane impedance, 
a step voltage (Uref) as input was applied The bio-functionalization layer exhibits an exponential transient response, 
which is reflected on the normalized IDS. After addition of target, this exponential behavior is slower (Figure 3a), 
and is related to the reduction of W/Rct. The analytical solution of this approximation is very close to the measured 
curves (Figure 3b). An approximation for ¨IDS is calculated with the inverse Laplace transform for fractional s, with 
application of the Mittag-Leffler function like in [9]. For the normalization, ¨IDS was multiplied by Rct/AUref: 
ο஽ௌሺሻൌܣࣦିଵ ቄ
௎೔೙ሺ௦ሻ
ோ೎೟ା௓ೈሺ௦ሻ
ቅ ൌ ஺
ோ೎೟
ࣦିଵ ቊ
௎ೝ೐೑
௦
௦భȀమ
௦భȀమା ೈೃ೎೟
ቋ ൌ
஺௎ೝ೐೑
ோ೎೟
݁ݔ݌ ൬ቀ ௐ
ோ೎೟
ቁ
ଶ
ݐ൰ ݁ݎ݂ܿ ቀ ௐ
ோ೎೟
ݐଵȀଶቁ  (3) 
915 N. Espinosa et al. /  Procedia Engineering  120 ( 2015 )  912 – 915 
 
 
 
 
 
 
 
 
 
 
 
 
 
Fig. 2. Rct and W and its ratio in dependence on the target DNA concentration. 
 
Fig. 3. (a) Dynamic response before and after target-DNA; (b) Comparison between a measured curve and analytic solution. 
Conclusions and outlook 
     The impedimetric properties of biofilms can be employed to enhance the performance of biosensor. This 
approach is applicable to other kind of transistors or biosensors. Integrating ¨IDS,norm (area over or under the curve) a 
calibration curve versus target concentration can be obtained. In future work, additional circuitry could be included 
to integrate ¨IDS,norm in a multi-transistor array.  
Acknowledgements The authors acknowledge the German Research Foundation grant GaBios (AM 105/16)  
References 
[1]   S. J. Pearton, et al., Recent advances in wide bandgap semiconductor biological and gas sensors, Prog. Mater. Sci. 55 (2010), 1-59.  
[2]   S.U.Schwarz, et al., DNA-sensor based on AlGaN/GaN high electron mobility transistor, Phys. Status Solidi A 208 (2011), 1626-1629. 
[3] A. Poghossian, et al., Possibilities and limitations of label-free detection of DNA hybridization with field-effect-based devices, Sens. 
Actuators. B 111-112 (2005), 470-480. 
[4] S. Ingebrandt, et. al, Label-free detection of single nucleotide polymorphisms utilizing the differential transfer function of field-effect 
transistors. Biosens. Bioelectron. 22 (2007). 2834-2840. 
[5] M. W.Shinwari, et. al, Study of the electrolyte-insulator-semiconductor field-effect transistor (EISFET) with applications in biosensor 
design, Microelectron. Reliab. 47(12) (2007), 2025-2057. 
[6] M. W. Shinwari, et. al, Analytic modelling of biotransistors, IET Circuits Devices & Syst. 2(1) (2008), 158-165. 
[7]  I.D. Rainstrick, Chapter 2: Theory, E. Barsoukov, J.R. Macdonald (Eds.) in Impedance spectroscopy: theory, experiment, and applications, 
2005, New Jersey, John Wiley & Sons., pp. 68-102 
[8] M. Bazant, 10.626 Electrochemical Energy Systems (2011), Massachusetts Institute of Technology: MIT OpenCourseWare, Lecture 35. 
[9] M. Slocinski, Modeling and parameterization, O. Kanoun (Ed.) in Lecture Notes on Impedance Spectroscopy. Vol. 4. 2014, London, CRC 
Press., pp. 3-16. 


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Impedance Characterization of DNA-functionalization Layers on AlGaN/GaN High Electron Mobility Transistors

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