Ultrathin GaN/AlN/GaN solution-gate field effect transistor with enhanced resolution at low source-gate voltage

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Abstract

The pH response of a GaN/AlN/GaN solution-gate field effect transistor (SGFET), with a GaN/AlN barrier of 7.5 nm thick, is analyzed and compared with standard GaN/AlGaN/GaN SGFETs with total barrier thicknesses of 19 and 23 nm. While all types of SGFETs show a similar surface sensitivity to H+ ions, a significant improvement in the transducive sensitivity of the SGFET source-drain current under pH changes is found when decreasing the barrier thickness, due to the increased transconductance of the FET structure. Resolution better than 0.005 pH can be estimated in the case of the ultrathin SGFET. Moreover, the maximum transconductance value shifts to gate-drain voltage close to 0 V, which eventually involves no need of reference electrode in less demanding applications, simplifying the final design of the device and making AlN barrier-based SGFETs highly recommended in the broad field of chemical sensors.

Introduction

There has been significant interest in group III-nitrides based chemical sensors during the last years, specially since the first evidence of pH-sensitivity of GaN surfaces [1] and its application in pH-sensitive AlGaN/GaN field effect transistors [2]. According to the site-binding model [3], the pH-sensitivity is due to amphoteric hydroxyl groups, in this case from the thin GaxOy surface layer formed after wet chemical oxidation or even after a simple exposure to atmosphere. In addition, the oxidic surface of group III-nitride devices allows the covalent immobilization of biomolecules after silanization [4], [5], and makes the surface naturally biocompatible [6], [7], [8]. For sensor applications in electrolyte solutions, high electron mobility transistors (HEMTs) based on AlGaN/GaN heterostructures have frequently been used [2], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. In such structures, the conductivity of a two-dimensional electron gas (2DEG), located typically 20–30 nm beneath the surface, is affected by chemically induced changes of the surface potential. In order to enhance the transducive sensitivity of devices based on AlGaN/GaN electrolyte gate FETs, some works have demonstrated the use of nanostructures (ZnO nanorods) on the gate area [13]. But from the HEMT heterostructure point of view, only the application of N-face polarity heterostructures, with a significantly reduced 2DEG-to-surface distance (z), has recently been proposed [14]. However, this approach presents two drawbacks: (i) it leads to structures with lower chemical stability, as compared to (0 0 0 1)-oriented surfaces [15], and (ii) the growth procedure of N-face heterostructures has been well established just recently, and it is not easy to obtain high quality material [16]. For these reasons, the application of AlN/GaN heterostructures as an alternative can be considered as a more promising approach. Due to the high difference in spontaneous and piezolectric polarizations between the GaN and AlN layers in these structures, 2DEGs with high electron concentration and mobility can be achieved employing a very thin AlN barrier. In spite of the obvious benefit that the reduction of barrier thickness has on the transconductance of transistors, and therefore on the device performance, sensor applications of AlN/GaN-based HEMT structures have not yet been studied. In this work we present an AlN-barrier solution-gate field effect transistor (SGFET) that, compared with standard SGFETs, exhibits much higher transconductivity at lower gate-drain voltages.

Section snippets

Experimental

The performance of three pH-sensitive transistor structures (differing by the distance z) is compared. Sample A is a GaN/AlN/GaN heterostructure with z = 7.5 nm, sample B consists of a GaN/Al0.27Ga0.73N/GaN layer sequence with z = 19 nm and sample C is a GaN/Al0.23Ga0.77N/GaN heterostructure with z = 23 nm. Samples A and B were grown by plasma-assisted molecular beam epitaxy (PAMBE) on semi-insulating GaN:Fe templates, while sample C was grown by metal organic chemical vapor deposition (MOCVD) on c

Results and discussion

Fig. 1b shows the dependence of the transconductance (gm) on Vgd at pH 7 for the three samples. The maximum transconductace (gmmax) equalsdIdsdVgdmax=WLVdsμmaxCiwhere Ci is the capacitance per unit area between the electrolyte and the 2DEG channel Ci = (ɛɛ0)/z, and μmax the maximum electron mobility. Leaving post-growth design parameters (W/L) and Vds aside, μ/z is the main key parameter to obtain a high transconductance value and therefore, high transducive sensitivity. Very recent improvements

Conclusion

In summary, we have demonstrated the sensing application of GaN/AlN/GaN HEMT structures, in the present case with a 2DEG-to-surface distance of 7.5 nm, as a pH-sensitive SGFET. Although the pH-sensitivity of the surface potential is very similar to the one obtained from standard AlGaN/GaN structures, the ultrathin AlN-barrier HEMT exhibits a significantly higher value for ΔIds/ΔpH than structures with thicker barriers (as expected due to the gmmax1/z), operating at Vgs values close to 0 V.

Acknowledgements

A. Bengoechea Encabo thanks B. Sepulveda for fruitful discussions. This work was partially funded by the Comunidad de Madrid within the project FUTURSEN (reference CAM S-0505/AMB-0374), and by a research grant from the Ministerio de Ciencia e Innovación of Spain (Grant No. BES-2005-8428). Financial support by the Deutsche Forschungsgemeinschaft (EI 518/4-1) and by the German Excellence Initiative via the Nanosystems Initiative Munich (NIM) is also acknowledged.

A. Bengoechea Encabo received the B.E. and M.S. degrees in Physics from the Universidad de Valladolid (2002). After that, she worked during 2 years on magneto-optics at Instituto de Microelectronica de Madrid (IMM-CSIC). In 2005 she moved to Instituto de Sistemas Optoelectronicos y Microelectronica (ISOM-UPM), where she is currently working toward the Ph.D. degree on nitrides growth and their sensing applications.

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A. Bengoechea Encabo received the B.E. and M.S. degrees in Physics from the Universidad de Valladolid (2002). After that, she worked during 2 years on magneto-optics at Instituto de Microelectronica de Madrid (IMM-CSIC). In 2005 she moved to Instituto de Sistemas Optoelectronicos y Microelectronica (ISOM-UPM), where she is currently working toward the Ph.D. degree on nitrides growth and their sensing applications.

J. Howgate is Doctoral candidate at Walter Schottky Institute of the Technical University of Munich.

M. Stutzmann graduated in Physics from the University of Marburg in 1982 and obtained his Ph.D. degree in 1983. Subsequently he worked as post-doctorate at Xerox Palo Alto Research Center, USA. From 1985 to 1993, he was a permanent member of the research staff at the Max-Planck Institute for Solid State Research, Stuttgart, Germany. Since 1993, he is Full Professor at the Technical University of Munich (Walter Schottky Institute) and Chair for experimental semiconductor physics. His main fields of interest are wide bandgap semiconductors, photo-voltaics, laser modification of thin semiconducting films, defect spectrocopy, sensors and bioelectronics.

M. Eickhoff has worked on SiC material and device development with DaimlerChrysler Research and Technology, Munich from 1995 to 2000. He received his Ph.D. in Experimental Physics from the Technische Universität München in 2000. After that he worked for Infineon Technologies AG, Munich on the development if surface micromachined integrated sensors. From 2001 to 2008 he was work group leader of the “Sensors and Materials” group at the Walter Schottky Institut, TU München. Since 2008 he is Professor at the Justus-Liebig-Universitaet Giessen. In his work he focuses on growth and characterization of wide bandgap semiconductors and their nanostructures, surface functionalization as well as the application in chemical and biochemical sensors.

M.A. Sánchez-García earned degrees in Electrical Engineering from Brown University (B.S. 1991, M.S. 1993) and from the Polytechnical University of Madrid (Ph.D. 2000). He has worked on MBE growth and characterization of III-V semiconductors at UPM since 1993. Since 2003 he is Associate Professor at the Department of Electrical Engineering at the School of Telecommunication (UPM). He carries out his current research at the Institute for Systems based on Optoelectronics and Microtechnology at UPM, focusing on the growth and characterization of III-nitrides layers and nanostructures for sensor applications and optoelectronic devices.

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