GaN Electrochemical Probes and MEMS on Si
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GaN Electrochemical Probes and MEMS on Si Ulrich Heinle1, Peter Benkart1, Ingo Daumiller2, Mike Kunze2, and Ertugrul Sönmez2 1 MicroGaN GmbH, Albert-Einstein-Allee 45, Ulm, 89081, Germany 2 MicroGaN GmbH, Lise-Meitner-Strasse 13, Ulm, 89081, Germany ABSTRACT AlGaN /GaN HEMT structures were used for liquid and gas sensors. The sensitivity of pH sensors was determined after clean-in-place tests. A multigas measurement at 330°C is presented. A piezo-driven mechanical actor is demonstrated and used as a variable capacitor. GaN on silicon logic circuitry is demonstrated at 250°C. INTRODUCTION Gallium nitride based materials are suitable for use at high temperatures and in harsh environments due to their physical and chemical properties. These attributes, combined with piezoelectric and mechanical characteristics, are the base of a considerable interest in sophisticated GaN sensors and actors. Furthermore, the market need for electrochemical sensors, mechanical sensors/actors, as well as electronic circuits, has led to a rapid progress of GaN based devices. Basic construction elements for electrochemical and mechanical sensors/actors, as well as the corresponding GaN-based circuitry, will be presented. The exemplary devices presented are robust and highly sensitive fluid and gas detectors and for the first time a mechanical GaNbased tunable capacitor. AlGaN/GaN heterostructures on silicon are the bases for all presented devices. EXPERIMENTAL DETAILS pH sensors The AlGaN/GaN material system is suitable for the fabrication of chemical sensors due to its chemical inertness. The principle operation is based on the modulation of the HEMT's 2DEG by surface charges caused by adsorption of ions on the device surface. GaN-based ionsensitive field effect transistors (ISFETs) have been demonstrated using GaxOz as ionsensitive layers [1]. These devices are very sensitive to small variations of pH values, but the native or thermal gallium oxides need to be replaced by other dielectrics because their chemical stability is not sufficient for cleaning procedures in industrial applications, like food processing. It is not practical to remove the sensors from the processing equipment before cleaning and mounting them afterwards, because this will increase cost of ownership and that is not acceptable. Therefore, the development of chemically very stable dielectrics is indispensable. The cleaning processes are simulated with clean-in-place (CIP) tests. During these tests the sensors are exposed to aggressive acids and bases in a cyclic manner. In our case the sensors are exposed to nitric acid (pH 1) and caustic soda (pH 13) for 30 cycles. The acid cycle lasts 5 minutes at room temperature and the base cycle takes 15 minutes at 60°C. Figure 1 shows the basic layout of an AlGaN/GaN ISFET with an deposited metal oxide
layer. In principle we have a HEMT structure where the area between the ohmic contacts is covered with an ionsensitive dielectric. High demands are also made to the packaging material. It has not only to withstand
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