GaN-on-Si Calorimetric Thermal Conductivity Gas Sensor

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Introduction Thermal conductivity gas sensors are used in a multitude of fields where identifying concentrations of certain gasses within mixtures is important, such as in exhaust emission monitoring, chemical process control to name some examples. There is also a growing demand to be able to place sensors in more extreme environments closer to the source of the reaction/emissions. Traditionally, silicon-on-insulator (SOI) is the material of choice for MEMS for use in harsh environments, however SOI has a limit of around 300°C before it can no longer reliably sense [1].In this work, we present a Gallium Nitride-on-Silicon (GaN-on-Si) thermal conductivity calorimetric gas sensor. Depending on the specific process, devices fabricated from GaN can survive temperatures up to 1000°C [2] [3]. As a wide band-gap semiconductor, GaN devices do not suffer the adverse effects of excessive carrier generation that silicon-based devices do [4]. Using GaN, it is possible to fabricate heterogenous devices by growing an alloy layer on top of the GaN layer. In the case of this work, the alloy is Aluminium Gallium Nitride (AlGaN). Just below the interface of the two materials, piezoelectric polarisation induced by the lattice mismatch of the two materials causes a thin layer (angstroms) of electrons to form a 2-dimensional electron gas (2DEG) [5]. Currently several factors limit GaN’s mass market appeal, such as yield and cost, although with more development, GaN is a candidate for the harsh environment sensing domain.This work presents for the first time a thermal conductivity calorimetric gas sensor fabricated in a GaN-on-Silicon process, with 2DEG sensing elements on a membrane structure. Thermal Conductivity Sensing Thermal conductivity gas sensors use heat transfer principles to detect specific gasses or gas concentrations. A heating element used in conjunction with one or more temperature sensing elements can be used to measure the transfer of heat from heater to the gas mixture. Measuring this heat transfer is the basis for this method of gas detection. The Sensor The sensor die is square at 1.6 mm x 1.6 mm. The device consists of a heating element placed in the centre of the membrane with thermopile temperature sensor upstream and downstream of the heater [6][7]. The thermopiles are made up of 5 thermocouples where the junction materials are gold and the AlGaN/GaN 2DEG. The combination of upstream and downstream temperature monitoring along with the heating from the hot wire allow the temperature profile of the gas to be detected and thus the concentration. Figure 1 shows an optical image of the sensor die with the specific devices highlighted. Experiment The experiment was conducted using a custom gas rig consisting of 3 mass flow controllers (MFCs) interfaced with a Raspberry pi. The different MFCs enable the gas concentration to be controlled so that measurements can be conducted to measure the sensitivity of the sensor to various gas concentrations. In this work, zero air is used as a carrier gas with Hydrogen being the gas of interest. Hydrogen was chosen principally due to its high thermal conductivity (185 mW/mK) compared to zero air (28 mW/mK). Flow rate was kept constant at 200 SCCM/Min so that the effect of forced convection due to flow velocity on the temperature profile can be discounted.Initial Results will be presented at the conference. Acknowledgments This work was supported by the Innovate UK project “GaN Sense” (file ref. 103446).