Detecting Volatile Organic Compounds in the ppb Range with Platinum-Gate SiC-Field Effect Transistors Christian Bur 1,2 , Mike Andersson, Anita Lloyd Spetz 1 Div. of Applied Sensor Science Dept. of Physics, Chemistry and Biology Linköping University SE-58183 Linköping, Sweden {chrbu; mikan; spetz}@ifm.liu.se Nikolai Helwig, Andreas Schütze 2 Lab for Measurement Technology Dept. of Mechatronics Saarland University D-66123 Saarbrücken, Germany {c.bur; n.helwig; schuetze}@LMT.uni-saarland.de Abstract— In this work, the use of a platinum gate gas-sensitive SiC Field Effect Transistor (SiC-FET) was studied for the detection of low concentrations of hazardous Volatile Organic Compounds (VOC). For this purpose, a new gas mixing system was built providing VOCs down to sub-ppb levels by permeation ovens and gas pre-dilution. Measurements have shown that benzene, naphthalene and formaldehyde can be detected in the ppb range and indicate a detection limit of 1-2 ppb for benzene and naphthalene. A sensitivity of 5.5 mV for 10 ppb naphthalene in a humid atmosphere (at 20% relative humidity) was found and with additional 2 ppm ethanol the sensitivity was still 1.3 mV. Formaldehyde can be detected down to approximately 100 ppb under humid conditions. This is the first time that a metal gated SiC-FET was used to detect hazardous VOCs in the low ppb range making SiC-FETs suitable candidates for indoor air quality applications. I. INTRODUCTION Indoor air quality has become a major issue in recent years since people spend most of their time (approx. 80-85%) indoors. Fresh air exchange is increasingly limited to reduce energy consumption for heating and air conditioning. Ventilation systems do not take the quality of the air into account but rather exchange the air based on a predetermined rate or, at best, based on carbon dioxide (CO 2 ) concentration. Lack of fresh air can lead to sick building syndrome (SBS) with symptoms like acute discomfort, headache, and irritations of eye, nose, throat and difficulty in concentrating. Besides CO 2 which can be measured using IR absorption, the quality of the air is strongly affected by volatile organic compounds (VOCs) which pose a serious health risk even at very low concentrations of a few ppb [1], [2]. The French Agency for Food, Environmental and Occupational Health & Safety (AFSSET) suggested 2006 the first guidelines for limiting the emission of VOCs [3]. A priority list divided into five classes of interest of air pollutions with an undeniable health impact is suggested. Class A “compounds of the utmost priority” contains the three VOCs benzene, naphthalene and formaldehyde. The mentioned VOCs need to be detected at concentrations down to a few ppb. Recent regulations in France set a threshold limit for benzene at 0.6 ppb by 2016 [4]. Measurements at these ultra-low concentrations pose severe requirements for future sensor systems: high sensitivity, good selectivity and lost cost at the same time are the main challenges. Gas sensitive field effect transistors based on silicon and more recently on silicon carbide (SiC-FETs) have been studied and improved for many years [5]. By using catalytically active gate materials like platinum or iridium excellent gas-sensitivity is achieved for SiC-FETs. The sensitivity of the sensor depends mainly on the gate material, and its structure, the underlying insulator as well as the operating temperature. The sensing principle of SiC-FETs is based on the formation of a polarized layer at the metal-insulator interface of the transistor (Fig. 1). For a dense gate metallization of e.g. palladium, hydrogen and hydrogen containing molecules (like hydrocarbons) can be detected. The molecules decompose after adsorption on the surface and H + -ions (i.e. protons) rapidly diffuse through the dense layer. At the metal-insulator interface a polarized layer is then formed influencing the density of mobile carriers in the channel of the transistor. However, in order to be able to detect other gases like carbon monoxide a dense layer is not suitable. In that case a porous gate is needed allowing direct interaction of the gas with the insulator surface. Dipoles for instance can adsorb on bare patches of oxide and thus directly create a polarized layer. Additionally, spill over from the metal catalyst to the insulator is possible. For ammonia detection the three phase boundaries between metal, insulator and gas are especially important for the molecule ionization and hence the gas response [6], [7]. In oxygen containing atmospheres the sensing mechanism towards hydrogen and non-hydrogen containing gases is similar to metal oxide gas sensors and can be explained by spill-over effects of adsorbed oxygen. Negatively charged oxygen ions on the sensor surface will influence the electric field and hence the density of mobile charge carriers in the underlying oxide, i.e. the conducting channel of the transistor [8].