Flip-chip suspended gate field effect transistors for ammonia detection

Ammonia sensitive field effect transistors (FET) realised in a hybrid flip-chip technology have been produced and investigated. The ammonia sensitive element, the suspended gate (SG) in the transistor structure, is a thin polyacrylic acid (PAA) layer prepared by spray deposition on a gold plated alumina substrate. The transistor platform was designed for and fabricated by the standard complementary-metal-oxide-semiconductor (CMOS) technology, which offers good industrialisation prospects. The intrinsic transconductance of the suspended gate setup is about 16 μA/V. The access of the analysed gaseous sample to the air gap of the device occurs, spontaneously, by diffusion. The electrical output of the device to ammonia exposure is in the range of 50 mV/decade of concentration variation. The sensor can be operated at low temperatures (25–60 °C) and presents good performance.