Download 18-2 Exquisitely Sensitive Gas Sensor

Exquisitely Sensitive Gas Sensor: Detecting the presence of ammonia (NH3) and nitrogen dioxide (NO2) in the environment is
important because even in low concentrations they are harmful. Both can be detected with dedicated instruments, but in limited areas
and at high cost. Fujitsu researchers will describe the use of graphene to build a compact, simple, highly sensitive and selective NO2
and NH3 gas sensor. They replaced the metal gate electrode of a standard MOSFET transistor with graphene, the idea being that when
NH3 and NO2 molecules are adsorbed on the graphene its electrical properties change. As a result, when exposed to NO2 the drain
current of the MOSFET decreases, and when exposed to NH3 it increases.
Above is a schematic of the graphene-gate transistor.
At room temperature the sensor can detect a NO2 concentration of 7 ppb (more than an order of magnitude greater than conventional
sensors) and an NH3 concentration 420 ppb. The graphene-gate sensor exhibits almost no response to SO2, H2S, and acetaldehyde,
demonstrating high selectivity. The researchers say doping the graphene gate can modulate sensor sensitivity, and that the device’s
threshold voltage can be changed by up to ~620 mV without degrading transistor performance.
(Paper #18.2, “Graphene-Gate Transistors for Gas Sensing and Threshold Control,” N. Harad et al, Fujitsu)