Amplifying the Signal of Metal Oxide Gas Sensors for Low Concentration Gas Detection

Coupling p+n Field-Effect Transistor Circuits for Low Concentration Methane Gas Detection

“…The transfer curves ( I DS –V GS ) of typical n-type FETs and p-type FETs are measured by the Keithley 4200 semiconductor analyzer shown in Figure S1 , where both 2SK184 and 2SK364 show larger I DS –V GS slopes (the smallest subthreshold swings), when compared with other n-type FETs and so do both 2SJ44 and 2SJ45 FETs. The larger slope makes the FET more sensitive to the gate voltage change, leading to the larger amplification effect according to our previous work [ 20 ]. Thus, FETs (2SK184, 2SK364, 2SJ44 and 2SJ45) are utilized to form the coupling p+n FET circuit.…”

“…The resultant amplifications are shown in Figure 2 b by testing at least four sensors, where it is obvious that a maximum MF of ~6.5 is obtained using the 2.0 kΩ resistor. In the meantime, it is noteworthy that every MF of the FET circuit with 2.0 kΩ is larger than that of the FET circuit with 1.0 and 3.0 kΩ because high R L would result in high ( R L + R FET,a ), while low R L would lead to low ( R L + R FET,g ) leading to low MF of ( R L + R FET, g )/( R L + R FET, a ) [ 20 ].…”

“…In our previous work, a single n-type FET amplification circuit with 2SK544 has amplified the signal of three kinds of MOX gas sensors (Figaro TGS2602 sensor for toluene, Hanwei MQ3 sensor for ethanol, Hanwei MP502 sensor for acetone) successfully [ 20 ]. In this paper, 2SK364 (~0.05 kΩ in the ON state) is exploited in the n-type FET amplification circuit (seen in Figure S2 ).…”

“…Thus, the response of MP-4 to methane is defined as Response = R a/R g [ 25 , 26 , 27 ] for the traditional circuit without FET, while the response is defined as Response = Ra/Rg × MF for the amplification circuit with FET. The magnification factor ( MF ) of FET is defined as MF = ( R L + R FET , g )/( R L + R FET , a ) [ 20 ], where R FET, a and R FET, g represent FET resistance in the air and methane gas respectively.…”

“…In our previous work, an n-type field effect transistor (FET) circuit [ 20 ] has been reported to amplify the voltage signal of the MOX gas sensor. For example, this n-type FET circuit is capable of magnifying the response of TGS2602 sensors to toluene by 5–6 times, so that TGS2602 sensors are able to respond effectively to ~0.1 ppm toluene, lower than the highest permissive limit (0.26 ppm) of toluene in the indoor air of cars.…”