Fig. 1 Schematic diagram of the HBT fabricated with a planar-type extended base as hydrogen sensitive sensor.
Results and Discussion: Fig. 2 shows common-emitter sensing characteristics of the hydrogen sensing transistor at various temperatures. The biased voltage is VBE = 3 V. Obviously, sensing currents flowing through the base-emitter junction (i.e., IDN = IBN andIDH = IBH in Fig. 1) will be dominated by the MSM diode. They will then be amplified as the collector currents (ICN andICH ). We firstly find thatICN for the hydrogen sensing transistor in N2 are as small as 0.0125, 0.095, 0.14, and 0.23 µA at 25℃, 50℃, 80℃, and 110℃, respectively. In facts, measuredICNs are very close toIBNs . This is because IBNsare so small that no gain is available for the HBT. Thus, stand-by power consumption smaller than 2 µw is expected at VCE= 5 V. Considering effects of ambient temperatures on the sensing properties, ICH increases from 13.5 µA at 25℃ to 60.1 µA at 50℃, then to 163 µA at 80℃, and finally to 220 µA at 110℃, atVCE = 3 V in a 0.01% H2/N2. ImprovedICHs are due mainly to the fact that the thermal emission current of the MS diode increases with increasing temperature. On the other hand, experiments also reveal thatICH at VCE = 3 V at 50℃ increases from 60.1 µA to 137 µA and then to 181 µA when the hydrogen concentration increases from 0.01% to 0.1% and then to 1%, respectively. Since more dipoles will be formed at MS interface in higher H2/N2, a larger Schottky barrier-height variation (ΔφBn ) is achieved to bring about a higher IDH and hence a higherICH [4].
Fig. 3a shows dynamic-state sensing currents of the MSM diode biased atVBB’ = 1 V at 25℃ (solid line) and 50℃ (dashed line). A sensing diode current starts to increase from theIDN = 1.57×10-3 µA at the moment the 1% H2/N2 is introduced at 25℃. After the sensing diode current saturates atIDH,sat = 0.35 µA, air and then pure nitrogen gas are used to remove the 1% H2/N2. The sensing diode current will then return to theIDN . Measurements have been repeatedly performed by introducing 0.1% H2/N2, air/N2, 0.01% H2/N2, and air/N2 to the MSM hydrogen sensor at 25℃. MeasuredIDH,sats are 0.23 and 0.14 µA in 0.1% and 0.01% H2/N2, respectively. Similar experiments have also been performed at 50℃. GDs were thus calculated to be 87 (314), 146