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