Fig. 3 Dynamic-state sensing properties at 25℃ (solid line) and 50℃ (dashed line) in 0.01%, 0.1%, and 1% H2/N2.
a IDH, MSM diode biased atVBB’ = 1 V.
b IBH, hydrogen sensing transistor biased atVBE = 2 V and VCE = 5 V.
c ICH , hydrogen sensing transistor biased at VBE = 2 V andVCE = 5 V.
Another key merit used to evaluate a gas sensor is response time (ta ) defined as the time required for the sensing current to reach (1−e−1) of total change [9]. Table 1 summarize current gains and response times obtained from measured sensing diode, base, and collector currents. A higher hydrogen concentration or a higher temperature produces a shorter response time.ta,D and ta,B obtained from IDH and IBH at 25℃ in 0.01% H2/N2 are 485 and 490 s whileta,C obtained from ICH is 745 s. At 50℃, ta,C is still larger thanta,D and ta,B . When 0.1% and 1% H2/N2 were introduced, relationship among ta,D ,ta,B , and ta,C at both 25℃and 50℃ was not changed. That is, ta,B is nearly equal to ta,D , which is shorter thanta,C . The extended base determines bothIDH and IBH . As a result, the response times from IDH andIBH reasonably equal each other. A new finding is that the response time from ICH is different from that from IBH . Accordingly, the sensing base current at IBN has to reach (1−e−1) × IBH,sat to finish its response while it is from ICN to (1−e−1ICH,sat for the sensing collector current. Owing to the current gain (β) at (1−e−1) × IBH,sat is smaller than that at ICH,sat . We thus concluded that (i) it will take more time for ICH to satisfy the quantitative definition on response time and (ii) the HBT integrated to the MSM EB sensor does not produce time delay in sensing.
Conclusion: We have proposed a new hydrogen sensing transistor with low-power consumption in stand-by mode and high-sensitive gains in common-emitter mode. The hydrogen sensing transistor is fabricated by using an HBT with a MSM Schottky diode as an extended-base hydrogen sensor. Measured ICNs 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℃ while ICHs atVCE = 3 V in 0.01% H2/N2 are increased 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℃. In addition, experiments reveal that the power consumption in stand-by mode is smaller than 2 µW and the sensing collector current gains at 25℃ in 0.01%, 0.1%, and 1% H2/N2 are as high as 512, 977, and 2.89×104, respectively.
Table 1: Sensing diode, base, and collector current gains and corresponding response times for our hydrogen sensing transistor.