Transition metal-gate MOS gaseous detectors

MOS capacitors with gates of palladium, platinum, and nickel are shown to be sensitive detectors of H
2
, CH
4
, C
4
H
10
, and CO gases. 1-MHz and 10-Hz
C-V
characteristics change by - 1240 and - 215 mV when the Pd-gate MOS capacitor is exposed to H
2
gas at 760 and 2 × 10
^-8
torr, respectively. Platinum-gate MOS capacitors exhibit approximately one-half the change of Pd-gate devices. Nickel shows a response of - 120 and - 140 mV in 1 atm of H
2
find CO gas, respectively. When the capacitors are operated in a constant-capacitance mode by varying the bias, absorption time characteristics are obtained. Pd-gate devices absorb H
2
gas in 190-200 ms at 760 torr of H
2
but require 200 min to reach 63 percent of the total change at 2 × 10
^-8
torr H
2
pressure. Both Pt- and Ni-gate devices are slower than Pd. The detection mechanism is attributed to the change in work function upon hydrogen absorption. This is established by demonstrating no change either in the density of interface states or in the distribution across the gap or in the number of states at midgap upon absorption. Also, no accumulation of additional charges in the SiO
2
or Si
3
N
4
dielectric is found upon hydrogenation of devices with either Pd or Pt gates.