D. Extended Range Operation
All of the measurements presented above were performed inside
the humidity chamber with humidity levels ranging from
30% to 70% RH. This range has been chosen primarily due to the time it takes to perform the measurements at lower humidity
levels. For instance, the minimum lower limit for the test
chamber at 37 C is 20% and it takes a significant amount
of time (on the order of days) to reach this low level. Nevertheless,
to verify extended-range operation, one 1200-Å device
was tested for the maximum possible range determined by the
humidity chamber. The measured results are shown in Fig. 9.
As evident, the sensor response is fairly linear and representative
of previous measurements for humidity levels of up to 70%
RH. However, the sensor displays a nonlinear behavior beyond
70% RH. As mentioned before, the polymide sensors are not
stable and do drift at high humidity levels.