Moisture sensors exist in a number of varieties: capacitive [1-7], resistive ,
microwave based [7, 13, 14, 16], in addition to other more sophisticated vari-
eties [8-10, 12, 14-16, 24]. Among the technologies used at present, the variable-
capacitance-based moisture sensor is particularly noteworthy because of its low cost
and the simplicity of its design. A drawback of this type of sensor, however, is that
the variable capacitor used must be considerably large in order to obtain sufficient
sensitivity to the presence of moisture [1-7]. This limits the range of applications
for this type of sensor. This chapter introduces a new ultraminiature, ultrahigh sensi-
tivity moisture sensor that is based on ultracapacitor technology (a direct nanotech-
nology application). An ultracapacitor is assembled from ordinary activated carbon
electrodes; however, unlike ordinary ultracapacitors, no liquid electrolyte exists in-
side the device. The ultracapacitor electrodes are placed on both sides of a layer of
porous silicon in which KOH (potassium hydroxide) in powder form is embedded.
As moisture penetrates the porous silicon layer, a liquid electrolyte starts to form.
The conductivity of the electrolyte, and hence the capacitance of the ultracapacitor
assembly, increase as the amount of moisture increases. In the present prototype, an
increase in the relative humidity from 5% to 80% results in a capacitance variation
from 0 to 17 µF. The sensor can be very useful for monitoring moisture penetration inside small electronic devices and packages that are sensitive to moisture.