Low power strain sensor based on MOS tunneling current.

Abstract

Sensors, such as pressure sensors, accelerometers and gyroscopes, are very important components in modern portable electronics. A limited source of power in portable electronics is motivating research on new low power sensors. Piezoresistive and capacitive sensing technologies are the most commonly utilized technologies, which typically consume power in the µW to mW range. Tunneling current sensing is attractive for low power applications because the typical tunneling current is in the nA range. This dissertation demonstrates a low power strain sensor based on the tunneling current in a metal-oxide-semiconductor (MOS) structure with a power consumption of a couple of nano-Watts (nW) with a minimum detectable strain of 0.00036%. Both DC and AC measurements were used to characterize the MOS tunneling current strain sensor. The noise level is found to be smallest in the inversion region, and therefore it is best to bias the device in the inversion region. To study the sensitivity in the inversion region, a model is developed to compute the tunneling current as a function of strain in the semiconductor. The model calculates the tunneling current due to electrons tunneling from the conduction band of the semiconductor to the gate (ECB tunneling current) and the tunneling current due to electrons tunneling from the valence band of the semiconductor to the gate (EVB tunneling current). It is found that the ECB tunneling current is sufficient to explain experimental gate leakage current results reported in the literature for MOSFETs with low substrate doping concentration. However, for the tunneling current strain sensor with a higher substrate doping concentration reported here, a model using both ECB and EVB tunneling current is required. The model fits our experiments. During both DC and AC measurements, the MOS tunneling current is found to drift with time. The drift could arise from the trap states within the oxide. The current drift makes it difficult to obtain an absolute measurement of the strain. Combining the tunneling current strain sensor with a resonant sensor may be a good choice because it measures changes in the mechanical resonant frequency, independent of a drift of the tunneling current amplitude.