Experimental demonstration of strain detection using resonant tunneling delta-sigma modulation sensors

A strain sensor was fabricated based on a resonant tunneling diode (RTD) oscillator. This sensor uses strain dependent frequency change of the RTD oscillator, and the frequency delta-sigma modulation (frequency DSM, or FDSM) technique, which can convert a frequency to a digital signal with wide dynamic range and wide bandwidth. The sensor consists of an oscillator using an LC resonator and an RTD loaded on a beam of the cantilever. The cantilever was fabricated using selective wet etching of the InP substrate. In this sensor, the strain applied to the cantilever changes the current-voltage characteristics of the RTD due to the piezoelectric effect, and hence it changes the oscillation frequency. In this paper, it is demonstrated that the acoustic strain signal can be successfully detected by this sensor with noise shaping behavior, which is a unique nature of the DSM.