  There is a large interest to decrease the size of mechanical oscillators
since this can lead to miniaturization of timing and frequency referencing
devices, but also because of the potential of small mechanical oscillators as
extremely sensitive sensors. Here we show that a single crystal silicon
resonator structure spontaneously starts to oscillate when driven by a constant
direct current (DC). The mechanical oscillation is sustained by an
electrothermomechanical feedback effect in a nanobeam, which operates as a
mechanical displacement amplifier. The displacement of the resonator mass is
amplified, because it modulates the resistive heating power in the nanobeam via
the piezoresistive effect, which results in a temperature variation that causes
a thermal expansion feedback-force from the nanobeam on the resonator mass.
This self-amplification effect can occur in almost any conducting material, but
is particularly effective when the current density and mechanical stress are
concentrated in beams of nano-scale dimensions.
