  We investigate the effect of hypersonic (> 1 GHz) acoustic phonon wavepackets
on electron transport in a semiconductor superlattice. Our quantum mechanical
simulations demonstrate that a GHz train of picosecond deformation strain
pulses propagating through a superlattice can generate current oscillations
whose frequency is several times higher than that of the strain pulse train.
The shape and polarity of the calculated current pulses agree well with
experimentally measured electric signals. The calculations also explain and
accurately reproduce the measured variation of the induced current pulse
magnitude with the strain pulse amplitude and applied bias voltage. Our results
open a route to developing acoustically-driven semiconductor superlattices as
sources of millimetre and sub-millimetre electromagnetic waves.
