  Graphene membranes act as highly sensitive transducers in
nanoelectromechanical devices due to their ultimate thinness. Previously, the
piezoresistive effect has been experimentally verified in graphene using
uniaxial strain in graphene. Here we report experimental and theoretical data
on the uni- and biaxial piezoresistive properties of suspended graphene
membranes applied to piezoresistive pressure sensors. A detailed model that
utilizes a linearized Boltzman transport equation describes accurately the
charge carrier density and mobility in strained graphene, and hence the gauge
factor. The gauge factor is found to be practically independent of the doping
concentration and crystallographic orientation of the graphene films. These
investigations provide deeper insight into the piezoresistive behavior of
graphene membranes.
