An apparatus comprising: a first layer (512) configured to enable a flow of charge carriers from a source electrode (505) to a drain electrode (506); a second layer (513) configured to generate a voltage in response to a physical stimulus, the second layer (513) positioned so that the generated voltage can affect the conductance of the first layer (512); and a third layer (514) positioned between the first (512) and second (513) layers to prevent a flow of charge carriers therebetween. The third layer (514) comprises a material configured to form electric double-layers (516, 517) at the interfaces with the first (512) and second (513) layers in response to the generated voltage. The formation of the electric double-layers (516, 517) enhances the effect of the generated voltage on the conductance of the first layer (512) such that determination of the conductance of the first layer (512) can be used to allow the magnitude of the physical stimulus to be derived.