Digital sensors are very linear for the vast majority of the response curve, with deviations at the very bottom (due to noise, depending on where the black point is set) and at the very top near saturation. Digital sensors are particularly linear when compared to film, which has a pronounced 'S' shape response curve.

The reason for this is that incoming photons free up electrons in the photodiode generating a charge which is ultimately read out to form the image. The number of freed electrons depends on the number of incident photons hence the linear response to light intensity. However when a photosite is nearing saturation various electrical effects reduce the charge generated by incident photons leading to a roll-off of the response curve (i.e. it becomes shallower). After saturation no further increase in output can be achieved so the curve becomes highly nonlinear.

However it is important to realise that RAW converters, both in software and in camera will probably apply some form of tonecurve to the image to give a more contrasty film-like output.