We have calculated the effects of finite thickness on electron or hole layers in double-quantum-well systems. In particular, we apply our model to calculate the Eisenstein ratio and the interlayer capacitance of a biased bilayer device; these are direct measures of the compressibility of the charge carriers in the layers. We show that our model agrees well with the experimental layer-occupancy data for a device of this type. We present results for the regime of negligible interlayer tunneling, zero applied magnetic field, and low layer densities, when the compressibility of one or both layers is negative
