These capacitances are a function of the voltage applied to various terminals. Figure
8 shows how the values change as the voltage from drain to source increases. The
reason for the drop in capacitance as VDS goes up is that the free electrons in the GaN
are depleted away. For example, the initial step down in COSS (Output capacitance)
is caused by the depletion of the 2DEG near the surface. Further increase in VDS extends
the depletion region deeper into the body of the device thus increasing the
distance between the plates of the capacitor.