Techniques are disclosed for forming tunable capacitors including multiple two-dimensional electron gas (2DEG) and three-dimensional electron gas (3DEG) structures for use in tunable radio frequency (RF) filters. In some cases, the tunable capacitors include a stack of group III material-nitride (III-N) compound layers that utilize polarization doping to form the 2DEG and 3DEG structures. In some instances, the structures may be capable of achieving at least three capacitance values, enabling the devices to be tunable. In some cases, the tunable capacitor devices employing the multi-2DEG and 3DEG structures may be a metal-oxide-semiconductor capacitor (MOSCAP) or a Schottky diode, for example. In some cases, the use of tunable RF filters employing the multi-2DEG and 3DEG III-N tunable capacitor devices described herein can significantly reduce the number of filters in an RF front end, resulting in a smaller physical footprint and reduced bill of materials cost.