Left: Optical microscope image of the MOSCAPs and diamond deep depletion MOSFETs (D2MOSFETs) of this work. Top right: Scanning electron microscope image of a diamond D2MOSFET under electrical investigation. S: Source, G: Gate, D: Drain. Bottom right: D2MOSFET concept. The on-state of the transistor is ensured thanks to the accumulation or flat band regime. The high mobility channel is the boron-doped diamond epilayer. The off-state is achieved thanks to the deep depletion regime, which is stable only for wide bandgap semiconductors. For a gate voltage larger than a given threshold, the channel is closed because of the deeply and fully depleted layer under the gate.