Abstract: Embodiments of a Low Dropout (LDO) regulator are provided in which an n-channel Metal Semiconductor Field Effect Transistor (MESFET) is utilized as a pass transistor of the LDO regulator. In one embodiment, the LDO regulator is implemented on an integrated circuit die and includes an n-channel Semiconductor-on-lnsulator (SOI) MESFET pass transistor. A voltage applied to a substrate of the SOI MESFET pass transistor is controlled to configure the LDO regulator in either an ultra-low dropout voltage mode or a high Power Supply Rejection (PSR) mode. In another embodiment, the LDO regulator includes an re-channel MESFET pass transistor and a switch that operates to disconnect the MESFET pass transistor from a supply voltage of the LDO regulator when the LDO regulator is desired to be shut off.