The LMG341xR150 utilizes a series FET to ensure the GaN module stays off when VDD is not applied. When this FET is off, the gate of the GaN transistor is held within a volt of the FET's source. As the DRAIN pin voltage increases, the silicon FET blocks the drain voltage, and the VGS of the GaN transistor decreases until it passes its threshold voltage. Then, the GaN transistor turns off and blocks the remaining drain voltage.
When the LMG341xR150 is powered up, the internal buck-boost converter generates a negative voltage (VNEG) that is sufficient to directly turn off the GaN transistor. In this case, the silicon FET is held on and the GaN transistor is switched with gate at VNEG to turn off and at SOURCE pin voltage to turn on. During operation, this removes the switching loss of silicon FET.