The LMG341xR070 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 silicon FET blocks the drain voltage, 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 LMG341xR070 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 gated directly with the negative voltage. During operation, this removes the switching loss of silicon FET.