For the purposes of this data sheet, the following terms are defined below. The SOURCE pin is assumed to be at 0 V for these definitions.
First-Quadrant Current = Positive current flowing internally from the DRAIN pin to the SOURCE pin.
Third-Quadrant Current = Positive current flowing internally from the SOURCE pin to the DRAIN pin.
First-Quadrant Voltage = Drain pin voltage – Source pin voltage = Drain pin voltage
Third-Quadrant Voltage = SOURCE pin voltage – DRAIN pin voltage = –DRAIN pin voltage
FET On-State = FET channel is at rated RDS(on). Both first-quadrant current and third-quadrant current can flow at rated RDS(on).
FET Off-State = FET channel is fully off for positive first-quadrant voltage. No first-quadrant current can flow.
While first-quadrant current cannot flow in the FET Off-State, third-quadrant current still flows if the DRAIN voltage is taken sufficiently negative (positive third-quadrant voltage). For devices with an intrinsic p-n junction body diode, current flow begins when the DRAIN voltage drops enough to forward bias the p-n junction.
GaN FETS do not have an intrinsic p-n junction body diode. Instead, current flows because the GaN FET channel turns back on. In this case, the DRAIN pin becomes the electrical source and the SOURCE pin becomes the electrical drain. To enhance the channel in third-quadrant, the DRAIN (electrical source) voltage must be taken sufficiently low to establish a VGS voltage greater than the GaN FET threshold voltage. The GaN FET channel is operating in saturation and only turns on enough to support the third-quadrant current as its saturated current.
LMG342xR030 GaN FET On-State = GaN FET internal gate voltage is held at the SOURCE pin voltage to achieve rated RDS(on). The GaN FET channel is at rated RDS(on) with VGS = 0 V because the LMG342xR030 GaN FET is a depletion mode FET.
LMG342xR030 GaN FET Off-State = GaN FET internal gate voltage is held at the VNEG pin voltage to block all first-quadrant current. The VNEG voltage is lower than the GaN FET negative threshold voltage to cut off the channel.
To enhance the channel in off-state third quadrant, the LMG342xR030 DRAIN (electrical source) voltage must be taken sufficiently close to VNEG to establish a VGS voltage greater than the GaN FET threshold voltage. Again, because the LMG342xR030 GaN FET is a depletion mode FET with a negative threshold voltage, this means the GaN FET turns on with DRAIN (electrical source) voltage between 0 V and VNEG. The typical off-state third-quadrant voltage is 4.5 V for third-quadrant current at 20 A. Thus, the off-state third-quadrant losses for the LMG342xR030 are significantly higher than a comparable power device with an intrinsic p-n junction body diode. The ideal-diode mode function described in Ideal-Diode Mode Operation can help mitigate these losses in specific situations.