Analysis of Drain-Dependent Threshold Voltage and False Turn-On of Schottky-Type p-GaN Gate HEMT in Bridge-Leg Circuit

To assess GaN power transistors' capability to maintain a decent enhancement-mode operation under high voltage switching operation, the impact of negative threshold voltage (Vth) shift on false turn-on of the Schottky-type p-GaN gate high-electron mobility transistor (HEMT) is analyzed. The negative Vth shift of the device at high VDS is firstly investigated by pulsed IV measurement. A customized double-pulse tester circuit is then built to evaluate the false turn-on problem. When the high-side transistor is turned on, the fast switching exerts a large overshoot current ID into the drain of the low-side device (SW2). The overshoot current consists of the displacement current that flows into Coss of the transistor and the current flowing through the channel due to false turn-on of the device. As an indicator of the false turn-on phenomenon, the peak value of overshoot current exhibits a 0.9 A difference comparing to the pure displacement current when dV/dt is 26 V/ns. The corresponding peak gate-source voltage of SW2 is 0.9 V, which is far from its 1.53 V Vth. The result indicates that the negative Vth shift in Schottky-type p-GaN gate HEMT aggravates the false triggering problem and special care should be taken for the design of high-voltage switching circuit.