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Characterization of 650 V enhancement mode GaN HEMT

Fig. 1. GaN Systems GaN transistor (left: Transistor package; right: Device Schematic).
Gallium Nitride (GaN) power transistors are divided into three categories: depletion mode, enhancement mode, and cascode configuration. The depletion mode (normally on) power mosfets are not preferred for a switching operation, hence the cascode configuration is packaged by GaN on Silicon technology. A low blocking voltage Si transistor is cascaded with the high blocking voltage depletion mode GaN transistor to make the configuration a normally off one. The Transphorm 600V, 17 A GaN transistor is based on the cascode configuration. Another high blocking voltage GaN High Electron Mobility Transistor (HEMT) is manufactured by EPC with a rated blocking voltage of 200 V. Recently, the GaN systems enhancement mode GaN transistor became commercially available. Rated at 650 V, 30 A it looks very promising since it has a considerably lower on-resistance, especially when compared to a Transphorm GaN transistor with similar ratings, which is a normally-off device.

The static characterization results show that the device has an on-resistance of 55 mω at 9 A drain current and 7 V gate to-source voltage. The transistor package also has a separate source-sense pad to keep the gate to source loop independent of the drain-source loop, as shown in Fig. 1. The blocking capability of the device is also tested, and the blocking voltage is found to be 650 V with a leakage current of 2.4 µA at 25 C. The reverse characteristics (as shown in Fig. 2) are similar to that of a MOSFET with a body diode, but the GaN HEMT has no intrinsic body diode; the junction conducts during the off-state. The on-state reverse characteristics are similar to the on-state forward characteristics.

From these measurements, the on-resistance at 9 A drain current and 7 V gate to source voltage during reverse conduction is found to be 54.5 mω. The on-resistance during forward and reverse conduction at 100 C is found to be 136 mω with similar operating conductions. The transconductance is found to be 15 S at 25 C and 7.95 S at 100 C. In conclusion, by reviewing the output characteristics, the recommended on state gate to source voltage should be between 6-10 V.

Fig. 2. I-V Transfer curves (top: Forward characteristics bottom: Reverse characteristics)