Gallium Nitride (GaN) Power Transistors, Gate Driving Techniques and Next Generation Integrated Power Converters

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Gallium nitride (GaN) power transistors promise to be the game changer for the next generation integrated power converters. Traditional silicon based power MOSFETs are already reaching their material limits for power conversion. Aluminum gallium nitride (AlGaN)/GaN high-electron-mobility transistors (HEMTs) with high breakdown field, high-mobility 2-D electron gas (2DEG), high saturation velocity, and low intrinsic carrier density are emerging as the ideal candidate for the implementation of high frequency (10’s to 100’s MHz) and high efficiency (>90%) converters. The wide bandgap materials also allow GaN power devices to operate with a high junction temperature (> 200°C), relaxing the need for expensive heat removal mechanisms. The availability of GaN on silicon substrates can further reduce the cost of fabrication by making good use of existing silicon-based manufacturing facilities. This proposal is focused on the development of two key enabling technologies to fully exploit the potential of integrated GaN power converters: enhancement mode metal insulator semiconductor field effect transistors (MISFETs) and intelligent gate driving techniques. Currently, majority of the GaN based power transistors are depletion mode HEMTs. The normally “on” characteristics made them less favorable when robustness and fail-safe characteristics are critical. Monolithic integration of silicon-based devices and GaN devices is still not practical at this time (although GaN on Si technology will eventually allow the co-existence of CMOS circuits and GaN power HEMTs). As a result, there is a critical need to develop novel methods/structures to implement true enhancement mode GaN power devices with silicon processing compatibility, low leakage current, long term reliability and simple gate drive requirements. The development of enhancement mode GaN power transistors will involve the investigation of suitable gate insulators and device structures, passivation techniques and silicon compatible ohmic contacts. In particular, the oxidation of AlGaN layer, recessed gate etching techniques, silicon nitride passivation and gold-free ohmic contacts such as recessed Ti/Al/W contacts will be studied. In order to take advantage of the inherent switching speed of the GaN power devices to implement integrated power converters with fast transient response, high efficiency, and compact form factor, dedicated gate driving techniques are essential. Due to the presence of parasitic inductance and capacitance, rapid switching between ground and supply voltage levels will produce ringing oscillation, leading to unwanted power losses (reduced efficiency) and electromagnetic interference (EMI). The finite turn-on and turn-off switching speeds between the high side and low side power transistors in a typical output stage could also lead to a momentary short between the supply voltage to ground (shoot through current) during every switching period. This would result in unwanted power loss. The second theme of this project encompasses the design intelligent gate driver ICs to provide precision control of the gate voltage during switching to suppress both EMI and shoot through current in GaN power converters. The development of gate driving ICs with continuous dead-time correction (in sub-nanosecond increment) and dynamically programmable output resistance will allow high speed switching (>100’s MHz) and low switching loss to be achieved simultaneously. Finally, the proposed work will also explore methods to combine the CMOS gate drive circuits and the enhancement mode GaN power transistors to realize the next generation high speed, high efficiency integrated GaN power converters on the same silicon substrate.