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

氮化镓（GaN）功率晶体管有望成为下一代集成电源转换器的游戏规则改变者。传统的硅基功率MOSFET已经达到了功率转换的材料极限。氮化铝镓（AlGaN）/GaN高电子迁移率晶体管（HEMT）具有高击穿场、高迁移率二维电子气（2DEG）、高饱和速度和低本征载流子密度，正成为实现高频（10 ~ 100 MHz）和高效率（>90%）转换器的理想候选者。宽带隙材料还允许GaN功率器件在高结温（> 200°C）下工作，从而减轻了对昂贵的散热机制的需求。硅衬底上GaN的可用性可以通过充分利用现有的硅基制造设施来进一步降低制造成本。该提案的重点是开发两项关键技术，以充分发挥集成GaN功率转换器的潜力：增强型金属绝缘体半导体场效应晶体管（MISFET）和智能栅极驱动技术。目前，大多数GaN基功率晶体管是耗尽型HEMT。当鲁棒性和故障安全特性至关重要时，通常的“开”特性使它们不太有利。硅基器件和GaN器件的单片集成目前仍不实用（尽管Si上GaN技术最终将允许CMOS电路和GaN功率HEMT共存）。因此，迫切需要开发新的方法/结构来实现具有硅加工兼容性、低漏电流、长期可靠性和简单栅极驱动要求的真正增强型GaN功率器件。增强型GaN功率晶体管的发展将涉及合适的栅极绝缘体和器件结构，钝化技术和硅兼容欧姆接触的研究。特别是，AlGaN层的氧化，凹陷栅极蚀刻技术，氮化硅钝化和无金欧姆接触，如凹陷Ti/Al/W接触将被研究。为了利用GaN功率器件的固有开关速度来实现具有快速瞬态响应、高效率和紧凑外形的集成功率转换器，专用栅极驱动技术是必不可少的。由于寄生电感和电容的存在，接地和电源电压电平之间的快速切换将产生振铃振荡，导致不必要的功率损耗（效率降低）和电磁干扰（EMI）。在典型输出级中，高侧和低侧功率晶体管之间的有限导通和关断切换速度也可能导致在每个切换周期期间电源电压到地之间的瞬时短路（直通电流）。这将导致不必要的功率损耗。该项目的第二个主题包括设计智能栅极驱动器IC，以在开关期间提供对栅极电压的精确控制，从而抑制GaN功率转换器中的EMI和击穿电流。具有连续死区时间校正（以亚纳秒为增量）和动态可编程输出电阻的栅极驱动IC的开发将允许同时实现高速开关（>100 MHz）和低开关损耗。**最后，本研究亦将探讨将CMOS闸极驱动电路与增强型氮化镓功率电晶体结合的联合收割机方法，以在同一硅基板上实现下一代高速、高效率的集成氮化镓功率转换器。

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.