15.6 A 10MHz i-Collapse Failure Self-Prognostic GaN Power Converter with TJ -Independent In-Situ Condition Monitoring and Proactive Temperature Frequency Scaling

With superior figure of merits, GaN switchs are highly anticipated to replace MOSFETs in high-performance power circuits [1], [2]. However, GaN technology today still faces formidable reliability challenges [3]. While GaN device aging and failure mechanisms are not as well-studied as silicon counterparts, its unique structure and operation also induce new aging and failure problems. Use a GaN switch M_H in a buck converter of Fig. 15.6.1 as an example. As a high-side switch, it faces large-switching-current and high-input-voltage stress in each charge phase. After repetitive switching actions, a number of electron carriers can be injected into the AlGaN barrier and buffer layers, known as hot-electron injection. In discharge phase, M_H is off, but M_L becomes conductive, which shorts the source of M_H to ground, creating high VDS stress on M_H. This induces charge traps in the insulator and buffer layers, known as charge trapping. As a joint effect of both mechanisms, trapped or injected electrons in the insulator, AlGaN barrier and buffer layers repel free electrons in the channel when M_H is on, weakening the 2-dimensional electron-gas (2DEG) layer and further elevating hot-electron injection. This effect, known as current collapse or i- collapse for short, degrades channel conductivity, increases the on-resistance RDS_ON, and is a major cause of GaN-switch aging and failure [3]. On the other hand, another aging cause is thermal effect. To reduce manufacture costs and improve technology compatibility, it is common to fabricate GaN transistors on a silicon substrate. Accordingly, to reduce lattice mismatch, an AlGaN buffer layer is often inserted (Fig. 15.6.1). However, this increases the junction-to-ambient thermal resistance R θJA, which, together with the increased R_{DS_ON} due to the i- collapse, causes higher power and heat generation, elevating the junction temperature, T J. According to Arrhenius' Law, as T J increases, the mean-time-to-failure (MTTF) drops exponentially [4]. Even worse, the elevated T_J deteriorates the i- collapse effect with even higher R_{DS_ON}, significantly reducing device lifetime. ©2019 IEEE.