A Reliability Study Of P-GaN Gate HEMTs

The GaN-based HEMTs have demonstrated promising performance for the next-generation high-efficiency and high-frequency power electronic applications.This is ensured by the polarization-induced two dimensional gas(2DEG)and the unique material characteristics of GaN,such as wide bandgap,large critical electric field for breakdown,high electron mobility and high electron saturation velocity.In order to achieve fail-safe operation,simplified gate drive IC design and reduced power loss in practical power switching applications,researchers have intensively investigated the normally-off HEMTs over the last decade.A number of techniques were reported previously,comprising of the adoption of the ultra-thin AlGaN barrier layer,the recessed-gate structure,the fluorine-plasma treatment,the MIS gate structure,and the p-type gate structure.Among these techniques,MIS gate structure was proved to be one of the most promising technique permitting a sufficiently large threshold voltage and gate voltage swing higher than 1 V.However,the associated reliability concern was simultaneously raised due to the dielectric/nitride as well as the dielectric quality.On the other hand,the p-type gate structure can not only ensure a large threshold voltage and gate voltage swing,but is also capable of obtaining low on-state resistance through channel conductivity modulation.As a result,the p-type gate structure has drawn growing attention of both the academia and the industry.In recent years,though high-voltage-switching related current collapse or dynamic on-resistance of p-type gate HEMTs have been intensely studied,reliability issues associated with carrier injection under gate voltage stress requires further investigation.In this thesis,the channel modulation instability of commercial p-GaN gate devices was studied in detail by investigating the non-self-heating RDS(ON)variations under different gate voltage stresses and different device temperatures.In addition,reliability issues were revealed experimentally under recommended operating gate voltage,including RDS(ON)variation,threshold voltage shift and blocking capability degradation,all suggesting carrier trapping related instability.The corresponding mechanism is proposed concerning the combined modulation of channel conductivity by trapped holes and electrons by donor-traps and acceptor traps in the AlGaN layer,respectively.Moreover,numerical simulation is performed to help explain the mechanism from the perspective of energy band.