(Invited) Vertical GaN High Voltage Transistors: Comparison with SiC Switches

Abstract

Wide bandgap (WBG) semiconductor high-voltage power switching devices, especially those made on silicon carbide (SiC) and gallium nitride (GaN), promise transformative advances in electrical power switching systems because of superior electrical and thermal properties of these materials compared to the semiconductor silicon [1]. Both SiC and GaN semiconductors contain a high density of crystal defects and the role of defects on the performance and reliability of electrical power switching devices under extreme operating environment is not clear.  The approach and results for achieving wide bandgap semiconductor (GaN) based high voltage “vertical” switches is reported. The GaN and the related ternaries, AlGaN and InGaN are the materials of choice due to their superior properties.  While the performance of the current generation of GaN devices is limited by material and reliability issues, the promise still remains an elusive goal.  The results reported in the present work address the issues of performance and reliability for future GaN and related compound power devices, and will enable the development a new class of devices based on V-HEMTS and Trench HFETs to exceed the state of the art of 2.5 kV for silicon and SiC devices for blocking voltages up to 10 kV. We report results which include the design, fabrication and testing of structures exploiting HEMT GaN/AlGaN structures as well as a new and unique GaN Trench MOS design shown in Figure 1. In this comparison, we review the key reliability issues facing SiC High Voltage MOSFETS, as well as the role of threading edge dislocations and threading screw dislocations affecting SiC MOSFET reliability. These issues are compared to similar defect related limitations for GaN vertical devices as well. Material-device-performance correlation methodology ensures that the reliability is designed in through the physics of failure approach.  The research results reported also assesses other novel structures with blocking voltages in excess of 10kV as well as the evaluation of the reliability of these devices under system environments. We note that crystallographic defects when present in active areas of SiC-based devices have a negative influence on device performance, reliability and reproducibility. Micropipes are well-known as device killers while the detrimental influence of Basel Plane Dislocations (BPDs) on device performance is also established [2]. For the present work we have investigated a vertical 4H-SiC Junction Barrier Schottky (JBS) diode where a slightly higher doped n-type buffer layer is used to constrict the spreading of the space-charge region beyond the lightly doped n- - type drift-region. The diode was subjected to increasing reverse voltage stress starting at the rated 600V. The reported experimental data for high-voltage SiC semiconductor strongly suggests that material defects have profound impact on its characteristics under extreme environments. Since GaN semiconductor has even higher density of material defects, this problem may be further exacerbated. REFERENCES [1] K. Shenai et al, IEEE Trans. Electron Devices 36, 1811-1823, 1989. [2] R. L. Myers-Ward, D. K. Gaskill, R. S. Stahlbush, N. A. Mahadik, V. D. Wheeler, L. O. Nyakiti, and C. R. Eddy, Jr., Electro Chemical Society Transactions, vol. 50, no. 3, ISBN 978-1-62332-002-7, pp. 103-108, Oct. 2012.