Active Devices for Power Electronics: SiC vs III-N Compounds – The Case of Schottky Rectifiers

Abstract

The main rectifier device structures for power electronics based on SiC and on GaN are compared and the main issues for each structure are evaluated in terms of performance and manufacturability. The driving volume markets for power electronics devices correspond to the systems working on 127, 240 and 400 V energy supply networks, setting the device voltage handling to 300, 600, and 1200V respectively. We have limited the scope hereafter to the 600 V typical target, for which SiC Schottky rectifiers are now commercially available from at least 3 sources. The key physical properties for any semiconductor material used as the active layer of a unipolar device for power electronics are the breakdown field and carriers mobility. The bulk values are very similar for SiC and GaN. Two main other key issues are related to quality of the ohmic and Schottky contacts. For the ohmic contacts, adequate solutions have been found for both SiC and GaN. Surprisingly, on hetero-epitaxial GaN layers on sapphire despite of the very high crystal defects density ( ≥ 109cm-2 ), the ideality factor of the best Schottky contacts seems very promising. On the other hand, improving this ideality factor and the reverse leakage current for Schottky contacts on GaN layers grown on silicon substrate remains a fierce challenge. For the SiC Schottky rectifiers, cost and availability of the SiC substrates appear as the main residual limiting factors today. For GaN based rectifiers, although engineering device prototypes have already been published [1], there are both basic issues to be validated regarding reverse leakage current and reliability, and also difficult manufacturing issues to be solved in relation with device reliability, directly resulting from the nature of the possible substrates: mainly sapphire and silicon.