HV-PhotoSw

The deployment of renewable energy involves a mutation of the electric grids to account these new sources. Particularly, HVDC and MVDC grids are new technologies that demand high voltage power electronic converters. Silicon carbide (SiC) is a good material for high-voltage power devices while Gallium Nitride (GaN) is limited in voltage range and diamond remains a long-term target. 10 kV and above SiC-devices are targeted in research projects while 3.3 kV devices are under industrialization. However, to achieve high voltage power converters, it is needed to associate in series power modules or power components. So, one main issue is the design of highly insulated drivers (up to several 100 kV). There exists some solutions as the self-supplying but these solutions are complex and may decrease the global reliability of the system. On the other hand, the photo-transistor is a classic device, and it is very common for the silicon ones. If some optical controlled SiC thyristors have been demonstrated, to our knowledge an optical controlled SiC Bipolar Junction Transistors (BJT) has not yet been demonstrated for high voltage applications. The advantage of the photo-transistor is to avoid the use of a circuit driver because of the electrical insulation may be easily achieved by an optical fiber. So, the project proposes the design and fabrication of a high-voltage phototransistor. The interest of such a power device is very high because of the simplification of the drive insulation. Moreover, for a given die area, the BJT is the best component with the lower on-loss among all other SiC power semiconductor devices. The consortium consists of 2 academic partners (Ampere Lab. and ISL) and one SME (NovaSiC). NovasiC will optimize a fast chemical vapor deposition (CVD) for the low-doping thick epitaxial layer (n-type) needed to reach 10 kV devices. The target is to reduce the cost of this process which is a significant part of the global process costs for such high-voltage devices. The obtained results will be compared to commercial epitaxial-layers. The project proposes the fabrication and the test of high voltage photo-transistors, BJTs, on two batches of fabrications. To reduce the risks in the project, some transistors on the wafers will have a base metalization to enable electrical tests. This type of cells may be used for the 4 targeted devices of the project. All of them are high voltage vertical devices with the target of 10 kV. This blocked voltage target has the advantage to be interesting for numerous applications (e-transformer, modules for HVDC or MVDC applications, DC breakers …) and not to be so difficult to be fabricated in an academic clean-room. However for a efficient BJT, the requirement for accurate process is high in terms of photolithography, dry etching, passivation and packaging. So technological facilities of the ESIEE are targeted which is recognized for its professionalism and support of research projects developed on its 600 m2 clean-room, offering a full 100 to 150 mm technology which is ideal for the actual SiC wafer sizes (4 to 6 inches). To avoid strong constraints on the device fabrication, a solution is to develop all the high voltage BJTs on the same wafer (2 lots). Depending on the project results, some targeted products are expected • Large 10 kV photo-transistors for efficient and isolated converters (HVDC, MVDC), DC Breaker (solid state). • Large 10 kV Photo-Darlington for all applications where the efficiency is not critical : single pulse applications, DC breakers (hybrid), active snubber ... • 10 kV optical switch 10 kV, i.e. a large BJT with a low voltage silicon phototransisor and a self-supply : a bypass of the first target to compare phototransisor to more classical solution. • Large 10 kV (even 15 kV) or 3.3 kV BJTs, as an alternative of SiC-MOSFETs without the gate oxide reliability issues for applications where a high efficiency is required: HVDC, MVDC, electric traction…