SiC power devices for high voltage applications

Silicon Carbide device technology is now evolving from a pure vision to a real alternative to silicon devices. The feasibility of SiC devices has been shown for many different types of devices, the development of a working production technology has started, yield, reliability and costs now being the key issues. At present the high substrate prices keep the manufacturing costs of SiC high, making it very difficult to enter the device market with SiC on economic terms. Prime applications are those for which SiC offers substantial benefits or even a technological breakthrough on the system level. The main application is power conversion where the latest development efforts on silicon based power switches (e.g. IGBT) allow utilisation of much higher switching frequencies, putting very high demands on the free wheeling diode. The system performance is to a large extent limited by the diode recovery charge—a major source of switching losses. Depending on the voltage range, different device concepts are of interest: In the lower voltage range the junction–barrier controlled Schottky (JBS) device is a promising candidate while at voltages beyond 2.5 kV the PIN diodes is the device of choice. Different system requirements—e.g. surge current capability—make the PIN junction superior to a Schottky device for certain applications. With progress in material and technology development the ‘world’s best’ result is becoming less and less important and reproducibility is the issue. Failure analysis of defective devices needs to be established with a high number of substrate defects being still an obstacle in SiC. High leakage/soft reverse characteristics are often encountered and can usually be attributed to localised defects. It is important to identify their origin and to separate process-induced defects from those already present in the epilayer or substrate. In order to use the high power handling capability of SiC reduction of margins (e.g. in epilayer thickness and doping) is necessary. This requires narrow bandwidth of process and material variations. For paralleling of SiC devices equal current sharing under static and dynamic conditions is a fundamental requirement from the system side. Top-down calculation gives material specifications, which the supplier has to meet.