Analysis of Superjunction MOSFET (CoolMOS™) Concept Limitations-Part I: Theory.

The CoolMOS™ (Infineon Technologies AG, Munich, Germany) has been considered a device that alleviates high-voltage limitations of unipolar power devices, but although the theoretical considerations seem to confirm such a possibility, this expectation has not been fulfilled until now. This paper identifies limitations of the CoolMOS™ concept. The analysis was carried out in two steps. The first step aimed at the theory of high-voltage superjunction and its implementation into a power VDMOS transistor, which resulted in the modified construction called CoolMOS™. The investigations have shown that the superjunction effect is not an inherent feature of high voltage junctions formed as a characteristic meander-like p-n junction. Such a junction starts to work in SuperJunction Mode (SJM) just when the electric field strength reaches the magnitude of the threshold electric field Eth. Also, other theoretical constraints concerning the SJ diode and CoolMOS™ design have been presented. The second step aimed at the physical and technological limitations that have been identified, taking advantage of numerical investigations for CoolMOS™ structures developed on the basis of a typical VDMOS one.