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 E_th. 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.