Research in MOS-gated power semiconductor devices with combined bipolarlVIOS action has been motivated by the continuous industrial need for highly efficient devices with reduced power losses and high impedance gate control. The Insulated Gate Bipolar Transistor (IGBT) and the IVIOS Controlled Thyristor (MCT) have been among the first :tvIOS-gated bipolar devices with high blocking capability and reduced on-state power losses. Presently these devices are fabricated using the conventional DIVIOS technology. Recently, a new technology, trench technology, has emerged in the field of low voltage power :tvIOSFETs. This thesis discusses the fundamental physical and geometrical advantages of the trench technology over the conventional technologies for high voltage devices. New physical phenomena such as channel length modulation, enhanced channel charge and the PIN diode effect are reported for the first time. A physically-based on-state analytical model for Trench Insulated Gate Bipolar Transistors (Trench IGBTs) is developed. Furthermore, the present study proposes a novel bipolar-MOS concept, termed Inversion Layer Injection. Unlike in conventional bipolar-MOS devices where the bipolar emitter is formed by a "static" semiconductor layer (e.g. n+ type), in this case the bipolar emitter is constituted by mobile carriers ( e.g. electrons), which can be directly controlled by an external gate electric field. Based on this physical principle, a trench-gated device termed the Inversion Layer Emitter Thyristor is proposed. The ILET has the potential to achieve an optimum on-state/turn-off performance and is suitable for a very wide range of applications which require voltage ratings from 400 V up to 5 KV and operating current densities from 100 A/cm2 to 500 A/cm2 . The Inversion Layer Injection concept is further employed in a novel class of power semiconductor devices called Lateral In version Layer Emitter Transistors (LILETs ). The LILETs exhibit low on-state losses, very fast switching time, improved gate control and flexible mode operation. It is concluded that the Trench IGBT and ILET will play the major role in the next generation of high voltage vertical power devices. At the same time, the LILETs are regarded as potential CMOS compatible structures for power int egrated circui ts which can replace the conventional Lateral In sulated Gate Bipolar Transistors (LIGBTs).