This is a comprehensive resource of power device electrical characteristics in a cryogenic environment. Using theoretical and experimental knowledge, temperature dependence of fundamental silicon material parameters like intrinsic carrier concentration, carrier mobilities, lifetimes and bandgap narrowing was identified. The temperature dependent model of avalanche breakdown was developed using experimental data on numerous devices. A wide range of power devices, each with its own unique features, was chosen for theoretical and experimental analysis. Using these analyses, Schottky diodes, power MOSFETs, power BJTs, and power JFETs were optimized in the 300-77K temperature range. This text presents the different characteristics of power devices operated below -55`C (220K). It provides data and physics based models for power devices operated at temperatures down to 77K in a single source. All commercially available devices have been included to provide comprehensive coverage. Also, a fundamental analysis of devices identifies the suitability of various devices to applications requiring cryogenic operations. A quantitative analysis of the relative strengths and weaknesses of these devices is also presented.