[en] The power subsystems of large space equipment, radiation-generating equipment, and nuclear power sources are frequently required to handle high currents and voltages above the kilowatt range. In addition, the other subsystems require local regulation by, amongst other devices, power transistors, thyristors, and large rectifiers. Silicon power devices of the type described operate on the same general principles as junction devices discussed earlier, but their construction is often different: the chip is larger and may even be in the form of a 'pellet' having the full diameter of the source ingot. Epitaxial growth is used more extensively in power devices, while special doping (e.g. neutron transmutation) and unusual geometry are required to achieve a high junction breakdown voltage, low 'ON' resistance and good heat removal. In general, locally low levels of doping are required for high-voltage devices. In radiation-effects engineering terms, this is significant because carrier removal due to particle irradiation will be more noticeable. Also, in order to accommodate large depletion regions, large values of base width are required in the design of power transistors and thyristors. This can impart unusually high sensitivity to gain-degradation. In some respects, the power MOS device is also structurally different from its low-power relatives (see, for example, VMOS and HEXFET construction). Nevertheless, the response of the gate oxide layer -- probably the dominant effect in space and gamma radiation -- can still be predicted by the methods described earlier