The present invention solves the problem of how to form local regions of semi-insulating material within a single crystal substrate. It does this by irradiating the semiconductor with a high energy beam capable of producing radiation damage along its path. As a consequence of such radiation damage the resistivity of the semiconductor in the irradiated area is increased by several orders of magnitude, causing it to become semi-insulating. Semi-insulating regions of this type are effective as electrically isolating regions and can be used, for example, to decouple analog from digital circuits or to maintain high Q in integrated inductors after these devices have been made. The radiation used could be electromagnetic (such as X-rays or gamma rays) or it could comprise energetic particles such as protons, deuterons, etc. Confinement of the beam to local regions within the semiconductor is accomplished by means of suitable masks.