drain area or source area that are formed separately on a main surface of.........

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I know this is quite chokuyaku-ish, leaving a real proper translation to your decision. Just hope to clarify the structure of the Japanese sentence.

An MOS semiconductor device such as an n-channel MOS transistor comprises a p-type monocrystalline silicon substrate, an element isolation region (a field oxide film) formed on the main surface of the substrate to form a plurality of islands in the substrate, n+ -type source and drain regions formed in the main surface of this island substrate region to be mutually isolated, a gate oxide film formed on the surface of the channel region between the source and drain regions, and a gate electrode formed on the gate oxide film to oppose the channel region therethrough...

The surface of the substrate between source and drain regions 13 and 14 serves ... of the semiconductor substrate to be mutually separated, a channel region ...
www.patentstorm.us/patents/5175599-description.html - 27k

When it comes to semiconductor, I guess "隔離" usually refers to electrical isolation. See the following example from "English for the Understanding of Semiconductor."(『半導体がわかる英語』傳田精一著　工業調査会）


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The epitaxial wafer is prepared for an "isolation" diffusion step by means of a photographic process. In the process, the wafer is covered with a uniform film of photosensitive emulsion, generally termed photo resist.

"English for the Understanding of Semiconductor" p.151
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I tend to agree with everyone so far, but this would be what I say. 隔離して does not refer to the action of fabricating the source and drain regions, but it means that there is a distance between the two regions.

... drain region and source region which are fabricated apart from each other on a main surface ...