It's unsafe to use a semicolon anywhere else. A sentence should contain one idea, though that can be a complex or compound idea. The most obscure sentences in academic writing are sentences filled to bursting. If your writing lacks clarity , check to see if a long, bad sentence might make two short, good ones. This isn't to say that all sentences should be short.
Long sentences add variety, and some ideas are too complicated to fit into seven words.
But don't turn your simple ideas into monstrous sentences, devouring line after line without mercy. One idea, one sentence. A sentence fragment is a group of words passing itself off as a sentence without having a subject and a verb. Which is a bad habit. Picked up from advertising.