Of course, the word 'therefore' may appear at the end of a sentence or sentence-element and, consequently, there are circumstances in which it might have a semi-colon after it. For example, in a paragraph about the rules of writing, the following may appear...
"Grammar is a complex business. It is best to know the rules, therefore; not to do so leaves one open to error."
However, Linlam, as said by others above, there is no necessity for that to happen. It is generally separated from the rest of a sentence by a comma or commas.