When is a semi colon necessary

A semicolon is necessary when you write a sentence that contains a dependent clause that is related to the independent clause. Closely related ideas can be connected by a semicolon instead of a comma.  These ideas or interdependent clauses are closely related to each other. When two clauses are joined by a semicolon, they are considered to be equally important to each other. 
It is important not to use too many semicolons in a paragraph. The rule of thumb seems to be that only one semicolon should be used per paragraph.