A semicolon is necessary in a compound sentence if a coordinating conjunction is not used.

Using Coordinating Conjunctions

Use coordinating conjunctions to indicate that the elements they join are equal in importance and in structure.

Definition of Coordinating Conjunctions

Coordinating conjunctions join grammatically similar elements (two nouns, two verbs, two modifiers, two independent clauses):

andornorsobutforyet

How to punctuate coordinating conjunctions

When a coordinating conjunction joins two independent clauses, a comma is used before the coordinating conjunction (unless the two independent clauses are very short).

Conjunctions that are not followed by non-essential elements should never be followed by commas.

Perhaps no budget is without some fat, but university officials argue that their unique function requires special standards of evaluation.

When either independent clause in a compound sentence contains a comma to set off introductory or non-essential elements, a reader may be confused by a comma before a coordinating conjunction. In this case, a semicolon may replace the comma.

The figures at elite universities, particularly, are enough to cause sticker shock; yet the current increases at many schools are the lowest in a decade.

When NOT to punctuate coordinating conjunctions

If a sentence begins with a coordinating conjunction, it is not followed by a comma

Yet the typical tenured professor's salary of $43,500 still represents 10% less buying power than the equivalent salary in 1970.

Commas are not used between two verbs, two subjects, two complements, or two objects joined by a coordinating conjunction.

That confuses most analogies between universities and profit-making enterprises. [compound object of preposition]

Endowments and gifts make up the rest. [compound subject]

Georgetown, for example, has eliminated one-third of its graduate programs in the past five years and recently decided to close its dental school. [compound verb]