Practical English in Use 3rd edition by Swan.
section 566 - still, yet, and already: time
"Not yet" is used to say that something which is expected has not happened (but we think that it will).
"Yet" usually goes at the end of a clause, but it can go immediately after "not" in a formal style.
"Don't eat the pears - they aren't ripe yet."
"The pears are not yet ripe. " (more formal)