From the website - LINK
13.5 In the Semi-Final ties, if the aggregate score is level at the end of the second game an extra half-hour shall be played. If the aggregate scores are still level at the end of extra time the tie shall be decided by goals scored away from home counting twice. If the teams remain equal after this procedure the tie shall be determined by the taking of kicks from the penalty mark in accordance with the Laws of Association Football.
So it's aggregate, then extra time, then away goals, then PKs.