Davide Frattesi struck a 99th minute extra time winner to send Inter into the final 7-6 on aggregate.
Sommer, outstanding on the night, said afterwards: "It was a crazy game. I am really proud of this team and how we played.
"We were already kind of out and then we came back - amazing.
"It means a lot - you don't have a lot of chances to go to a final. We knew that in the last four minutes of extra time that it was our last chance to go through.
"We put everything on this pitch with the fans together."