@franzi My German is very rusty, but from the gist of what you're saying and what's on there, if you stick 3.3V on that base you'll likely blow both the transistor and the LED. Are you perhaps trying to control a power LED using a microcontroller?
@franzi Well, you need some kind of ballast resistor or constant current source like LEDs almost always do. Then just an N-channel MOSFET between that and ground. Assuming the LED is 7V, 700 mA then something like the pic attached. If you need higher efficiency then you'll want to use a switching constant current driver. That only really needed for battery powered stuff https://social.umeahackerspace.se/attachment/523383