how to - trigger that reacts when a gameobject passes through, but the gamobject cant have a functioning collider

so, I got this game in wich airbourne units move from left to right and right to left. A castle defence game.

The planes have triggers, so when a hostile unit enters it, it gets shot.I want the units to be able to pass through each other. But if I dont put a collider on the planes they can't fire a trigger.

In short-> a gameobject detectable by a trigger but not able to collide with anything.