Personally, I would stay with "Physical Design Engineer".
Speaking of terms more used in their English form, they might have a slightly different meaning literally translated. I mean just check the different translations for "physical" and "design" and imagine a personell manager gets dozens of applications for a job as "Ingenieur für physisches Design", "Technischer Gestaltungsingenieur", "Leibhaftiger Anordnungs-Pionier" and so on.
If the english term is used more, it's more distinct.
Also even if there is a 100%-safe translation, you still don't know if it's correct. Just because the job is named the same, that doesn't mean it is the same job (if not proofed by an international educational degree or something).
And check if the term does exist "as is" in German, as there are examples like the "Bachelor of Science/Art/etc." or "Gamedesigner" that are never translated or can't be translated at all.
@Christian Geiselmann pointed out another very important point to find the solution to a situation like this: adaptation. If it's written in the offer, it might be the safest option to copy it.