Does the Turkish AT/11 form replace health insurance in Germany?

AT/11 is not an insurance policy; it is an entitlement certificate under the German–Turkish social security agreement. It confirms that you are insured with the Turkish SGK and lets a German statutory health insurer provide treatment in Germany on behalf of the SGK.

For students it can play a role, under conditions:

• Universities require proof of health insurance for enrolment. Students who are insured through the SGK can present the AT/11 to a German statutory insurer (for example AOK, TK, Barmer), which then registers the entitlement and sends the university the electronic confirmation (the "M10" notification) – either as coverage via the agreement or as an exemption from the compulsory student insurance. Several university international offices (for example KIT and Hochschule Bochum) describe this route.
• The exemption from compulsory student insurance is final for the whole period of study in Germany; you cannot switch back into the cheap statutory student tariff later. Many students therefore choose the German statutory student tariff (a fixed monthly student rate that depends on the insurer's additional contribution; ask the insurer for the current amount) even if an AT/11 would be accepted.
• For the visa and residence permit the authorities require adequate health insurance for the whole stay. Whether coverage via the AT/11 is accepted is decided by the mission and the Ausländerbehörde; a German statutory confirmation based on the AT/11 is usually accepted, a plain travel insurance is not.
• The AT/11 must stay valid: your SGK status in Turkey (for example as a dependant of a parent) has to continue during your studies, and the form is issued for a limited period and must be renewed.

A short-term travel insurance is something different: it covers only the first weeks until you have proper insurance and does not satisfy the enrolment requirement.