You need a valid I-20 to return to the US with a valid student visa. Your SEVIS record needs to be in Active status. You may be refused entry or denied boarding with an expired I-20/terminated SEVIS record. If you are not able to obtain the new I-20 before you leave the US, you should have it sent to you at wherever you are traveling outside the US.
The USCIS web site advises:
If you need to travel on a terminated record, you must first visit your DSO. If your school has requested a correction request or data fix, the DSO will put your correction request or help desk ticket number on your Form I-20 and report your pending travel to SEVP.
There is no guarantee that Customs and Border Protection (CBP) will readmit you to the United States if you travel on a terminated record. In most cases, CBP inspectors will allow you to reenter the United States if you are otherwise admissible and your DSO has properly annotated your Form I-20. It is likely, however, that the CBP officer at the port of entry will send you to secondary inspection while they determine whether you are eligible to return to the United States.
If you have an expired visa and a terminated record, SEVP advises you not to travel outside the United States until your SEVIS record shows that you are in Active status. If you do travel, you may not be able to renew your visa or return to the United States.
