Yes you can join DRDO after completing M.Sc. in Biotechnology. Nowadays selection procedure is through GATE score which is followed by internal test by DRDO-RAC and personal interview. Sometimes they do not take the internal test and admission is on the basis of GATE score and interview. Through this you will get a post of scientist.

DRDO also opens some positions for technical cadre too which is done through DRDO-CEPTAM.

Hope this helps