After B.Tech if you want to study in IIST, IIST also offers Postgraduate Programmes like:
M.Tech in Soft Computing & Machine Learning
M.Tech in RF & Microwave Engineering
M.Tech in Geo informatics
M.Tech in Optical Engineering
M.Tech in Machine Learning & Computing
M.Tech in Solid State Technology
M.Tech in VLSI & Microsystems
M.Tech in Materials Science & Technology, etc.

So these are the branches that have better opportunities.  Opportunities depend upon ISRO Placements conducted by centralised recruitment committee.