Ask HN: Further studies with maximum ROI?

From my personal experience (MMV): You need a PhD these days for even a decent RCG/entry-level position (Google, Intel etc.) in the tech sector. Sad. Couple of years ago, if you had a Master's degree it was like "wow you have a Master's degree!" but now it's a commodity badge. Same thing is happening with the PhD badge. Now, everybody is trying to get a PhD just to secure their career (people have varied opinions on this though). I have met so many students who are doing a PhD just for the sake of hopefully landing up with a decent job offer -- and not at all for the sake of research/"fun." PhD in CS, ECE, EE etc. is popular and everyone is going after them. (I do understand that your research area may be very specific/unique)

Now, since everything is slowly going mobile, which areas you all feel a college student should seriously invest/focus for further studies (PhD/Masters) with a maximum possible ROI? I did my Bachelor's in CS and Math and Master's in Engineering Management and planning to do a PhD in HCI (the one with less CS-focus) or Information Management. HCI is going to be huge in the next 5-10 years IMO.