Hii!
It is usually recommended that you go for Master's after b.tech and then go for PhD. But, there are some Institutes which allow PhD directly after b.tech too, under FPM Program. The eligibility criteria for them is quite different.
Some of the Institutes which you can apply into are-
* IIT, Kanpur
* IIT, Gandhinagar
* IIMs
* TIFR
* BARC
* IUCA
* SINP etc.,
The admissions nare taken on the basis of GATE/ GMAT/ CAT scores.
Hope this helps.
Good luck!