Hello Kunal, 

If you are a an ECE student and have qualified GATE with it then you will need to score a rank lower than 40 to have an assured seat in IIT BOMBAY. You can also get in with an AIR rank between 41-100 but you will need to clear an interview to get admitted. 

The subject code for microelectronics is EE4 and the admission process for EE4 is as follows : 

1. For TA - direct admission

2. TAP/RA/RAP/FA - written test on the 7th of June and interview on 8th and 19th June

3. PS/IS/SW/IIT BTech - written test on 7th June and interview on 8th and 9th June. 

Hope this helps :)