…which type of mosfet we are using?
Use types that give results that most closely match your specific requirements. What are you requirements? What molecules are you wanting to detect? What are your sensitivity ranges? What are your environmental factors? (Temperature? Humidity? Pressure?) What do you want to ignore? Or if you are asking about commercial sensors, you will have to ask the vendors, or tear them apart and look. They will be the only ones who know. (And they probably won’t tell you their secrets.)
…which software we are using in computer to detect these things?
Since you will write it, you are the only one who knows. Or if you are asking about commercial sensors, see previous paragraph.