After we build an app using sencha, how do we package it to deploy to physical devices like IPhone. Can we do this in Sencha only or do we have to use Phone gap or Appcelarator.
This is really a distribution issue...if you want to package it up so that it can be deployed via the App Store or Android Market, you're going to have to use PhoneGap.
PhoneGap is very easy to use. You should be able to compile most apps to native within an hour.
You can do it in the PhoneGap Cloud Build, but for iOS, you'll need an Apple developer certificate. To get the certificate, you'll need a Mac. So, if you can get a Mac for a while, create a certificate for you, later if you configure your PhoneGap Build account, you will be able to compile it for many mobile OS.
FYI, PhoneGap Build compiles for iOS (after declare certificate), Android, webOS, Symbian and BlackBerry