You don't really need an external mic
I recently used the app called Field Recorder from Google Play.
I am using a Samsung galaxy S4 and was blown away by it's feature.
The best thing is it allow you to record in stereo by using your mobile mic and the receiver (The part that press towards your ear)
Very minimal electronic static noise is produce by my phone itself can be capture and heard by it is very easy to eliminate it by using any DAW.
Here a video I created that I have capture it raw inside my bathroom
Check it out: https://youtu.be/4REwmJlQ8nE