This app was mentioned in 3 comments, with an average of 1.00 upvote
Both Common FM Synthesizer and Common Analog Syntheziser by oxxxide are free and quite capeable. Keep in mind that most Android devices have pretty terrible input (and audio output) latency so really performing on them is next to impossible, sadly enough.
Have a look at Common FM Synthesizer for android. It's a pretty simple FM synth, but everything is assigned to knobs. A hardware version looks possible to me and would be so much fun!
Someone linked the one I use already but here's another.
https://play.google.com/store/apps/details?id=org.oxxxide.fmsynth
The same people that put that one out made an analogue synth app too.