How to define a (ideally: global) keyboard shortcut for a particular letter plus diacritic? Is there a way to define (OSX natively, or alternatively: through some program) a global keyboard shortcut for a single letter plus diacritic, say: ctrl+alt+A gives "ä", ideally globally?

Note: I am aware of various ways to access diacritics faster (e.g. the "key repeat" diacritics menu), but I'd like to know if there's some way to get the exact behavior described above.

Does anyone knows if this is possible in OSX, and if so, how?



(EDIT) Thanks to a comment, I realize I should clarify my question. 

I don't really care which modifier key I have to press, but what I want to do is:

[some modifier, e.g. alt or ctrl, or if necessary: two of them]

+

[some letter]

=> [a single letter with diacritic]

Now, I know that "alt-u plus letter" produces the corresponding "Umlaut" letter, but it's not exactly the same as the shortcut I ask about above. Ideally I would have the above version of a shortcut, because it is slightly unintuitive to me, and therefore a bit slower, to press a different vowel first, "u",  when I really want to produce another vowel afterwards, say, an "a" with diacritic, i.e. "ä". 

So I am really just wondering: can I bind a shortcut as described above producing a single (Umlaut) letter of my choice.
The Karabiner and Ukelele apps should be able to bind keys the way you are requesting:

https://pqrs.org/osx/karabiner/

http://scripts.sil.org/ukelele