Higher than 20 AC and Crits

If a character or creature has an AC that with all bonuses requires a 20 to hit does that mean that every hit is a crit? or alternately the creatures AC is so high that with all bonuses no hit would be possible except that a 20 always hits. So if a nat 20 is rolled you hit and crit and then the only way you can hit the impossible to hit creature always results in a crit. I feel like the impossible to hit would grant you a hit on a nat 20 but not the crit.