A natural 20 on an attack roll disregards armor class entirely.
From the rules for attack rolls (emphasis mine):
If the d20 roll for an attack is a 20, the attack hits regardless of any modifiers or the target’s AC. This is called a critical hit, which is explained later in this section.
A critical hit totally disregards armor class. Since this hit is called a critical hit, we consult the rules for critical hits:
When you score a critical hit, you get to roll extra dice for the attack’s damage against the target. Roll all of the attack’s damage dice twice and add them together. Then add any relevant modifiers as normal. To speed up play, you can roll all the damage dice at once.