I've learned that most Zener diodes require something on the order to 20mA in reverse bias in order for Zener breakdown to take place. However, devices such as the LM431 "Adjustable Zener" can begin regulating at as little as 0.4mA. I understand that the LM431 is basically a non-inverting op-amp with a reference (nowhere close to a passive Zener diode). How are low-power fixed voltage references established?
To clarify, I would like to know how a reference voltage can be generated inside of an IC such as the LM431. I'm mainly interested in the general design techniques used to generate reference voltages in linear/switching regulators, comparators, op-amps, supply supervisors, etc.
I have seen multiple "textbook" regulator designs, but many of them are designed for discrete implementation, and they generally use a Zener or a forward-biased diode to generate a semi-stable reference voltage connected to the output via negative feedback.