In voltage reference diodes

1. The breakdown mechanism is only due to zener effect
2. For low voltages and heavily doped material the breakdown is due to avalanche multiplication
3. For a limiting reverse voltage which can be used as reference, the reverse current increases depending on the circuit connected
4. The current flow at the reference voltage is due to minority carrier injection

For a given voltage reference diode the reverse current depends on the circuit through which this resistance is connected.