# Zener diodes in series

Hello, In this problem, i am not able to understand how I2 is getting replaced by I01. More over, will these zener diodes become ON as the applied voltage is less than Vz?

Because the zener voltage is 10V, and the applied voltage only 9V, so the two diodes all are not ON. They are series, so the current flow in them should be limited by the smaller reverse saturation current, that is, $I_{o1}$.