When two identical zener diodes are connected in series, the breakdown voltage of the combination is halved.

Explanation:
When a zener diode is connected in reverse bias, it operates in the breakdown region, allowing current to flow through it when the voltage across it exceeds its breakdown voltage. The breakdown voltage is a characteristic property of a zener diode and is typically specified by the manufacturer.

When two identical zener diodes are connected in series, the total voltage across the combination is divided equally between the two diodes. This is because the diodes are identical and have the same current flowing through them. Therefore, the voltage across each diode is half of the total voltage.

Now, let's consider the breakdown voltage. The breakdown voltage is the voltage at which the zener diode starts to conduct significant current. In the case of a single zener diode, this breakdown voltage is determined by the properties of the diode itself.

When two identical zener diodes are connected in series, the breakdown voltage of each diode remains the same. However, since the total voltage across the combination is divided equally, the voltage across each diode is halved. Therefore, the breakdown voltage for each diode is also halved.

In other words, if the breakdown voltage of a single zener diode is Vz, then the breakdown voltage for the combination of two identical zener diodes connected in series is Vz/2.

So, when two identical zener diodes are connected in series, the breakdown voltage of the combination is halved. This is because the total voltage is divided equally between the diodes, resulting in each diode experiencing half of the total voltage.