Zener breakdown occurs in a p-n junction having p and n both:

Explanation:

• The zener breakdown occurs in the heavily doped p-n junction diode.
• Heavily doped p-n junction diodes have narrow depletion region.
• The narrow depletion layer width leads to a high electric field which causes the p-n junction breakdown. 
• This heavy doping reduces the width of the depletion region, allowing a strong electric field to develop across the junction even with a small reverse bias voltage.
• When this electric field reaches a critical value, it can cause electrons to tunnel from the valence band of the p-side to the conduction band of the n-side, resulting in a sudden increase in reverse current.
• This phenomenon is known as Zener breakdown and is utilized in Zener diodes for voltage regulation.

∴ The correct option is 2