sentences:  Some Zener diodes have a sharp, highly doped p–n junction with a low Zener voltage, in which case the reverse conduction occurs due to electron quantum tunnelling in the short space between p and n regions − this is known as the Zener effect, after Clarence Zener.Diodes with a higher Zener voltage have a more gradual junction and their mode of operation also involves avalanche breakdown.Both breakdown types are present in Zener diodes with the Zener effect predominating at lower voltages and avalanche breakdown at higher voltages.