Zener Diode Breakdown Mechanism

A Zener diode is a special type of diode designed to operate reliably in the breakdown region. This breakdown occurs under reverse bias when a sufficiently high voltage is applied.

Dominant Carrier Transport

In the breakdown region of a Zener diode, the primary mechanism for current flow is quantum mechanical tunneling. This phenomenon is known as Zener breakdown.

• A very high electric field develops across the depletion region due to heavy doping.
• This strong electric field enables electrons to tunnel directly from the valence band on the p-side to the conduction band on the n-side.
• This tunneling allows for a significant reverse current to flow even at relatively low reverse voltages compared to a standard diode.

Comparison with Other Mechanisms

While drift and diffusion are common carrier transport mechanisms in semiconductors:

• Drift current is typically dominant under normal forward or reverse bias conditions, driven by electric fields or concentration gradients.
• Diffusion current is significant in forward-biased diodes, related to the movement of carriers from regions of high concentration to low concentration.
• Ballistic transport involves carriers moving without scattering, relevant in nanoscale devices, but not the primary mechanism in Zener breakdown.

Therefore, the unique, strong electric field effect in heavily doped junctions leads to tunneling being the defining carrier transport mechanism during Zener breakdown.