Zener effect

The I-V curve for a diode showing avalanche and Zener breakdown.

The Zener effect is a type of electrical breakdown in a reverse biased p-n diode in which the electric field enables tunneling of electrons from the valence to the conduction band of a semiconductor, leading to a large number of free minority carriers, which suddenly increase the reverse current. Zener breakdown is employed in a Zener diode.

Mechanism

Under a high reverse-bias voltage, the p-n junction's depletion region widens, leading to a high strength electric field across the junction. A sufficiently strong electric field enables tunneling of electrons across the depletion region of a semiconductor leading to a large number of free charge carriers. This sudden generation of carriers rapidly increases the reverse current and gives rise to the high slope conductance of the Zener diode.

Relationship to the avalanche effect

The Zener effect is distinct from avalanche breakdown which involves minority carrier electrons in the transition region which are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bound electrons. Either the Zener or the avalanche effect may occur independently, or both may occur simultaneously. In general, diode junctions which break down below 5 V are caused by the Zener effect, while junctions which experience breakdown above 5 V are caused by the avalanche effect. Intermediate breakdown voltages (around 5V) are usually caused by a combination of the two effects. This Zener breakdown voltage is found to occur at electric field intensity of about 3×10⁷ V/m. Zener breakdown occurs in heavily doped junctions (p-type semiconductor moderately doped and n-type heavily doped), which produces a narrow depletion region. The avalanche breakdown occurs in lightly doped junctions, which produce a wider depletion layer. Temperature increase in the junction increases Zener breakdown and decreases the contribution of avalanche breakdown.