Skip to main content

Avalanche vs Zener breakdown; Zener V-I curve

Short answer

Avalanche breakdown occurs in lightly doped junctions above ~6 V by impact ionization with a positive temperature coefficient. Zener breakdown occurs in heavily doped narrow junctions below ~5 V by quantum tunneling with a negative temperature coefficient. The Zener V-I curve shows forward conduction (~0.7 V), low reverse leakage, then sharp breakdown at Vz with nearly constant voltage.

Differentiate between avalanche and zener breakdown. Draw V-I characteristic curve of zener diode and briefly explain about it.

Difference between avalanche and Zener breakdown

PointAvalanche breakdownZener breakdown
CauseImpact ionisation by collision of fast carriersDirect breaking of covalent bonds by a strong electric field
DopingLightly doped junctionHeavily doped junction
Depletion regionWideVery narrow (below 10 nm)
Breakdown voltageAbove about 6 V (high)Below about 5 V (low)
Temperature coefficientPositive (VBR rises with temperature)Negative (VBR falls with temperature)
CharacteristicSharp knee, noisySmooth, rounded knee
MechanismCarrier multiplicationField emission (tunnelling)

V-I characteristic of a Zener diode

                I ^
                  |          /
                  |         /  forward
       -Vz        |        /
  ------+---------+-------/-------> V
        |  leakage (µA)  0.7 V
        +-- Izk (knee)
        |
        |   reverse current rises sharply
        |   at almost constant V = Vz
        v   Iz(max)
  • Forward region: it acts as a normal silicon diode, conducting above about 0.7 V.
  • Reverse region before Vz: only a tiny leakage current (µA) flows.
  • Breakdown (Zener) region: at the Zener voltage Vz, the reverse current rises very sharply at nearly constant voltage. The knee current Izk is the minimum current needed to stay in this region, and Iz(max) = Pz(max)/Vz is the upper safe limit.
  • In this region the voltage is nearly constant (small dynamic resistance rz), so the Zener is used as a voltage regulator and voltage reference. A series resistor limits the current.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Electronics Devices and Circuit chapter 6 questions →