Explore the complete I-V characteristics of a Zener diode — from forward conduction through reverse leakage to the sharp Zener breakdown region. Adjust parameters and watch the curve plot live.
In heavily doped Zener diodes (Vz < 5V), the strong electric field at the reverse-biased junction causes quantum mechanical tunneling — electrons tunnel directly from the valence band of the p-side to the conduction band of the n-side. This produces a sharp, reproducible breakdown at Vz.
In lightly doped Zener diodes (Vz > 8V), breakdown occurs by impact ionisation — carriers accelerated by the field collide with lattice atoms, creating new electron-hole pairs in an avalanche process. Higher Vz values use this mechanism.
When forward biased, the Zener diode behaves like a normal p-n junction. Current increases exponentially once the forward voltage exceeds ~0.65–0.7V (silicon). The knee of the curve appears around Vf = 0.65V for silicon Zener diodes.
Zener diodes below ~5V have a negative temperature coefficient — Vz decreases as temperature rises (tunneling dominates). Above ~8V, the positive temperature coefficient means Vz increases with temperature (avalanche dominates). Near 5.1V, the coefficients nearly cancel — making 5.1V Zener diodes ideal references.
In the breakdown region, the Zener diode has a small dynamic resistance rz = ΔVz/ΔIz. A smaller rz means better voltage regulation. Typical values: 1–100Ω. The slope of the I-V curve in the breakdown region represents 1/rz.
Every Zener diode has a maximum power dissipation Pmax = Vz × Iz(max). Exceeding this destroys the device. A series resistor Rs is always used to limit current and keep power within safe limits. The power bar above shows the current dissipation.
A Zener diode is a specially designed p-n junction diode that is manufactured to operate in the reverse breakdown region reliably and repeatedly. Unlike ordinary diodes where reverse breakdown can be destructive, the Zener diode breakdown voltage (Vz) is precisely controlled during fabrication through careful doping. This makes it invaluable as a voltage reference and voltage regulator in electronic circuits.
The I-V characteristics of a Zener diode have three distinct regions. In the forward bias region, the Zener diode behaves exactly like a normal silicon diode — negligible current flows until the forward voltage reaches approximately 0.65–0.7V, after which current rises exponentially. In the reverse leakage region (applied reverse voltage less than Vz), only a tiny reverse saturation current (microamperes) flows. In the Zener breakdown region, once the reverse voltage reaches Vz, the current increases sharply while the voltage across the diode remains nearly constant at Vz — this is the defining characteristic that makes Zener diodes useful.
Two distinct mechanisms cause Zener diode breakdown. The Zener effect (quantum tunneling) dominates at lower voltages (Vz < 5V) in heavily doped junctions — the intense electric field allows electrons to tunnel quantum mechanically through the thin depletion region. The avalanche breakdown mechanism dominates at higher voltages (Vz > 8V) in lighter doped junctions — thermally generated carriers are accelerated by the field and collide with atoms, generating new electron-hole pairs in a cascade. Between 5–8V, both mechanisms contribute.
The most important application of a Zener diode is as a voltage regulator. In the breakdown region, the voltage across the Zener diode remains approximately constant at Vz even when the current varies significantly. A basic Zener voltage regulator circuit consists of a Zener diode in reverse bias, a series resistor Rs to limit current, and the load resistance RL in parallel with the Zener. The output voltage equals Vz regardless of variations in input voltage (line regulation) or load current (load regulation), as long as the Zener remains in breakdown.
The series resistor Rs must be chosen such that: (Vin − Vz)/Rs provides sufficient current to keep the Zener in breakdown under all load conditions. The Zener current Iz = Is − IL, where Is is the series resistor current and IL is the load current. If Vin drops or IL increases excessively, the Zener may come out of breakdown and regulation is lost.
The temperature coefficient of Zener voltage is a critical specification. Zener diodes with Vz below ~5V have a negative temperature coefficient (Vz decreases with temperature), while those above ~8V have a positive temperature coefficient. Near 5.1V, the two mechanisms nearly cancel, resulting in an extremely stable reference voltage — which is why 5.1V Zener diodes are preferred as precision voltage references in temperature-sensitive applications.
The Zener diode I-V characteristics experiment is a core practical in Kerala University FYUGP Physics and Electronics. Students plot the characteristic curve by varying the applied voltage and recording the current, identifying the forward conduction knee, reverse leakage, and the sharp breakdown at Vz. The experiment demonstrates the concept of voltage regulation and the practical use of semiconductor devices in electronic circuits.