In one line
A Zener diode is a heavily doped silicon p-n junction diode designed to work safely in reverse breakdown, where its voltage stays almost constant; breakdown happens by the Zener effect at low voltages and by avalanche multiplication at higher voltages.
Explained simply
An ordinary diode is like a one-way valve that gets damaged if you push hard against it the wrong way. A Zener diode is a valve built to "give way" at an exact pressure and hold that pressure steady, like a pressure-relief valve on a cooker. That fixed reverse "give-way" voltage is called the Zener voltage \( V_Z \).
Technically, heavy doping makes the depletion layer very thin, so even a small reverse voltage creates a very strong electric field across it. Its symbol is a diode arrow with a bent (Z-shaped) bar, and it is operated in reverse bias with a current-limiting resistor.
How it works
| Feature | Zener breakdown | Avalanche breakdown |
|---|---|---|
| Doping | very heavy | lighter |
| Depletion layer | very thin | wider |
| Typical voltage | below about 5 V | above about 6 V |
| Mechanism | strong field pulls electrons directly out of covalent bonds (field emission) | minority carriers gain enough energy to knock out more carriers by collision, which multiply |
| Temperature coefficient | negative (\( V_Z \) falls as temperature rises) | positive (\( V_Z \) rises as temperature rises) |
- In forward bias the Zener diode behaves like a normal silicon diode (about 0.7 V drop).
- In reverse bias only a tiny leakage current flows until \( V_Z \) is reached.
- At \( V_Z \), breakdown starts and current rises sharply while voltage stays nearly constant.
- Breakdown is not destructive as long as power \( V_ZI_Z \) stays below the rated \( P_{Z(max)} \).
- Between about 5 V and 6 V both mechanisms occur and temperature effects nearly cancel, so such diodes are very stable.
Key formula
\( P_Z = V_ZI_Z, \qquad I_{Z(max)} = \dfrac{P_{Z(max)}}{V_Z} \)
\( V_Z \) = Zener (breakdown) voltage (V), \( I_Z \) = Zener current (A), \( P_Z \) = power dissipated (W), \( P_{Z(max)} \) = rated maximum power (W).
Quick example
Zener diodes are sold by voltage and power, e.g. 5.1 V, 0.5 W. Such a diode can carry at most \( 0.5/5.1 \approx 98 \) mA in breakdown. They are used as voltage references and to protect circuit inputs from voltage spikes.
Common mistakes
- Saying a Zener diode is used in forward bias for regulation; it is used in reverse bias.
- Saying breakdown always destroys the diode.
- Swapping the temperature coefficients of Zener and avalanche breakdown.
How MU asks it
Numericals are not expected in this module (MU syllabus), so questions are theory with diagrams. Typical: "Explain the Zener diode and differentiate between Zener and avalanche breakdown" (5 marks). For full marks draw the symbol, explain heavy doping and thin depletion layer, give both mechanisms with a comparison table, and mention the temperature coefficient and the power limit.