Zener Diode
Push an ordinary diode too hard in reverse and it breaks down — a fault to be avoided at all costs. The Zener diode takes that same breakdown effect and turns it into a feature: a device deliberately designed to be operated in reverse breakdown, holding a rock-steady voltage across itself no matter what else is happening in the circuit.
A diode designed to break down
An ordinary diode is normally used only in forward bias, where it conducts readily; in reverse bias it is designed to block current, and if the reverse voltage is pushed too high it eventually breaks down — usually something to be avoided, since it can permanently damage the device. The Zener diode is a specialised diode manufactured specifically to be operated in reverse breakdown deliberately and repeatedly, without damage, because that breakdown behaviour is exactly what makes it useful.
Like any diode, it has two terminals: the anode and the cathode, with the circuit symbol showing the direction in which conventional current flows easily (from anode to cathode) when forward biased. In a Zener diode's typical application, however, it is deliberately connected the other way around — reverse biased — so that it operates in its breakdown region.
The characteristic curve
The current-voltage characteristic of a Zener diode has two distinct regions:
- Forward bias: the device behaves just like an ordinary diode — negligible current flows until the forward voltage reaches a small threshold, after which current rises steeply with only a small further increase in voltage.
- Reverse bias: initially, only a very small leakage current flows as the reverse voltage increases, just as with an ordinary diode. But once the reverse voltage reaches a particular value — the breakdown voltage, $V_Z$ — the current rises sharply. Crucially, as this breakdown current increases over a wide range, the voltage across the diode stays very close to $V_Z$, barely changing at all.
This near-constant voltage during breakdown, across a wide range of currents, is the entire basis of the Zener diode's usefulness: however much current happens to be flowing through it (within its rated limits), the voltage across its terminals stays essentially fixed at $V_Z$.
Minimum operating current
To stay in this useful, voltage-regulating breakdown region, a Zener diode needs a certain minimum current flowing through it continuously. If the current through the diode falls below this minimum operating current, the diode drops out of stable breakdown and its terminal voltage is no longer reliably held at $V_Z$ — the voltage-regulating behaviour breaks down (in the everyday sense) exactly when the electrical breakdown (in the technical sense) can't be sustained. Any practical circuit using a Zener diode as a voltage reference has to be designed so that the current through the diode never drops below this minimum, across the full range of conditions the circuit is expected to see.
The standard practical circuit
The standard way of using a Zener diode is to connect it in series with a resistor across a supply voltage, with the diode reverse biased. The resistor's job is to drop the excess voltage — the difference between the supply voltage and $V_Z$ — and to limit the current flowing through the diode to a safe and appropriate level. Provided the diode is kept in breakdown (current above its minimum operating current, and below its maximum safe rating), the voltage across the diode itself stays fixed at $V_Z$, regardless of moderate changes in the supply voltage — extra or reduced supply voltage simply changes the voltage dropped across the resistor instead.
Without the series resistor, nothing would limit the current once the diode entered breakdown — the current would rise without bound and destroy the diode. The resistor's role is exactly analogous to a current-limiting resistor for an LED: it protects the device while still allowing the diode to sit in its useful breakdown region.
Constant-voltage reference source
This series resistor-and-diode arrangement is the standard way of building a simple constant-voltage reference source. Even if the main supply voltage fluctuates somewhat — due to a battery discharging, or ripple from a power supply, for instance — the voltage across the Zener diode (and across anything connected in parallel with it) stays close to $V_Z$, provided the diode remains in breakdown throughout. This gives the rest of the circuit access to a stable, known reference voltage that other parts of the circuit can rely on, even though the raw supply feeding the circuit isn't itself perfectly stable.
Key takeaways
- A Zener diode is a diode deliberately designed to be operated in reverse breakdown, unlike an ordinary diode where reverse breakdown is normally avoided.
- It has two terminals, anode and cathode, and in its usual application is connected reverse biased.
- In reverse bias, negligible current flows until the breakdown voltage $V_Z$ is reached; beyond that, current rises sharply while the voltage across the diode stays close to $V_Z$.
- A minimum operating current must be maintained to keep the diode in stable breakdown and hold $V_Z$ reliably.
- The standard practical circuit places a resistor in series with the reverse-biased diode across a supply: the resistor drops the excess voltage and limits the current.
- The diode's terminal voltage stays fixed at $V_Z$ even if the supply voltage varies moderately, since the resistor absorbs the change instead.
- This arrangement is used to build a simple constant-voltage reference source for other parts of a circuit.