A Zener diode acts like a regular diode, in that it lets current flow in only one direction, but it makes an exception. If the voltage in the reverse-bias direction is above a certain value, called the breakdown voltage, then the Zener diode allows current flow. Zener diodes are often used for voltage regulation, where an unstable or time-varying signal is turned into a near-constant voltage. Zener diodes work well for this purpose when placed in reverse bias, since they only allow current to flow when the voltage is above the breakdown voltage.
Like a regular P-N junction diode, the Zener diode has two terminals, called the anode and the cathode. Here is a schematic symbol:
To help remember which terminal is which, note that the “A” of Anode looks like a triangle and the of Cathode looks like a vertical bar (also referred to as a “k” bar or a "t" bar). This symbol differs from a regular diode because the line at the end of the arrow is slightly bent on each side.
The Zener diode has three main modes: forward biased, reverse biased, and breakdown/avalanche in reverse bias.
Forward Bias: When the voltage on the anode is higher than the threshold "knee voltage" on the cathode (~0.7 V for a silicon diode), then the diode is forward biased and it conducts current. When the diode is forward biased, the current flows in the direction of the triangle: from the anode toward the cathode. Although diodes can be considered a short circuit when forward biased and an open circuit when reverse biased, this is an ideal. In reality, when a diode is forward biased, it conducts as much current as the external circuitry demands, and adjusts its internal resistance so that the voltage drop across it is always 0.7 volts, the knee voltage.
Reverse Bias (Before Breakdown): When a diode is reverse biased and below the breakdown voltage, it has a very high resistance so it conducts almost no current.
Breakdown/Avalanche: After the breakdown voltage, a Zener diode conducts easily. When the voltage on the anode is more negative than the voltage on the cathode, and the difference is larger than the breakdown voltage, a zener diode conducts as much current as the external circuitry demands and adjusts its internal resistance so that the voltage drop across it is always the breakdown voltage. This mode of operation is unique to Zener diodes and is called avalanche or breakdown mode. Unlike a normal diode, Zener diodes are intended to operate beyond the breakdown voltage and are not damaged until much higher voltages are reached. Zener diodes are designed to have specific breakdown voltage values, often on the range of 5.6 V but can often be much higher when Zener diodes are used as part of a high-voltage regulator.
The main parameters for a diode are its threshold voltage (a.k.a knee voltage) and its breakdown voltage. The knee voltage for silicon diodes is about 0.7 volts, which is due to the properties of silicon when doped to form P-N junctions. Almost all diodes are made of silicon, except when specific other characteristics are needed (e.g., germanium diodes have lower threshold voltages around 0.3 volts).
The breakdown voltage of a Zener diode is a second major parameter. Unlike normal diodes, this parameter is precisely controlled and is important to how the diode functions in practice.
These parameters may be understood by considering the Voltage-Current response curve, shown below. At breakdown, current suddenly flows after nearly no current. Likewise, at the knee voltage, current starts to flow easily, with only a little resistance.
Zener diodes are constructed similarly to normal diodes, but with some key differences. As with a regular diode, a zener diode is made of P and N material with a junction between them. The P material is connected to the anode, and the N material is connected to the cathode. These form a "depletion region" that works like a one-way valve: current flows fairly easily in one direction, but acts as a barrier to current in reverse bias. However, Zener diodes can conduct under reverse bias using two mechanisms: the Zener effect and avalanche breakdown.
Forward Bias: When a positive voltage is connected to the P material and a negative voltage is connected to the N material, the voltages push the materials majority carriers (holes for the P; electrons for the N) toward the junction. This push shrinks the depletion region until it disappears and then current can flow. When the push is hard enough, namely greater than the knee voltage threshold, the diode becomes forward biased and current flows.
Reverse Bias (Below Breakdown): When a Zener diode has a negative voltage attached to the P material and a positive voltage attached to the N material, the voltages pull the majority carriers away from the junction. If the voltages are weak, then the majority carriers will not move far, because they are attracted to each other and want to stay near the junction. As this occurs, the depletion region stretches wider around the junction but does not break.
Breakdown (Avalanche): When the voltages are strong enough, namely greater than the breakdown voltage, they overcome the mutual attraction that pulls them toward the junction and break free. Avalanche breakdown occurs when enough voltage is present that free electrons have enough energy that their collisions break electron-hole pairs. These collisions free more electrons, causing more collisions, and forming an electron "avalanche" that allows current to flow. These collisions collapse the depletion region in the P-N junction, allowing current to flow in reverse bias. Avalanche breakdown also occurs in regular diodes, but is typically not controlled and damages the diode. By comparison, Zener diodes are built to withstand current flowing in reverse bias and the avalanche is controlled: current flows easily with increased voltage, but does not entirely short out the diode.
Breakdown (The Zener Effect): The Zener effect typically occurs under 5.6 V and occurs through a special mechanism known as quantum tunneling, where electrons "jump" from one side of the junction to the other. This is a special effect that occurs due to the increased electric field caused by pulling the majority carriers away from the junction.