Explore the versatile functionality of Zener diode, which allow current to flow in both directions, making them a crucial component of electronic circuits.
This article is a detailed guide on Zener diodes. It covers their definition, explanation, and how they operate in reverse bias. We also delve into Zener diodes’ two breakdown mechanisms: avalanche and Zener. The article explains the circuit symbol, V-I characteristics, and specifications of Zener diodes. Furthermore, we provide insight into the various applications of Zener diodes and answer frequently asked questions to comprehensively understand this important electronic component.
Zener Diode Explanation
A Zener Diode is a type of semiconductor device, also called a breakdown diode, designed to work in the reverse direction. Special doping allows it to conduct electricity in the opposite direction when the voltage across its terminals is reversed and reaches a certain level, known as the Zener Voltage or knee voltage. This phenomenon is called the Zener Effect, a defining characteristic of Zener diodes.
Zener Diode Definition
Zener diode is a semiconductor device and it functions in reverse bias. Its junction is heavily doped. It is available in a wide range of Zener voltages (Vz), and certain types can be adjusted for variable voltage regulation.
How does Zener Diode work in reverse bias?
A Zener diode operates similarly to a regular diode when it is forward-biased. However, a small leakage current flows through the diode when it is reverse-biased. Current flows through the diode as the reverse voltage increases and reaches the predetermined breakdown voltage (Vz). The current stabilizes and remains constant across a wide range of voltages after reaching a maximum level determined by the series resistor.
Two types of breakdowns in a Zener Diode are Avalanche Breakdown and Zener Breakdown.
Avalanche Breakdown
Avalanche breakdown can happen in normal diodes and Zener diodes when they are exposed to high reverse voltage. The free electrons acquire sufficient energy and move at high speed when the PN junction is subjected to a significant reverse voltage. These high-speed electrons hit other atoms, causing more electrons to be ejected. This continuous process generates a large number of free electrons, leading to a rapid increase in electric current through the diode. If a normal diode experiences this sudden increase in current, it can be permanently damaged. However, a Zener diode can withstand avalanche breakdown and it has capacity to handle the sudden surge in current. You can observe Avalanche breakdown in Zener diodes at Zener voltage (Vz) greater than 6V.
Zener Breakdown
In a Zener diode, when a reverse bias voltage is applied, and it approaches the Zener voltage, the electric field within the depletion region becomes strong enough to attract and remove electrons from their valence band. Due to the intense electric field, these valence electrons get energized and break free from their parent atoms. This happens in the Zener breakdown region. Even a minor increase in voltage in this region causes a sudden surge in electric current.
Circuit Symbol of Zener Diode
Depending on their power dissipation needs, Zener diodes are manufactured in various packaging options. The Zener is also designed for high-power applications. A surface mounting package is available to mount the Zener on an SMD PCB. The Zener diodes are commonly enclosed in a small glass package with a band indicating the cathode side. The symbol of Zener diode is shown in the image below,
symbol and package outlines
The symbol of a Zener diode is similar to that of a regular diode, with a unique addition. It has a triangle or arrowhead that points towards the cathode side (the side with the band) of the diode with two perpendicular lines at the cathode end. There are two lines, one extending upwards and the other extending downwards. These lines specify the Zener diode’s behavior and distinguish it from other diodes in circuit diagrams. The Zener diode symbol provides a clear visual representation for engineers and technicians to easily identify its presence in a circuit.
V-I Characteristics of Zener Diode
The following diagram illustrates the V-I characteristics.
The small current flows through the Zener when it is reversed bias. We can divide the V-I characteristics into two parts;
(i) Forward Characteristics
(ii) Reverse Characteristics
Forward Characteristics
The graph shows the forward characteristics of a Zener diode in the first quadrant, which is almost similar to that of a P-N junction diode.
Reverse Characteristics
When a Zener diode is subjected to a reverse voltage, a small current called reverse saturation current (Io) flows through it due to the thermally generated minority carriers. As the reverse voltage increases, the reverse current remains relatively constant until it reaches a certain value of reverse voltage. At this point, the reverse current increases rapidly and sharply. This is an indication that the Zener breakdown voltage (Vz) has been reached. The voltage at which this breakdown occurs is known as the Zener voltage and is denoted by Vz.
Zener Diode Specifications
Here are some specifications that are commonly used for Zener diodes:
- Zener/Breakdown Voltage: The Zener or reverse breakdown voltage can range from 2.4 V to 200V, with a maximum of 47V for surface-mounted devices.
- Current Iz (max): It is the maximum current at the rated Zener Voltage (Vz – 200μA to 200 A)
- Current Iz (min): It shows the maximum zener current at the rated Zener Voltage. The maximum zener current is in the range of 200μA to 200 A.
- Power Rating: The maximum power that a Zener can handle is equal to the product of its voltage and current.
- Temperature Stability: Diodes with a voltage rating of around 5 volts tend to exhibit the best stability
- Voltage Tolerance: ±5%
- Zener Resistance (Rz): It is the resistance exhibited by the Zener.
Application of Zener Diode
Below are the various applications.
Voltage Regulator:
Zener diodes are often used as a Shunt voltage regulator to control voltage across small loads. To achieve this, the Zener diode is connected parallel to the load, which makes it reverse bias. Once the Zener diode reaches its knee voltage, the voltage across the load will remain constant. The breakdown voltage of Zener diodes remains constant for a wide range of currents.
Over-voltage Protection:
The Zener diode can prevent a short circuit that occurs when the input voltage is higher than the Zener breakage voltage, causing the voltage across the resistor to drop.
Zener diode in clipping circuits:
Zener diodes are used in clipping circuits to limit one or both half cycles of an AC waveform.