Last Updated on October 22, 2023 by Kevin Chen
Zener diodes are a type of specialty diode that can be operated below its reverse-voltage threshold, or breakdown, to produce a stable, predictable, fixed voltage at an intermediate current. In other words, zener diodes can operate in the non-linear region and still perform a useful function.
Zener diodes are named for William Shockley, inventor of the transistor and fellow researcher reference as “The Traitorous Seven”. Their ability to operate in the non-linear region makes them useful for many applications. Read on to learn more about how Zener diodes work and their common uses.
How Zener Diodes Work?
What is the simple basic principle of a zener diode? You may ask.Zener diode working principle is quite straightforward. In the forward bias, the diode will allow current to flow through. In the reverse bias, it will block current from flowing.
So, how does this happen? Let’s take a look at the cross-section of a zener diode.
The PN junction of a zener diode is composed of two types of semiconductor materials, usually n-type and p-type. The n-type semiconductor material is on the anode side, or negative side. It is doped (impregnated) with impurities that have more electrons than protons to give it a net negative charge. The p-type semiconductor material is on the cathode side, or positive side. It is doped with impurities that have more protons than electrons to give it a net positive charge.
The region between these two materials, called the depletion zone, has no free electrons at all. This means there are no mobile electrons to conduct current in this region and no holes (positively charged vacancies) to carry current through this region either.
The depletion zone is responsible for zener breakdown behavior in a zener diode: when voltage across the diode exceeds its breakdown voltage, current begins flowing through the depletion zone and around the circuit path where it normally would not be able to flow because there are no conducting electrons or holes present in this region.
How does a zener diode work in reverse bias?
The depletion zone of a zener diode is much larger in reverse bias than in forward bias. When the voltage across the diode is negative (reverse bias), the depletion zone can extend far into the p-type and n-type semiconductor materials. This means that there will be no current flow through the p-n junction until the applied voltage exceeds a certain level, called the breakdown voltage.
The breakdown voltage for a zener diode is higher than it would be for a regular diode, because there are no free electrons or holes to carry current through this region.
Avalanche breakdown in zener diode
The avalanche breakdown in zener diode occurs when the reverse breakdown voltage is exceeded. The avalanche breakdown in zener diode has a sharp threshold and is characterized by thermal runaway caused by ionization avalanche and runaway electron-hole pair generation.
Zener diodes are designed to operate below the avalanche breakdown. This design ensures that the diode can easily withstand a sudden spike of current- a factor that leads to severe damage of the diode.
Circuit symbol of Zener diode
Zener diodes are packaged and presented in many different ways. The diagram below shows a circuit symbol of the zener diode.
From the photos, you can see that the zener diode is packaged in a glass case with a metal end cap. The glass is typically filled with an inert gas to prevent the formation of internal electric fields that could become high enough to cause damage to the device.
The majority of zener diodes are made from silicon, however, gallium arsenide and germanium are also used. Zener diodes are available in many different power ratings, depending on the application.
Understanding Zener Diode Terminology
When it comes to the zener diode usage and working principle, there are several terms that you should know. Let’s look at some of them and how they affect the performance of the diode.
-Zener region: This is the region between the zener voltage and the breakdown voltage. It is also called the zener plateau or zener range.
-Zener voltage: The zener voltage is the breakdown voltage of a diode that produces a desired current flow in the diode (also referred to as clamping voltage).
-Holding current: This is also called standby or leakage current. It represents the constant current that flows through a zener diode even when there are no changes in its input or output voltages. This type of conduction occurs because of an inherent property of diodes—the fact that they are constructed so that they conduct electricity even if there are no external forces acting on them.
-Zener current: This is the current that flows through a zener diode when it is in its zener region. The current may be either constant or non-constant, depending on the application.
-Power dissipation: This refers to the amount of power consumed by a device during operation. It is measured in watts and depends on the voltage and current that are applied to it.
-Zener impedance: This represents the opposition that a diode offers to a certain type of flow in it. For example, if there were no zener impedance in a zener diode, then any amount of current could flow through it without any difficulty or opposition from the device itself. However, this does not happen because diodes have some level of resistance against current flow as well as other types of electrical forces such as voltage and power dissipation. This type of opposition is referred to as “zener impedance.”
-Voltage coefficient: This refers to how much a diode’s forward voltage changes with respect to changes in an input voltage applied to it. A typical semiconductor diode has a voltage coefficient value between 0 and -0.5 volts per 1-volt change in an input voltage (for PN junction diodes).
What is the difference between avalanche breakdown vs zener breakdown?
Avalanche breakdown occurs when the current through a diode increases to a point where it causes the diode to break down, or in other words, to become forward-biased. Zener Breakdown occurs when the current through a diode increases so much that it causes the voltage across it to go above its zener voltage.
Applications of a Zener Diode
Zener diodes are useful in a number of applications.Let’s highlight some of them and you will find out which one fits you.
Zener diode as a voltage regulator
This is among the most common applications of zener diodes. They are used in voltage regulator circuits to maintain a constant voltage across load.
For example, if you may have an LED connected across a battery and you want to make sure that the maximum voltage that it sees is no more than 3.3V, then you can use zener diode as a clamping device. The LED will be protected from any higher voltage spikes.
Zener diode as an overcurrent protection
This application of zener diodes is not very common but still they are used in some applications where overcurrent protection is required. For example, they are used in transistor amplifiers to protect transistors from damage due to large currents through them.
Zener diodes for clipping circuits
Zener diodes are also used to limit some parts and elements of electric circuits. This is done by clipping the output of the circuit. For example, they are used to clip the unwanted positive and negative values of signals to zero.
Zener diodes for voltage reference
Zener diodes are also used as voltage references. They act like a standard source of fixed voltage in many applications.
For example, they are used as references in digital circuits and also in some analog circuits such as voltage comparators etc. In digital circuits, they are used to provide fixed voltages across different parts of the circuit so that they can be compared with each other by using logic gates such as AND gate or OR gate etc.
Zener diodes for oscillator applications
Zener diodes can be used as an active element in oscillator circuits just like transistors or BJT’s can be used. They can generate regular pulses at a very high frequency when biased properly with proper biasing current and resistor values etc. The frequency of these pulses is determined by the breakdown voltage measured on the zener diode which is nothing but a breakdown region inside the Zener diode itself.
How to Calculate the Forward Voltage of a Zener Diode
The voltage a zener diode can breakdown at is dependent on the amount of current flowing through it. For example, a 1A zener diode may breakdown at 10 volts while a 10mA diode may breakdown at 1 volt. When choosing the right diode for a particular application, it’s important to know what voltage the diode will breakdown at so you can select a zener diode with the right breakdown voltage.
You can calculate the breakdown voltage of a zener diode using this equation: Where V is voltage across the diode, I is the amount of current flowing through the diode, and R is the zener impedance. As you can see from the equation, the breakdown voltage depends on the current. This means that the breakdown voltage varies based on the current running through a diode. This is why you need to know the current running through a diode when choosing a zener diode for a circuit.
Zener diode specifications
When buying a zener diode, it is important to know its specs. Zener diode manufacturers design and make their products using different specs which include:
-Zener voltage: As we have already defined, this is the breakdown voltage of the diode. It is important to choose a zener diode with the right breakdown voltage for your application.
-Zener current: This is the amount of current flowing through a zener diode when it’s under rated breakdown voltage. The current can vary from 3mA to 1A and beyond, depending on the manufacturer and rating of the zener diode.
-Maximum power dissipation: This is one of the most important specifications for choosing a zener diode, since it determines how much heat will be dissipated by a particular diode. The maximum power dissipation depends on the breakdown voltage and maximum current running through the device.
-Maximum reverse voltage: This indicates how much reverse voltage a particular device can withstand before breaking down or shorting out. -Maximum forward current: This indicates how much forward current a particular device can handle before breaking down or shorting out.
-Maximum junction temperature: This is an important specification that determines how hot a particular device will get while in operation under rated conditions (breakdown voltages, currents etc.)
-Zener Resistance (Rz): This is the electrical resistance of a zener diode.
-Voltage tolerance: This is the tolerance of the voltage across a particular zener diode.
Where to buy zener diodes?
Will your application require the use of zener diodes? You should strive at buying these diodes from reputable sources. By this we simply mean reputable zener diode manufacturers and suppliers.
This way, you will be assured that the diodes are of high quality, tested for reliability, and guaranteed to be free from defects. If you have a hard time trying to find a reputable zener diode manufacturer and supplier, you can always use the services of sourcing agents.
They are reputable companies that will assist you in sourcing high-quality zener diodes at competitive prices.
For example if you want to buy zener diodes in China, we at ICRFQ can do the sourcing for you. We will find a reliable and competent zener diode manufacturer and supplier in China that can supply you with zener diodes of your choice.
Final Words
Zener diodes are very useful in many applications. However, it is important to understand their limitations so you know when they are the best choice. Zener diodes are particularly useful when you need to regulate voltage. They are a great choice for applications where you need to avoid excess voltage or where you need a consistent voltage.
With the information in this article, you should be able to choose the best zener diode for your application.
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