Within the electrical and electrical world exists a myriad of different components each with their own unique capabilities.

One of these components is the Diode. 

But, there isn’t just one type of diode out there. It has a number of different variations, and the two commonly used in circuits and devices are the PN Junction and Zener Diodes.

So, what is the difference between a PN Junction and Zener diode? The main difference between a PN Junction diode and a zener diode, is that the PN junction only allows current to flow in one direction, whereas the zener diode allows current to flow in both directions.

Other notable differences include;

• Schematic Symbol
• Materials used
• Doping level and 
• Application

This article shall take a close look at these two types of diode, as well as a more in-depth look at the key differences. 

What are diodes?

Before we dive into looking at the differences between a PN Junction and Zener diode, it will first help to learn about the diode (however if you are well versed with diodes, you can skip this section). 

Resistors help limit current, capacitors store charge, switches turn circuits on and off, etc.

So what is a diode and what is its purpose?

A diode is a device which is constructed using semiconducting material and only allows current to flow in one direction. 

It acts like an open switch (no current flow) in one direction, and a closed switch (allows current to flow) in the other direction. 

Diodes are passive devices and have a polarity ( have a positive and negative terminal and need to be wired the right way in a circuit).

The positive terminal of the diode is known as the anode, and the negative terminal is known as the cathode. 

Below is an image of a conventional diode along with its schematic symbol highlighting the terminals. 

Different types of diodes

The diode has a number of different variations, each having unique characteristics that will set them apart from their peers.

These unique characteristics will also make each type of diode suitable for a certain type of application.

The different types of diode include;

• PN Junction Diode
• Zener Diode
• Tunnel Diode
• Schottky Diode
• Varactor Diode
• Diac
• Triac
• SCR
• Light Emitting Diode (LED)
• Photodiode

This article shall take a closer look at the PN Junction and Zener diode and the key differences between them.

What is a PN junction diode

Let’s start our journey with the PN Junction Diode.

So what exactly is this diode all about? 

They are aptly named PN Junction diodes because they are composed of a p-type and n-type semiconducting material which are fused together and form a junction.

The P-region and N-region are separated by a depletion region. 

These diodes are also often referred to as rectifier diodes because they are commonly utilized in applications where rectification is necessary (have the ability to convert an alternating current (AC) into a direct current (DC)). 

It is a two terminal device with positive (anode), and negative (cathode) terminals and has the same schematic symbol we saw earlier. 

How a PN junction diode works

PN junction diodes are the simplest of the semiconducting devices and have the ability to only allow current to pass through them in one direction.

This can only happen when they are forward biased.

So what exactly does it mean to be forward biased?

When we talk about diodes, there are three possible modes of operation, or biases;

• Zero Bias
• Forward Bias
• Reverse Bias

Zero bias occurs when there is no voltage applied across the diode.

Forward bias is when a voltage is applied to the diode. However this is when the positive and negative terminals match up with the positive and negative terminals of the voltage source respectively (positive to positive, and negative to negative).

Reverse bias is when a voltage is applied to the diode but in the reverse direction (positive to negative, and negative to positive). 

IV characteristics of diodes

Diodes do not have a linear relationship with voltage, but rather have an exponential  Current (I) – Voltage (V) relationship.

This IV relationship is known as the IV Characteristic and is given in the form of a graph as seen below.

The IV characteristic shows us how the diode performs under zero, forward and reverse bias conditions. 

How a PN junction operates in the forward bias

When discussing the operation of a PN junction diode, we are only concerned about the zero and forward bias conditions (as they can only allow current to flow in the forward direction).

So how does a PN Junction diode work?

To allow current to flow in the forward direction, the diode needs to first be forward biased.

But first, a sufficient amount of voltage needs to be applied in order for current to flow freely. The voltage level required is known as the knee or forward voltage.

For PN junction diodes this is usually 0.3V (Germanium), and 0.7V (Silicon).

When the supply voltage equals (or exceeds) the forward voltage, current starts to flow through the PN junction diode. 

A little increase in voltage after the forward voltage sees a large increase in current.

When a reverse voltage is applied to the PN junction diode making it reverse biased, its depletion layer increases thereby increasing its overall resistance and blocking current flow. 

What is a zener diode  

Next up we have the Zener Diode.

So what awesome powers does this type of diode hold?

Let’s take a look.

The zener diode is also constructed using semiconducting material like the PN Junction diode. However it has the ability to conduct current in the forward and reverse direction.

It has the ability to conduct in the reverse direction thanks to a special heavily doped p-n junction. 

Zener diodes are also two terminal devices with an anode (+) and cathode (-), however their schematic symbol is different. 

The zener diode was discovered by Clarence Zener who discovered its electrical properties and is how it got its name.  

How a zener diode operates

While the PN junction is only limited to allowing current to flow in the forward direction, the zener diode is capable of conducting current in both the forward and reverse direction.

When operating in the forward bias, the zener diode has the same operation as the PN junction diode. 

So it requires a sufficient voltage which equals the forward or knee voltage before it can allow current to start flowing in the forward direction. 

However, if a reverse voltage is now applied to the zener diode thus causing it to be reverse biased its operation changes. 

If we look at the IV characteristic graph from before (in the third quadrant), we can see what happens to current when a reverse voltage is applied.

Initially there is a small leakage current that flows through the zener diode. 

We know that in the forward bias there is a critical voltage known as the knee or forward voltage, which is the amount of voltage needed before current can flow.

In the reverse bias, this voltage is known as the breakdown voltage (Vz). 

When the reverse voltage equals, or is greater than the breakdown voltage, a large reverse current flows through the zener diode. 

Key differences between a PN junction diode and a zener diode

Now that we have had a close look at the PN Junction and Zener diode, we can delve into their key differences.

Difference #1 between a PN junction and zener diode: Direction of current flow

The first most obvious and main difference between these two diodes is direction of current flow.

A PN junction diode only has the ability to conduct current in the forward direction (when the voltage applied across its terminals is greater than the forward voltage of the diode).

Zener diodes on the other hand have the capability to conduct current in the forward and reverse direction. 

In the forward bias zener diodes operate the same as PN junction diodes. However, when reverse biased, a small leakage current flows until its breakdown voltage is matched or exceeded, which then sees a steep rise in current. 

PN junction diodes are always used in the forward bias, whereas zener diodes are commonly used in the reverse bias. 

Difference #2 between a PN junction and zener diode: Schematic symbol

There are many different types of components that exist in the electrical and electronic world. Each of them has its own unique purpose.

But, when designing circuits, or analyzing circuit diagrams we need a way of distinguishing between the different types of components (otherwise things would get confusing real fast).

This is where schematic symbols play a crucial role. 

They help us identify particular electrical/electronic components as each of them will have their own unique schematic symbol. 

Since the PN junction and Zener diode are two variations of the diode, and have separate functionalities, each gets its own schematic symbol.

This will help us distinguish between a PN junction and zener diode in circuit diagrams. 

Difference #3 between a PN junction and zener diode: Doping level

Diodes are semiconducting devices that are constructed using, well, semiconductors. 

But what exactly are semiconductors and doping?

Semiconductors are materials which have properties of both a conductor as well as an insulator. 

Doping of semiconductors is a process used to add or remove the number of electrons and holes in semiconductors.

This process alters the electrical, optical and structural properties of semiconductors. Increasing the doping level of a semiconductor makes it act more like a conductor. 

This is the next difference between a PN junction and Zener diode is their doping levels. 

A PN junction diode has lower doping levels compared to zener diodes which are heavily doped. 

Difference #4 between a PN junction and zener diode: Materials used

The next difference is the materials that each type of diode is composed of. 

Common materials used to construct diodes include Silicon, Germanium and Gallium Arsenide. 

However, PN junction diodes will only be constructed using a single material, while zener diodes will use a mix of these materials. 

Difference #5 between a PN junction and zener diode: Applications

As you have just seen, while they are both diodes, there are some key differences between the PN junction and zener diode. 

Each has their own unique abilities and is why they share separate applications (which is our next and final key difference).

The PN junction diode is mostly used in the applications seen below;

• Rectification
• Voltage-Controlled oscillator
• Mixing signals
• Lighting systems
• Detection of signals 
• Solar cells

Whereas zener diode can be seen in applications which include;

• Voltage Regulation
• Reference elements
• Surge Suppressors
• Clipping circuits

PN junction vs Zener; which is better?

So, you need a diode for your circuit. 

What do you choose, the PN Junction or Zener diode?

Which is the better option?

There is no right answer to this as one diode is not better than the other. Picking the right diode comes down to the needs of your application.

Each diode is used for different applications according to how its characteristics best help.

For example, you will not be able to use a PN junction diode for a voltage regulation application, as it will not be able to perform the job.

A zener diode is the best option here.

So, it really depends on the needs of your application when selecting the right diode.

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However, a normal diode can only allow current to flow in one direction, whereas the zener diode has the ability to conduct current in forward and reverse directions.

The advantage of a zener diode is that it is able to maintain a stable reference voltage for a wide variety of voltages. 

Because of these unique abilities, the zener diode is utilized in a wide range of applications. 

But, what is a zener diode used as?

The zener diode is used in applications which include;

• Voltage Regulation
• Overvoltage protection
• Clipping circuits

This article shall take a closer look at the zener diode and its applications mentioned above. 

What is a zener diode

Before we take a look at its applications and what the zener diode is used as, let’s take a quick look at the zener diode and how it operates. 

As mentioned at the start, the zener diode is a variation of the commonly used electronic component being the diode. 

It is a semiconducting component which consists of heavily doped P-N Junction.

Unlike a normal diode (which can only conduct current in the forward direction, or when forward biased), a zener diode has the capability of conducting current in the forward and reverse direction (forward and reverse bias).

Other unique characteristics include being able to conduct a large current at a certain voltage known as the breakdown voltage, and being able to provide a stable voltage for a wide range of voltages.

Later you will see how these characteristics come in handy in certain applications.

How does a zener diode work?

So, how does a zener diode conduct current in both directions?

Let’s take a look.

A zener diode can be biased in three ways;

• Zero Bias
• Forward Bias and
• Reverse Bias

A zener diode is said to have zero bias when there is no external voltage applied to it. 

When forward biased, the zener operates like a normal diode. In the forward bias, a zener will start to conduct current when the voltage across it is between 0.3 or 0.7 volts.

In the forward bias, the zener diode’s positive terminal (anode) is connected to the positive terminal of a voltage source, and its negative terminal (cathode) is connected to the negative terminal of the voltage source. 

Now, if we reverse the voltage across the diode, the zener diode is in the reverse bias mode. 

When reverse biased, a small leakage current flows through the zener diode. This current remains close to zero until the breakdown voltage is reached. 

When the breakdown voltage is reached, current starts to flow in the opposite direction. 

For example, a 6V Zener diode tells us the breakdown voltage value is 6V. It will block current to flow in the reverse direction until the breakdown voltage of 6V is reached. 

V-I Characteristics of a zener diode

The operation of the zener diode can be best depicted in a graph known as the V-I characteristic graph as seen below. 

This graph shows us what happens when the zener is forward and reverse biased. 

In the first quadrant, the zener diode functions like a normal P-N junction diode.  

The third quadrant shows us what happens when we reverse the voltage across the zener diode. Initially there is the small leakage current until the zener breakdown voltage is reached which then sees the current start to rise.

Zener diode in the reverse bias

The zener diode behaves like a normal diode in the forward bias, however its true superpowers lie in the reverse bias. 

As we just learnt, a small leakage current flows through the zener diode when it is reverse biased. When this voltage reaches a critical voltage (breakdown voltage), current starts to flow in the reverse direction and will reach its maximum after a certain time.

This maximum current is controlled by a series resistor. Once the current has reached its maximum, it will remain constant for a wide range of voltages.

There are two major breakdowns that occur when the zener diode is reverse biased;

• Zener Breakdown and
• Avalanche Breakdown

Zener breakdown

The first of the two breakdowns is the zener breakdown. 

This happens when the reverse voltage reaches the zener voltage (or breakdown voltage). The electric field within the depletion region strengthens and starts to pull electrons from their valence bands.

Valence electrons that gain a sufficient amount of energy break free from their parent atom.

In the zener breakdown region, a small increase in voltage results in a rapid increase in current. 

Avalanche breakdown

The avalanche breakdown in a zener diode occurs at a high reverse voltage (higher than the zener voltage) after the zener breakdown. 

When this high voltage is applied to its PN junction, free electrons gain energy and start to accelerate.

These free electrons start to collide with other atoms knocking more electrons free. These constant collisions cause a large number of free electrons resulting in an increase in current. 

This avalanche breakdown exists in the zener diode at a zener voltage greater than 6 volts. 

What a zener diode can be used as; 3 common applications of a zener diode

Ok, now that we have learnt a bit about the zener diode, let’s take a look at what it can be used for in a variety of applications. 

What a zener diode can be used as; Application #1: Voltage Regulation

First on the list, and one of the most common applications where a zener diode is used is Voltage regulation.

Voltage regulation is when a system is able to generate a constant (or near constant) output voltage level for a variety of input voltages.  

A voltage regulator is an electrical/electronic device whose task is to provide this constant voltage regardless of any changes of voltage at its input. 

They are an essential component as they maintain voltages within the desired ranges of the system without damaging it. 

How a zener diode is used as a voltage regulator

One of the main abilities of the zener diode is to maintain a stable voltage across it as the input voltage may vary (which is exactly what is needed in voltage regulation). 

This happens at the zener voltage in the zener breakdown region. 

To get a desired output voltage, you would have to select a zener diode with the breakdown voltage that matches that value. 

So, for example, if a voltage of 5V (or less) is required at the output, a zener diode with a breakdown voltage of 5V is required. 

The zener diode would need to be connected in parallel with the load as seen below.

As you can see, the zener diode is connected so that it is reverse biased (so that it can enter the zener breakdown region). 

Since the load is connected in parallel to the zener diode, the voltage across it equals the zener breakdown voltage (in this instance 5V).

If the voltage source increases, the output voltage is maintained at the desired 5V.

This is why zener diodes are perfect for voltage regulation applications. 

What a zener diode can be used as; Application #2: Over-voltage protection

Next up of the applications of a zener diode is Overvoltage protection.

Before we look at what overvoltage protection is, we need to learn about overvoltage. 

So what exactly is overvoltage? 

In general a circuit will have three voltage ratings; minimum, recommend/standard, and maximum.

In the example in the previous section we wanted an output voltage of 5V (which we can call our standard voltage). 

We can say the minimum voltage is 4.5V and the maximum voltage is 5.5V.

Overvoltage occurs when the voltage levels in a circuit (or part of a circuit) exceeds the maximum voltage level which it was designed for. 

It can appear as a spike, transient or could be permanent. 

So, if the voltage levels in the circuit exceeded 5V to say 6V, this would be considered overvoltage. This is an unwanted scenario as it could have the potential of damaging components within the circuit.

Overvoltage can be a result of internal as well as external conditions which include, wiring errors, failure of electrical insulation/isolation, varying circuit loading, or poor voltage regulation of a power source. 

Overvoltage protection is a means of protecting the electrical/electronic system from these overvoltage scenarios mentioned above. 

How a zener diode is used for over-voltage protection

A zener diode is a cost-effective means of overvoltage protection. 

This comes down to the zener’s capability to accept a large current at a certain voltage (breakdown voltage), and also being able to maintain a stable output voltage for a wide variety of input voltages (as we just saw earlier).

At the breakdown voltage the zener diode ‘opens up’ and accepts an increase in current. This is the exact trait that is required for overvoltage protection.

The circuit setup for using a zener diode for overvoltage protection is similar to the one used in voltage regulation but with the addition of a series resistor. 

Say our output load operates at a voltage between 10 and 12 volts, we would need to select a zener diode rated at a breakdown voltage of 12 volts.

If there is a spike in voltage at the input above 12 volts, the zener diode will protect the load by limiting the output to only 12 volts.

What a zener diode can be used as; Application #3: Clipping circuits

Clipping circuits are the next major application where a zener diode can lend its helping hand. 

What are clipping circuits and why are they necessary? 

A clipping, or clipper circuit is a circuit with the purpose of stopping a signal from exceeding a reference voltage level. 

They remove a small part of the input signal which is usually intended for transmission and are most commonly used in Alternating Current (AC) circuits. 

How a zener diode used as a clipper

Clipping circuits consist of linear and non-linear components and none that store energy. 

One of the most widely used clipping circuits uses a zener diode as seen below.

The operation of a zener diode clipping circuit goes as follows; during the positive cycle of the AC waveform (which has a peak of 5V), the zener diode is reverse biased and thus does not conduct. 

The waveform is unchanged at the output. 

During the negative cycle of the waveform, the zener diode is forward biased and is now conducting current. 

The negative cycle of the waveform is clipped off at -0.7 volts due to forward voltage drop of the zener diode. 

So when forward biased, it clips the voltage, and when reverse biased, it blocks the current. 

Is a zener diode used in AC or DC applications?

As we just saw, the zener diode is a very versatile component with unique capabilities that can be utilized in many different electrical and electronic applications.

The zener diode can also be used for both DC (Direct Current) and AC (Alternating Current) applications.

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