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What is a Zener diode and how does it work?


Have you ever wondered why we operate a Zener in reverse bias unlike normal diodes that are operated in forward bias? This is because Zener diodes are meant to ‘break down’. Most of us are familiar with general-purpose and rectifier diodes. However, there are several other types of diodes that are designed for special purposes. One of these is the Zener diode. So, what is a Zener diode and how it is different from a typical diode?

What is a Zener diode?

A Zener diode is a silicon pn junction device that allows current to flow not only in the forward direction like a typical silicon or germanium diode, but also in the reverse direction if the voltage is greater than the breakdown voltage known as Zener knee voltage or simply Zener voltage, named after Clarence Melvin Zener, the discoverer of this electrical property.

The schematic symbol for a regular diode has a straight line representing the cathode, while the Zener diode has a bent line that resembles the letter Z (for Zener). It makes a lot of sense, right?

How does a Zener diode operate?

Zener diodes act like normal diodes when forward-biased. However, they are designed to allow current to flow in reverse once the reverse voltage equals its rated Zener Voltage. Unlike ordinary rectifier diodes, which are never intended to be operated at or near breakdown, a Zener diode is designed to operate in the breakdown region. Breakdown of a diode happens when you apply a reverse bias voltage across the diode.

A Zener diode operating in breakdown acts as a voltage regulator because it maintains a nearly constant voltage, which is equal to the Zener voltage, across its terminals over a specified range of reverse-current values. This constant voltage drop across the Zener diode produced by reverse breakdown is represented by a DC voltage symbol.

Avalanche and Zener Breakdown

To understand more how Zener diodes operate, let’s look at two types of reverse breakdown in a Zener diode: avalanche and Zener breakdown. The avalanche effect occurs in both rectifier and Zener diodes at a sufficiently high reverse voltage. On the other hand, Zener breakdown occurs in a Zener diode at low reverse voltages. A Zener diode is heavily doped to reduce the breakdown voltage. This causes a very thin depletion region. As a result, an intense electric field exists within the depletion region. Close to the Zener breakdown voltage, the field is sufficiently able to pull electrons from their valence groups and create current.

Zener diodes with breakdown voltages of less than approximately 5 V operate typically in Zener breakdown. Those with breakdown voltages greater than approximately 5 V operate typically in avalanche breakdown. Both types, however, are called Zener diodes. Zeners are commercially available with breakdown voltages from less than 1 V to more than 250 V with specified tolerances from 1% to 20%.

Breakdown Characteristics

As the reverse voltage (VR) is increased, the reverse current (IR) also increases until it reaches the Zener knee current (IZK). This time, the breakdown effect begins. The Zener impedance (ZZ), which is the internal Zener resistance, begins to decrease as the reverse current increases rapidly.

From the bottom of the knee, the Zener breakdown voltage (VZ) remains relatively constant although it increases slightly as the Zener current (IZ), increases. VZ is usually specified at a value of the Zener current known as the test current.

Zener Diode Specifications

To ensure proper operation of Zener diode in a circuit, we have to keep in mind these important specifications.

1. Zener Voltage (VZ)
The breakdown voltage,commonly called the Zener voltage, is the reverse-biased voltage that causes the diode to conduct current. Breakdown voltages usually range from 2.4 V to hundreds of volts.

2. Test Current (IZ)
For each Zener diode, the Zener voltage (VZ) is measured at a specified Zener test current (IZ). For example, the Zener voltage for a 1N4732A ranges from 4.465 to 4.935V with a typical value of 4.7V at a test current of 53mA.

3. Knee Current (IZK)
There is a minimum current required to keep the diode in breakdown for voltage regulation. Typical values are around 0.25 to 1mA for a 1 watt Zener diode. If this current is not reached, the diode will not break down sufficiently to maintain its rated voltage.

4. Maximum Current (IZM)
The Zener diode maintains a nearly constant voltage across its terminals for values of reverse current ranging from IZK to IZM. If IZM is exceeded, the Zener diode may be damaged due to excessive power dissipation.

5. Leakage Current
Reverse leakage current is specified for a reverse voltage that is less than the knee voltage. This means that the Zener is not in reverse breakdown for these measurements. For example is for a reverse voltage of 1V in a 1N4728A.

6. Power Rating (PZ)
The power rating tells you the maximum power (voltage x current) the Zener diode can handle. (Even diodes designed to break down can break down for real if you exceed their power ratings. So, be careful!)

7. Zener Resistance (ZZ)
ZZ is the maximum Zener impedance at the specified test current, IZ. For example, for a 1N4728A, ZZ is 10Ω at 76mA. At the knee of the characteristic curve, the maximum Zener impedance ZZK is specified at IZK which is the current at the knee of the curve. For example, ZZK is 400Ω at 1mA for a 1N4728A.

8. Temperature Coefficient (TC)
Zener diodes are affected by temperature changes associated with their voltage temperature coefficient. The temperature coefficient specifies the percent change in Zener voltage for each change in temperature. The formula for calculating the change in Zener voltage for a given junction temperature change (%/℃), for a specified temperature coefficient is:

Vz is the nominal zener voltage
TC is the temperature coefficient
ΔT is the change in temperature

If the temperature coefficient is expressed in mV/℃, ΔVz is given as:

A positive temperature coefficient means that the change in Zener voltage is directly proportional to the change in temperature. Consequently, a negative TC means that the Zener voltage is inversely proportional to the change in temperature.

9. Junction Temperature Specification
In order to ensure the reliability of the diode, the temperature of the diode junction is key. Even though the case may be sufficiently cool, the active area can still be very much hotter. As a result, some manufacturers specify the operating range for the junction itself. For normal design, a suitable margin is normally retained between the maximum expected temperature within the equipment and the junction. The equipment internal temperature will again be higher than the temperature external to the equipment. Care must be taken to ensure that individual items do not become too hot despite there being an acceptable ambient temperature outside the equipment.

10. Package
Zener diodes are specified in a variety of different packages. The main choice is between surface mount and traditional through-hole devices. However the package chosen will often define the package heat dissipation level. The choices available will be detailed in the Zener diode datasheet specification.

And that's it! I hope you've learned something from this tutorial about Zener diodes and how they work. If you’ve found this tutorial interesting or helpful, give it a like and if you have any questions, leave it in the comments below. See you in our next tutorial!

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