Zener Diode is a special type of silicon semiconductor device that operates in Zener breakdown region. This post will discuss in detail about what is Zener Diode,its parameters, how it works, applications as voltage regulator, clipper and voltage shifter.
What is Zener Diode
In general, Diodes are semiconductor devices that allow the flow of current when forward biased and offer resistance when reverse biased. Zener Diode is a special type of diode in which a critical reverse voltage and larger reverse current can flow.
It starts conducting in a reverse biased condition when the applied voltage is above the breakdown voltage and this voltage is called Zener breakdown voltage. It is named after Clarence M Zener, an American physicist who invented it in 1905.
Fig. 1 – Introduction to Zener Diode
It contains a heavily doped P-N junction and hence has a thin depletion region. It allows electrons from the valence band of P-type material to the conduction band of N-type material. In other words, it allows more electric current than the other P-N junction Diodes. It starts conducting current at reverse breakdown voltage, and continues to operate in the reverse-bias mode.
This type of Diode is advantageous as the voltage drop over a wide range of voltages remain constant. The volt-ampere (VI) characteristic curve of the Zener Diode and its symbol is shown in the Fig. 2. It shows that the reverse resistance is large and the reverse leakage current is extremely small, when the reverse voltage is lower than the reverse breakdown voltage. But when the reverse voltage exceeds, the reverse current will rise suddenly, called breakdown, and the reverse resistance will suddenly drop to a small value.
Fig. 2 – VI Characteristics and Symbol
Parameters of Zener Diode
Various parameters affect VI characteristics. They are:
- Maximum Zener Current
- Minimum Zener Current
- Nominal Voltage
- Power Dissipation
Other parameters, which influence its performance are Forward Current, Forward Voltage and Packaging type, etc.
Maximum Zener Current
It is defined as the current that can pass through to the Zener Diode at Zener breakdown voltage.
Minimum Zener Current
It is the current that is required for conducting Zener Diode in the breakdown region.
Zener Breakdown Voltage is also called the Nominal Operation Voltage. It is one of the important parameters for Zener Diode selection.
It is the maximum amount of power that the Zener Diode can dissipate. Power Dissipation is an important parameter to be considered for Zener Diode selection because excessive power rating leads to excessive increase in the temperature which results in the permanent damage of the device.
How does Zener Diode Work
The device works on the principle of the Zener effect, which means reverse conduction occurs due to electron Quantum Tunneling in the short space between highly doped P-N regions. In normal diodes, when the applied voltage exceeds the breakdown voltage; it results in the permanent damage of the diode. However, in the Zener diode, the breakdown voltage is not as high and does not lead to the permanent damage of the device.
Fig. 3 – Schematic Representation of Working Principle
Basically, the voltage regulation value below 6 Volt is called as Zener breakdown and at this stage the temperature coefficient of the diode is negative i.e. When the temperature rises, the depletion layer decreases. In this narrow depletion region, the energy of the valence electrons of the atom rises, and electric field with smaller intensity can excite the valence electrons from the atoms to generate Zener breakdown.
When the depletion layer is wide with greater electric field strength, the increase in temperature; increases the energy of the lattice atoms which obstructs the movement of carriers, generating Avalanche breakdown. Avalanche Breakdown occurs only when the reverse voltage is increased and hence the temperature co-efficient is positive.
Applications of Zener Diode
It’s capacity to withstand High voltages without getting damaged create several applications in modern-day electronic circuits. The three major applications are listed below:
- Voltage Regulator
- Clipping Circuit
- Voltage Shifter
Use of Zener Diode as Voltage Regulator
The primary application of a Voltage Regulator is to provide a constant output voltage irrespective of the variations in the energy drawn by the load current or instability of the supply voltage.
Figure. 4 illustrates the voltage regulating property of Zener Diodes. The R1 resistor is connected to the Zener Diode in series. The Diode is connected in Reverse bias to regulate the voltage.
Fig. 4 – Voltage Regulator Circuit
The Resistor R1 is calculated by using the formula below.
R1= (Vin – Vz) / Iz
- Iz = Zener Current
- Vz = Zener/Output Voltage
- Vin = Input Voltage
- R1 = Resistance
Based on the formula, it is easy to ensure that the value of the resistor selected doesn’t lead the flow of current higher than the Zener current.
Use of Zener Diode as Clipper
Clipping Circuit is used to prevent the output voltage from exceeding pre-determined voltage without altering the input signal or waveform. Zener Diode acts as a normal diode when the applied voltage is less than the Zener breakdown voltage. Hence it is widely used in Clipping Circuits.
Fig. 5 – Clipper Circuit
Clipping Circuits could be designed to clip the signal in the positive, negative or both regions. Zener diode is used to clip the output signal irrespective of the input waveform. If the device is used to clip the positive swing of the clipping circuit, then it prevents the voltage exceeding the Zener breakdown voltage without altering the input waveform.
During the negative swing of the Clipping Circuit, it acts as a normal Silicon diode and clips the output voltage. In order to Clip the output signal in both positive and negative swing; double Zener clipping circuit is used.
Use of Zener Diode for Voltage Shifting
Voltage Shifter circuit which help to convert signal from one voltage domain to another. They have the ability to maintain the steady output voltage irrespective of the input voltage and makes them an ideal component as Voltage Shifter. This device in Voltage Shifter circuit minimizes the output voltage by a value equal to the Zener breakdown voltage. An example of Voltage Shifter Circuit is illustrated below in the Fig. 6.
Fig. 6 – Voltage Shifter Circuit
Advantages of Zener Diode
The advantages include:
- They are cheaper than other types of diodes.
- It maintains a stable output voltage irrespective of the input voltage.
- It can be used as a normal Silicon Diode when it is forward biased.
- They have very high power dissipation capacity.
- They have very high-performance standards.
- Due to their small size they can be used in smaller electronic devices.
Disadvantages of Zener Diode
The disadvantages are:
- These diodes have low efficiency at Higher load currents.
- They have relatively poor voltage regulation ratio.
- When used as Voltage Regulator there is always a slight change in the DC output due to the Zener resistance.