What is a Zener Diode?
Zener Diode sometimes also known as breakdown diode is a pn junction diode with quite a high doping level especially formulated to operate in reverse biased condition. These are known as special type of semiconductor devices that allows flow of current in both the directions i.e., forward and reverse.
Before elaborating Zener diode in detail let us first get some idea about diodes.
What is a Diode?
A diode is an electronic device formed of p and n type of semiconductor that is designed in a way that it conducts current in forward direction. Diodes are known for their ability to act as one-way switch for the current.
Zener is known to be a special type of diode as mainly conducts in reverse biased condition.
Basics of Zener Diode
A normal pn junction diode when operated in reverse biased condition acts as Zener diode when operated in breakdown region. The phenomenon of breakdown was explained by an American scientist C. Zener thus it is named so.
The figure given below shows the symbolic representation of Zener diode:
The operational mechanisms of diodes classify them into different categories.
Constructional details of Zener Diode
A Zener diode is specifically designed to operate when it is in reverse biased condition. However, when forward voltage is applied it acts as a normal working diode.
The figure here shows the diffused structure of Zener diode:
It is clearly shown over here that here we have n and p types of substrates that are diffused combinedly. Also, a metallic layer is deposited over it so that it gets connected to cathode and anode terminals.
When there is alloy diffused structure then the junctions are covered with layer of silicon oxidized i.e., SiO2.
Working of Zener Diode
We have already mentioned in the beginning that this diode acts as normal pn junction diode while when the diode is put under reverse biased condition then there is consistency in the current flowing through the diode over a wide range. This facilitates it to behave as voltage regulator in DC circuits.
The breakdown of Zener diode is achieved by two mechanisms namely, avalanche breakdown and Zener breakdown. Let us understand each mechanism separately.
Zener breakdown: This breakdown occurs at quite a high reverse voltage. In forward biased condition, Zener diode acts as normal pn junction diode however, when a high reverse voltage is provided then the width depletion region of the negatively biased diode increases.
With the increase in the depletion width the potential barrier also increases thereby giving rise to high electric field. Due to such high level of electric current the covalent bond at the depletion region gets break and minority current starts flowing through the circuit. Gradually, the minority current increases and after some amount of time this completely breaks down the junction.
This mechanism of Zener diode is known as Zener breakdown and is achieved at voltage less than 5 V.
Avalanche Breakdown: This mechanism is the result of quite a high reverse biased voltage condition of Zener diode. In this mechanism what basically happens is the minority charge carriers of the covalent bond gets very high kinetic energy and get detached. The released free electrons then further break more covalent bond present at the junction, thereby liberating more charge carriers. This flow of carriers gives rise to a large flow of current thereby leading to the junction breakdown.
At reverse voltage higher than 5V, avalanche breakdown occurs.
In these two operating mechanisms Zener diode operates.
The figure below shows the behaviour of Zener diode in forward and reverse biased conditions:
We know that when reverse biased voltage increases then this leads to breaking down of the junction thereby resulting in large amount of current. The particular voltage in the reverse biased condition where the diode starts conducting is known as Zener voltage or breakdown voltage while the current is called Zener current.
The doping concentration of the diode decides the breakdown voltage.