The article explains the working principles and characteristics of Zener diode, focusing on their operation in the reverse-bias direction. It discusses the unique breakdown processes—avalanche and Zener breakdown—that allow Zener diodes to regulate voltage without damage, along with their electrical characteristics and applications.
Reverse biasing of a P-N Junction diode does not produce conduction unless the peak reverse voltage $P_{RV}$ is exceeded. When this occurs, a normal PN junction diode is usually destroyed. It is therefore a rule that the operation of a PN junction diode is generally related to forward conduction applications.
In this article, you will be introduced to a group of special diodes referred to as avalanche breakdown or Zener diodes. This type of diode operates in the reverse direction. The reverse breakdown voltage characteristic of this device is used to a unique advantage: voltage can be regulated. The reverse breakdown of a semiconductor is an important solid-state operational characteristic.
Zener Diode Structure and Symbol
The Zener diode is a PN junction device that is different from the PN junction diode. Zener diodes are designed by modifying their crystal structure during manufacture. The breakdown voltage of a Zener diode is set by carefully controlling its doping level. The symbol of a Zener diode is slightly different from a PN junction diode. On the Zener diode symbol, the cathode is usually drawn with a bent line in the form of a Z to distinguish it from the PN junction diode symbol. It only appears as a Z, however, when the symbol is oriented horizontally in a diagram.
Figure 1. Zener Diode and Symbol
Note that the physical appearance of Zener diodes is similar to that of PN junction diodes. However, the diode symbol is rarely printed on the case of a Zener diode as it is for a PN junction diode. If a diode symbol is printed on the case of a diode, the diode is ordinarily a PN junction diode.
Zener diodes are normally connected in a circuit in the reverse-bias direction (see Figure 2.). This means that the anode must be connected to the negative side of the voltage source and the cathode to the positive side of the voltage source. This action causes the depletion zone of the junction to widen.
Figure 2. Zener Diode Connections
Figure 3. The Widening Depletion Region of Reverse Biased Zener Diode
Under normal circumstances, we would expect a reverse-biased junction to be nonconductive. In a Zener diode, two special conditions are responsible for causing this device to become conductive in the reverse-biased direction.
Avalanche Breakdown
This condition occurs when thermally generated holes and electrons gain enough energy from the reverse-biased source to produce new carriers. These current carriers are the result of removing valence electrons from their normal bonding. These new carriers, in turn, produce additional carriers that disrupt the bonds of other atoms. The process continues with the number of carriers building up in increasing numbers. The end result is high current flow through the reverse-biased junction. Avalanche breakdown is primarily responsible for reverse current conduction above 5 V. Diodes of this type were once called avalanche diodes.
Zener Breakdown
This condition is the result of a barrier potential that appears across the PN junction. This field causes covalent bonds near the junction to break apart. As a result, a large quantity of new holes and electrons are produced. These newly generated holes and electrons represent a substantial increase in current. Zener breakdown is primarily responsible for conduction of a reverse-biased junction below 5 V. Diodes that conduct in this voltage range are called Zener diodes. Zener diodes are heavily doped so that they can begin conduction at relatively low reverse-bias voltages. Today, the term Zener diode is primarily used to describe all diodes that operate in the reverse-bias direction.
Characteristics of Zener Diodes
The characteristics of a PN junction diode show that it is designed for operation in the forward direction. Forward biasing produces a large value of forward current ($I_{F}$) for a rather small value of forward voltage ($V_{F}$). Reverse biasing generally does not cause current conduction until higher values of reverse voltage are reached. If reverse voltage ($V_{R}$) is great enough, however, breakdown occurs and causes a reverse current flow. PN junction diodes are usually damaged when this occurs. Zener diodes, however, are designed to operate in the reverse direction without being damaged. The value of the voltage across the Zener diode and the current flowing through it can be graphed.
The I–V characteristics of a Zener diode is shown in Figure 4. As the value of the applied reverse voltage ($V_{R}$) is gradually increased, the amount of current ($I_{R}$) flowing in the circuit is measured. The voltage across the Zener diode will remain constant after the Zener breakdown voltage ($V_{Z}$) has been reached.
Figure 4. I–V characteristics of Zener diode
In the reverse-bias direction, there is practically no reverse current (labeled $I_{Z}$) until the breakdown voltage (labeled $V_{ZK}$) is reached near the “knee” of the curve. When this occurs, a very large change in Zener current is accompanied by only a small change in Zener voltage. This means that varying amounts of $I_{Z}$ can pass through a Zener diode without causing damage. The reverse Zener voltage ($V_{Z}$) across the diode remains relatively constant even when the source voltage is increased.
The Zener current has a safe operating limit or maximum value that must be observed. It is determined by the wattage rating of the diode. Nominal Zener voltage ($V_{ZT}$) is specified on datasheets at a value of reverse current called Zener test current ($I_{ZT}$).
When operated in the forward-bias condition, the Zener diode behaves like an ordinary silicon diode. A large value of forward current ($I_{F}$)is produced for a rather small value of forward voltage ($V_{F}$).
Zener Diode Review Questions
- The of a Zener diode symbol is drawn as a bent line or bar.
- A Zener diode is normally connected in the – bias direction.
- When a Zener diode is reverse biased, it will go into at a prescribed voltage value.
- The breakdown is the result of thermally generated holes and electrons gaining enough energy from the source to move valence electrons out of their bonding.
- The breakdown is the result of covalent bonds breaking up due to a strong electric field across a reverse-biased PN junction.
- The characteristics of a Zener diode show that after a minimum value of $I_{Z}$ is reached, the __ ___ remains at a constant value.
Answers
- cathode
- reverse
- breakdown
- avalanche
- Zener
- voltage
Zener Diode Key Takeaways
Zener diodes are crucial components in electronic circuits due to their ability to regulate voltage in the reverse-bias direction without damage, making them ideal for use in voltage regulation, surge protection, and voltage clamping applications. Their unique characteristics, such as the Zener and avalanche breakdowns, allow them to maintain a constant voltage despite variations in input, ensuring stable operation in power supplies and other critical electronic systems.
