What Is a Zener Diode?
A Zener diode is a diode consisting of an n-type semiconductor and a p-type semiconductor connected together, and has a relatively small reverse voltage and a stable voltage value. They are also called constant voltage diodes.
Normal diodes exhibit a rectifying effect in which the degree of conduction changes according to the polarity of the bias applied to both ends, and no current flows even when a very high reverse bias voltage is applied. However, a special type of diode called a Zener diode exhibits diode characteristics that allow a large amount of current to flow rapidly when a relatively small reverse bias voltage above a certain threshold is applied.
By utilizing the unique rectification characteristics of Zener diodes, a constant voltage can be maintained in the circuit.
Uses of Zener Diodes
Zener diodes are characterized by the fact that when a reverse bias above a certain value is applied, a rapid flow of current occurs and further voltage increases are suppressed. For this reason, they are used to maintain the voltage of unstable power supplies at a constant level and to protect circuits from surge currents.
For example, a Zener diode is connected in parallel with an unstable power supply with a reverse bias. As a result, if the voltage of the power supply is higher than the breakdown value of the Zener diode, a large current flows through the Zener diode, thus lowering the voltage and keeping the voltage across the circuit constant.
Principle of Zener Diode
The origin of the unique characteristics exhibited by Zener diodes is said to be related to two factors: the Zener effect and the avalanche effect. The former is a phenomenon (unique to Zener diodes) in which the depletion layer generated when a reverse bias is applied becomes thinner by intentionally creating a pn junction using a semiconductor with a high concentration of impurities, and at a certain threshold value, electrons jump through the depletion layer due to the tunneling effect, causing electrical conduction to occur.
The latter is a phenomenon in which electrons accelerated more strongly than those with a higher bias collide with semiconductor atoms, knocking out many carriers, and the electrons knocked out repeatedly collide with semiconductor atoms, knocking out more carriers, causing an electron avalanche and large current flow.
When the bias exceeds the threshold, the avalanche effect occurs, in which electrons that jump over the depletion layer due to the Zener effect cause a high reverse bias that results in a large current, which in turn causes a voltage drop, thus lowering the voltage to the threshold.
The voltage applied to the circuit is then kept constant, stabilizing an unstable power supply and protecting the circuit from external surges. Currently, the reverse breakdown voltage of these Zener diodes can be fabricated with extreme controllability, depending on the impurity concentration ratio and semiconductor process treatment.
A wide range of types and tolerances are available on the market, from 1 V to several hundred V, with narrow tolerances of ±0.05% depending on the voltage value.
Other Information on Zener Diodes
1. Series and Parallel Connection of Zener Diode
Series Connection
When connecting Zener diodes in series, pay attention to the value of the Zener current lz that flows. The maximum allowable current for the entire series corresponds to the smaller allowable current of the connected Zener diode. Therefore, use it within the smaller allowable loss.
Also note that if the Zener current for the Zener voltage specification is different for each diode, the voltage value will be different from the Zener voltage you want to obtain. The reason for this is that one of the Zener diodes will not have the specified Zener current value.
Parallel Connection
Zener diodes cannot be connected in parallel because it increases the allowable losses of the Zener diode. Note that when connected in parallel, the zener current may concentrate in whichever has the lower zener voltage and exceed the allowable power dissipation.
2.Characteristics of Zener Diode
Temperature Characteristics
Temperature characteristics mean that the characteristics change with temperature. In the case of Zener diode, this temperature characteristic changes depending on the Zener voltage. The reason for this is the “tunneling effect” and the “avalanche effect.”
The temperature coefficient of the tunneling effect is negative, while that of the avalanche effect is positive. Therefore, if the Zener voltage is low, the Zener voltage will decrease as the ambient temperature rises. On the other hand, those with high Zener voltage are characterized by an increase in Zener voltage as ambient temperature rises.
A low Zener voltage here generally refers to a voltage lower than 5 V, while a high Zener voltage here generally refers to a voltage higher than 5 V. At around 5 V, the Zener phenomenon is caused by the combined effects of the tunneling and avalanche effects. At this time, the temperature characteristics are also about the same, making the Zener voltage less sensitive to ambient temperature.
Noise
In Zener diodes, the higher the Zener voltage, the higher the noise, and the higher the current, the lower the noise. To prevent noise, multiple elements with low Zener voltage should be connected in series. Noise can also be removed by connecting a capacitor in parallel with the Zener diode.
3. [By Application] How to Select a Diode
Although the characteristics and uses of Zener diodes have been described, there are various other semiconductor diode devices. Here is a supplementary explanation of the differences from other diodes and their characteristics.
One device that uses reverse characteristics is the TVS (Transient Voltage Suppressors) diode. Like Zener diodes, these diodes are used to provide over voltage protection, but unlike Zener diodes, TVSs are normally turned off and only turn on when a surge voltage is applied.
Schottky barrier diodes, which utilize a metal-semiconductor Schottky barrier, have even lower voltage values and are often used for rectifying applications with high switching speeds. PIN diodes with reduced terminal capacitance for RF (high frequency) applications are also used.