Breakdown voltage is a central concept in electrical engineering that describes the behaviour of diodes under certain voltage conditions. To understand what happens during the breakdown of a diode, one must be familiar with the fundamental mechanisms and the various effects that occur in the diode when this voltage is exceeded. This article looks at the breakdown itself, the associated effects and their practical applications.
What is the breakdown voltage of a diode, explained simply?
The breakdown voltage of a diode is the point at which the diode, operated in reverse bias, suddenly begins to conduct a significant current. Under normal conditions a diode only conducts current in the forward direction. In reverse bias it should block the current, but when the applied voltage exceeds a certain limit, breakdown occurs and the current suddenly rises sharply.
Basic explanation of breakdown voltage
Diodes are semiconductor devices consisting of two different semiconductor materials, a p-type and an n-type. The junction between these two materials is called the PN junction. Under normal operating conditions the diode conducts current in the forward direction while blocking in reverse bias, unless the voltage exceeds a critical limit, the breakdown voltage.
Breakdown voltage occurs when the applied reverse voltage becomes so high that the electric field strength in the PN junction is strong enough to accelerate electrons or holes so they cause impact ionization. At this moment a very high current suddenly flows through the diode.
Behaviour of the diode when reverse voltage is exceeded
Under normal conditions the diode blocks current flow in reverse bias. However, when the applied voltage is exceeded and reaches the breakdown voltage, electrons and holes in the semiconductor material begin to accelerate. This causes the resistance of the diode to drop rapidly and the current through the diode to rise abruptly.
Why does the current through the diode rise abruptly?
The sudden rise in current is the result of two effects that occur during the breakdown of a diode: the Zener effect and the avalanche effect. These effects cause the diode to become capable of conducting the current that would otherwise be blocked by the reverse voltage.
Which effects cause breakdown in diodes?
There are two main mechanisms that cause the breakdown of a diode: the Zener effect and the avalanche effect. Both effects are closely linked to the electric field strength in the PN junction of the diode.
Zener effect and its impact on the PN junction
The Zener effect occurs when the voltage becomes so high that the electrons have enough energy to break through the PN junction. In this case electrons can move from the valence band into the conduction band, enabling a current flow in reverse bias. This effect is mainly used in diodes with a lower breakdown voltage, such as Zener diodes.
The avalanche effect in electrical engineering explained
The avalanche effect occurs when the electric field in the PN junction becomes strong enough to accelerate electrons so forcefully that they cause impact ionization. These new electrons generate still more electrons, which likewise trigger a chain reaction that causes the current to rise quickly. This effect mainly occurs in diodes with a higher breakdown voltage.
Difference between Zener effect and avalanche effect
The Zener effect occurs when the voltage quickly reaches a certain threshold and the electrons jump directly across the PN junction. The avalanche effect, by contrast, is a slower process that takes place in diodes with a higher breakdown voltage and a stronger electric field, ionizing a larger number of electrons.
How does the doping concentration influence the breakdown voltage?
The doping concentration in the diode directly affects the magnitude of the breakdown voltage. A higher doping concentration leads to a higher number of charge carriers and therefore a stronger electric field strength in the PN junction. As a result, the breakdown voltage is increased, since the diode requires a stronger voltage to reach breakdown.
Relationship between doping concentration and breakdown voltage
The higher the doping concentration, the more charge carriers are present in the semiconductor material. This reduces the resistance of the diode in reverse bias, while at the same time raising the breakdown voltage. This relationship is decisive when selecting diodes for specific applications that require a particular breakdown voltage.
Parameters that influence the dielectric strength
The dielectric strength of a diode is influenced by various parameters, including doping concentration, temperature and the geometry of the PN junction. Higher temperatures or a greater thickness of the dielectric (the insulating material) can cause the diode to enter the breakdown region at lower voltages.
What happens when the breakdown voltage is reached?
When the breakdown voltage is reached, the current through the diode begins to rise dramatically. There is no longer any insulation and the diode behaves like a conducting element, allowing current to flow through the circuit. This sudden rise in current can lead to overheating and destruction of the diode if the current is not controlled.
Rise in current upon reaching the breakdown voltage
When the breakdown voltage is reached, the current rises exponentially. This is the moment at which the resistance of the diode drops virtually to zero and it begins to act as a current conductor. This can cause damage, particularly if the current is not dissipated in a controlled manner.
Can exceeding the breakdown voltage cause damage?
Yes, exceeding the breakdown voltage can cause permanent damage to the diode. If the current does not remain within the specified limits, it can lead to overheating, destruction of the diode and, in the worst case, failure of the entire circuit.
How is a diode used in the breakdown region?
Diodes designed for the breakdown region, such as Zener diodes, are deliberately operated in the breakdown region to enable voltage regulation. They are used in voltage stabilizers and protection circuits to ensure a constant voltage in electronic devices.
Practical applications of diodes in the breakdown region
A classic example of the use of diodes in the breakdown region is the Zener diode. These special diodes are designed to operate in the breakdown region and supply a stable voltage, which is used in many voltage regulator circuits and stabilization devices.
Zener diodes: specialty diodes for the breakdown region
Zener diodes are diodes that have been specifically developed for use in the breakdown region. They exploit the Zener effect to maintain a stable voltage at a certain level. These diodes are particularly useful in stabilization and protection circuits.
Uniform voltage regulation through controlled breakdowns
Through the controlled breakdown voltage, Zener diodes offer an excellent means of ensuring uniform voltage regulation, which is indispensable for electronic devices and circuits.
How does the reverse current behave at high voltages?
The reverse current behaves very characteristically at high voltages: it initially remains minimal but rises abruptly as soon as the breakdown voltage is reached. This sudden change is typical of diodes in the breakdown region.
Development of the reverse current as the voltage is increased
As the voltage is increased further, the reverse current rises progressively, indicating that the diode is moving into the breakdown region.
Critical values and their importance in practice
The critical values of breakdown voltage and reverse current are decisive for guaranteeing the service life and reliability of electronic components. Exceeding these values can lead to faults in the circuit and damage to the components.
GOBA Takeaway
The breakdown voltage of a diode is a critical point that is decisive for the functioning and safety of electronic components. Understanding the mechanisms that lead to breakdown is of central importance for the development of safe and efficient electronic circuits. Zener diodes and avalanche diodes allow this effect to be used deliberately for voltage regulation.