What is Breakdown Voltage in Junction Diodes & Zener Diode

Depending upon their electrical characteristics, materials are categorized as Conductors, Semiconductors, and Insulators. Conductors are materials that can conduct electricity easily. In contrast, the materials that cannot conduct any electricity are categorized as Insulators. The characteristics of semiconductor materials lie between conductors and insulators. While working with insulators, researchers have observed that Insulator material can be made to behave as a conductor when a certain amount of electricity is applied to them. This phenomenon was named Breakdown, and the minimum voltage at which this occurs is known as Breakdown Voltage. These voltage levels are different for different materials and also depend on their physical properties.

What is a Breakdown Voltage?

Breakdown Voltage is the characteristic of Insulator materials. The minimum voltage level at which an insulator starts behaving as a conductor and conducts electricity is known as “Breakdown Voltage.”It is also known as the Dielectric Strength of the material.

Conduction of electricity is possible only when there are mobile electric charges in the materials. Insulators cannot conduct electricity because there are no free mobile electric charges in them. When a potential difference is applied across the insulator, it does not conduct any electricity.

When the value of the applied potential difference is increased beyond certain levels some electron pairs are broken and the ionization process starts in the material. This leads to the formation of free mobile electrons. These mobile charges start moving from the positive end towards the negative end causing the flow of electricity.

Thus, the insulator starts conducting electricity and behaves as a conductor. This process is known as the Electrical Breakdown of the material and the minimum voltage at which this phenomenon starts is known as “Breakdown Voltage of the material”. This voltage level varies for different types of materials depending upon material composition, shape, size, and length of material between electrical contacts. The breakdown voltage value of a material given by the manufacturers is usually the Average Breakdown Voltage value.

Diode Breakdown Voltage

Diodes are the semiconductors and their electrical properties lie in between that of conductors and insulators. A PN junction diode is formed using a P-type and N-type material. PN junction diodes contain a bandgap through which exchange of charge carriers takes place. When a forward bias is applied current flows in the forward direction and conduction take place. When a reverse bias is applied no conduction should take place. But due to the presence of minority charge carriers, a small reverse current flows through the diode known as leakage current.

Due to the flow of reverse current the width of the junction barrier increases. When this applied reverse bias voltage is increased gradually at a certain point a rapid increase in the reverse current can be observed. This is known as Junction breakdown. The corresponding applied reverse voltage at this point is known as Breakdown Voltage of the PN junction diode. This is also known as  Reverse Breakdown Voltage.

The essential factor to determine the breakdown voltage of the diode is its doping concentration. Exceeding this voltage level causes the exponential increase in the leakage current of the diode. When a diode breakdown’s, overheating can be observed. So, when operating with reverse voltages heat sinks and external resistors are used.

Breakdown Voltage of Zener Diode

Zener diodes are used as basic building blocks in electronic circuits. They are popularly used to provide a reference voltage to the electronic circuits. They are designed to work in the breakdown regions of the diode.

Zener diodes are heavily diodes that can work reliably in the reverse-biased regions. Here breakdown takes place due to the Zener effect. In the Zener effect when the electric field of the reverse-biased P-N diode is increased, tunneling of the valence electrons into the conduction band takes place. This leads to an increase in the minority charge carriers thereby increasing the reverse current. This phenomenon is known as the Zener effect and the minimum voltage at which this phenomenon starts is known as Zener Breakdown voltage.

Avalanche Breakdown

In lightly doped diode breakdown takes place due to the Avalanche effect. Here in the Avalanche effect, when a diode is operated in reverse bias due to increased electric filed the minority charge carriers gain kinetic energy and collides with the electron-hole pairs, thereby breaking their covalent bond and creating new mobile charge carriers. This increase in the number of minority charge carriers leads to an increase in reverse current causing breakdown. Here, the breakdown voltage is known as Avalanche breakdown voltage.

The breakdown voltage of the commonly available Zener diode varies between 1.2V to 200V. Zener diode exhibits a controlled breakdown and does not require any external circuitry to limit the current. The V-I characteristics of the diode with avalanche breakdown increases gradually whereas for a diode with Zener breakdown the V-I characteristics are sharp.

Breakdown in Solids, Liquids, and Gases

Besides solids, many gases and liquids also have insulator properties and also seen to undergo breakdown phenomena. The minimum dielectric strength of silicon at room temperature can be calculated using the below formula.

|ε_br| =(12×10⁵)/(3-log(N/10¹⁶))V/cm

Air also acts as an insulator at standard atmospheric pressure conditions. It breakdown’s when the voltage increases beyond 3.0kv/mm. Breakdown voltages of gases can be calculated using Paschen’s Law. In partial vacuum conditions the breakdown voltage of air decreases. When air undergoes breakdown lightning, sparking occurs. These voltages are also known as Striking voltages.

The breakdown voltage of transformer oil is also known as it’s Dielectric strength. It is the voltage value at which sparks are observed between two electrodes which are separated by a gap and immersed in the transformer oil. When moisture or other conducting substances are present in the oil, lower values of breakdown voltages are observed. The minimum dielectric strength of the ideal transformer oil is 30KV.

The breakdown can also be observed in cables that carry current. The breakdown voltage of cable depends on the presence of moisture around it, time of application of voltage and the working temperature of the cables. What is the minimum breakdown voltage of a Zener Diode?