Brief Introduction of Zener Diode

May 11, 2024

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What is Zener Diode?

Zener Diodes are silicon-based semiconductor devices that allow current to flow bidirectionally- either in a forward or backward direction. The diode usually consists of a heavily doped p-n junction. The diode is designed to conduct the flow of electricity in the reverse direction once it reaches a certain threshold voltage.

The reverse-breakdown voltage of the Zener diode determines when it begins to conduct electric current and remains constant in the reverse-bias mode without incurring damage. This characteristic of the Zener diode makes it excellent for voltage regulation because the voltage drop across the diode always remains constant depending on the applied voltage.

Zener Diodes Symbol

In a Zener diode, current flows from the anode to the cathode and back again. The vertical bar’s bent edges distinguish the Zener diode symbol from the typical p-n junction diode symbol.

Zener Diodes Circuit

The example below shows the circuit diagram for a Zener diode. A Zener diode is used in reverse bias. Reverse biasing involves attaching the P-type material of the diode to the supply’s negative terminal and the n-type material to its positive terminal. The depletion area is small since the diode is made of heavily doped semiconductor material.

Zener Diode Characteristics

The Zener diode operates precisely like a conventional diode in the forward-bias condition. They have a turn-on voltage between 0.3 and 0.7 V.

However, a small amount of leakage current may flow when connected in the reverse mode, which is usual in most of its usage. As the reverse voltage approaches the predetermined breakdown value, a current starts to flow through the diode (V_z). Before stabilizing, the current reaches the maximum current set by the series resistor, which remains constant over a wide range of applied voltage.

Voltage almost stays constant in the diode no matter what amount of current is passing through the diode. This also holds true in large current variations, providing the diode current remains between the maximum current and the breakdown current.

A Zener diode’s excellent self-control is quite helpful when it comes to regulating and stabilizing changes in load or supply versus a voltage source. So diodes are used in many electrical regulator applications, and this makes them an important property.

Zener Diode Specifications

Different Zener diodes will have different specifications. These consist of maximum reverse current, nominal working voltage, and power dissipation. Apart from this, there are other specifications which are explained below.

Zener Voltage

Zener Voltage relates to the reverse breakdown voltage. Depending on the particular diode, this can be anywhere between 2.4V to 200V.

Current (maximum)

It is the highest current when the Zener voltage is rated. This might range from 200µA to 200A.

Current (minimum)

It is the minimum current required to break down at the Zener voltage. It usually ranges from 5 to 10 µA.

Power Rating

It is the maximum power that a diode can dissipate based on the voltage across it and the current that flows through it. Some of the standard values include 400 mW, 500 mW, 1 W, and 5 W. Typical values for surface-mounted diodes are 200mW, 350mW, 500mW, and 1W.

Voltage Tolerance

Typically ±5%

Temperature Stability

The most robust diodes are typically 5V.

Zener Resistance

Resistance that the diode displayed.

How Does Zener Diode work?

The reason for the diode’s failure under the reverse bias condition yields information about the Zener diode’s working principle. There are normally only two types of mechanisms by which PN junction breaks: Avalanche and Zener.

Avalanche Breakdown

The reverse saturation current carries on the Avalanche breakdown mechanism. P- and N-type materials combine to form the PN junction. There is the formation of depletion regions when P and N-type materials overlap.

P-type material consists of electrons, and N-type material consists of holes. They both combine, which makes up the PN junction, with some impurities. The width of the depletion region varies. The bias given to the terminal of the P and N region affects their width.

The reverse bias increases the electrical field across the depletion zone. Minority charge carriers transit the depletion area more quickly when there is a strong electric field. These carriers strike the atoms of the crystal. The charge carrier removes the atom’s electrons due to the forceful collision.

The electron-hole pair expands as a result of the collision. They immediately split and collide with the other atoms of the crystals as the electron-hole pair induces a strong electric field. The procedure is ongoing, and as the electric field increases, the reverse current begins to flow in the PN junction. This is the Avalanche breakdown process. Because the diode has entirely burned off after the breakdown, the junction cannot return to its previous state.

Zener Breakdown

As we know, a PN junction is formed when a p-type semiconductor combines with an n-type semiconductor, which results in a depletion region.

The Zener breakdown event occurs in the extremely thin depletion region. In the narrow depletion area, there are more free electrons. The reverse bias applied across the PN junction creates the electric field intensity over the depletion area. The electric field becomes substantially stronger.

The strength of the electric field increases the kinetic energy of the free charge carriers. The carriers begin to jump from lower regions to higher regions. This creates holes in the depletion region because these powerful charge carriers collide with the p- and n-type material’s atoms.

At this stage, reverse current starts flowing in the junction, because of which the depletion region disappears entirely. Thus, this process is known as Zener Breakdown.