Zener Effect vs Avalanche Effect in PN Junction Diode

Introduction

When the reverse bias voltage applied to the PN junction increases to a certain value, the phenomenon that the reverse current density suddenly begins to increase rapidly is called PN junction breakdown. From the mechanism, it can be divided into three categories: avalanche breakdown, tunnel breakdown and thermoelectric breakdown. Among them, there are two physical mechanisms for forming reverse breakdown in PN junction: zener breakdown and avalanche breakdown. Generally, both breakdowns coexist. So what is the difference between them?

Avalanche Breakdown and Zener Breakdown Effect Explained

Catalog

Introduction

Ⅰ Basic Characteristics

1.1 Avalanche Effect

1.2 Zener Effect

Ⅱ Zener Effect vs Avalanche Effect

Ⅲ Transistor Secondary Breakdown and Protection

3.1 A Brief Description

3.2 Cause of Breakdown

3.3 Precaution

3.4 Snubber Circuit Examples

Ⅳ FAQ

Ⅰ Basic Characteristics

1.1 Avalanche Effect

As the reverse voltage increases, the electric field in the space charge region strengthens, and the energy obtained by the carriers in the barrier region also increases. When the reverse voltage is close to the breakdown voltage, these carriers with higher energy meet the neutral atoms in the space charge region and cause collision ionization, generating new electron-hole pairs. These newly generated electrons and holes will regain energy under the action of the electric field, collide with other neutral atoms to ionize them, and generate more electron-hole pairs. With reaction continues, causing the number of carriers in the space charge region to increase sharply, just like an avalanche, what’s more, the reverse current also increase sharply, resulting in breakdown. So this breakdown is called avalanche breakdown (or avalanche effect).

This breakdown generally occurs in PN junctions with lower doping concentration and higher applied voltage. Because a PN junction in this state has a wider space charge region and more opportunities for impact ionization.

Zener Breakdown and Avalanche Breakdown

Figure 1. Zener Breakdown vs Avalanche Breakdown

1.2 Zener Effect

When the reverse voltage increases to a certain value, a strong electric field can be established in the barrier region, which can directly pull out the valence electrons bound in the covalent bond, so that a large number of electron-holes are generated in the barrier region. Then a large reverse current is formed, resulting in breakdown. At this time, atoms in the barrier region are directly excited under the action of a strong electric field is called Zener effect/breakdown. It is caused by the tunneling effect in quantum mechanics. Giving a simple metaphor, the simple understanding is that the two lines are too close, and they pass through directly. At this time, the potential barrier loses its function of blocking electrons, and a breakdown occurs.

Zener breakdown generally occurs in PN junctions with higher doping concentrations. This is because the PN junction under this situation has a large charge density and a narrow width in the space charge region. As the temperature increases, the energy gap decreases, and a breakdown can be resulted in with a small reverse voltage.

PN Junction

Figure 2. PN Junction

Ⅱ Zener Effect vs Avalanche Effect

(1) Zener effect mainly depends on the maximum electric field in the space charge region, and in the collision ionization mechanism is related to both the field strength and the collision accumulation process of carriers. Obviously, the wider the space charge region, the more times of multiplication, so the avalanche breakdown is not only related to the electric field, but also related to the width of the space charge region, which requires the thickness of the PN junction.

(2) Because avalanche breakdown is the result of impact ionization. If we increase the electrons and holes in the space charge region by means of illumination or fast particle bombardment, they will also have a multiplier effect. However, the above external effects will not have a significant impact on the Zener breakdown.

(3) The breakdown voltage is determined by the tunnel effect, and its temperature coefficient is negative, that is, the breakdown voltage decreases with the increase of temperature, which is the result of the decrease of the forbidden band width with the increase of temperature. The breakdown voltage determined by avalanche multiplication decreases with the increase of temperature due to the impact ionization rate (the ionization rate represents the number of electron-hole pairs generated by a carrier drifting a unit distance under the action of an electric field), and its temperature coefficient is positive. That is, the breakdown voltage increases with temperature. Zener with voltage lower than 5-6V is mainly due to Zener breakdown; Zener with voltage higher than 5-6V is mainly due to avalanche breakdown. Zener diodes with a voltage between 5-6V have similar breakdown degrees and the best temperature coefficient, which is why many circuits use 5-6V Zener tubes. The principle of the Zener tube determines that its response speed is not very fast, so a tube reference voltage is used in occasions with high speed requirements.

(4) For the PN junction with higher doping concentration and thinner barrier, it is mainly Zener breakdown. The PN junction with lower doping and therefore wider potential barrier is mainly avalanche breakdown, and the breakdown voltage is relatively high.

The PN junction breakdown is an important electrical property, and the breakdown voltage limits the working voltage of the circuit, so semiconductor devices have certain requirements for the breakdown voltage. However, a variety of devices such as Zener diodes, avalanche diodes, and tunnel diodes can be fabricated by using the breakdown phenomenon.

Under normal circumstances, the avalanche breakdown and Zener breakdown are within a certain range of conditions (breakdown voltage, time), with the normal working conditions are restored, are reversible. If it is only for protection, the TVS voltage regulator tube is mainly used for voltage regulation. The smaller the current passing through, the better. When the instantaneous voltage exceeds the normal working voltage of the circuit, the TVS diode will avalanche, providing an ultra-low resistance path for the instantaneous current, which is diverted through the diode, avoiding the protected device. In additional, the protected circuit keeps the cut-off voltage until the voltage returns to normal value. When the instantaneous pulse ends, the TVS diode automatically returns to the high resistance state, and the entire circuit entering the normal voltage, the failure mode of the TVS tube is mainly short circuit. But when the overcurrent passed is too large, it may also cause the TVS tube to be burst and open.

TVS Diode

Figure 3. TVS Diode

Ⅲ Transistor Secondary Breakdown and Protection

3.1 A Brief Description

In most switching power supplies, power switching transistors work under high-voltage, high-current high-frequency pulses, and switching on and off under such conditions will cause a great impact on the transistors. Secondary breakdown is one of the important causes of transistor damage. To design a high-performance, high-reliability switching power supply, it is necessary to have a clear understanding of the secondary breakdown of transistors and avoidance measures.

3.2 Cause of Breakdown

The secondary breakdown is mainly caused by the high local temperature in the device body. The temperature rise is caused by thermal imbalance when forward biased and avalanche breakdown when reverse biased.

Because the thermal resistance of the transistor is unevenly distributed throughout the tube, in some weak areas, the temperature rise will be higher than other parts, forming a so-called "hot spot", and so on until a critical temperature, causing the breakdown of the tube. The secondary breakdown caused by the avalanche breakdown is a phenomenon in which the electric field distribution of the junction is changed due to the excessive current density at some points after the primary avalanche breakdown occurs, resulting in a negative resistance effect and the local temperature is too high.

3.3 Precaution

Turn-on and turn-off losses are important factors that affect the normal operation of switching devices. In particular, the transistor is prone to secondary breakdown in the dynamic process, and this phenomenon is directly related to the switching loss. Therefore, reducing the switching loss of the self-shutdown device is a necessary measure for the correct use of the device. There are two ways to reduce losses:

(1) Turn off the transistor at the lowest possible collector-emitter voltage (Vce).

(2) When the transistor is turned off during the rise of the emitter voltage, the emitter current should be minimized. For example, introducing a buffer circuit is one of the ways to achieve the above purpose.

3.4 Snubber Circuit Examples

The following snubber circuits can be used in the design of switching power supplies to ensure that the transistors operate within a safe area.

(1) The commonly one is an energy-consuming shutdown snubber circuit. Although it consumes more energy, this circuit is simple.

Commonly Used Shutdown Snubber Circuit

Figure 4. Commonly Used Shutdown Snubber Circuit

It consists of an RCD network connected in parallel with transistor switches. When the transistor is turned off, the load current charges the capacitor C through the diode D, so that the collector current of the tube gradually decreases. Because the voltage across the capacitor C cannot be abruptly changed, its collector voltage is restrained. The situation where the collector voltage and current reach their maximum values at the same time is avoided, so there is no maximum instantaneous power consumption spike. When the tube is turned on, the capacitor releases energy and dissipates it in the resistor.

(2) Two commonly used energy-consuming turn-on snubber circuits.

a. An inductor-diode network is connected in series with the transistor collector to form a turn-on snubber circuit. When the tube is turned on, the inductance Ls controls the current rise rate di/dt during the collector voltage drop. When the tube is turned off, the energy stored in the inductor Ls 1/2 freewheels through the diode Ds, and its energy is dissipated in the resistance of Ds and the reactor.

Open Snubber Loop with Unsaturated Reactance

Figure 5. Open Snubber Loop with Unsaturated Reactance

b. Turn-on snubber circuit with saturable reactor: The purpose of using turn-on snubber circuit is to make the collector voltage drop to 0 when the collector current of the transistor is small, so as to minimize the turn-on loss. Especially for inductive loads, the effect is more significant. The designed saturable reactor should be: in one hand, after the collector voltage drops to zero, the buffer reactor is in a saturated state; in the other hand, before saturation, the collector voltage drops to zero, the reactor presents a high resistance, and the magnetizing current flowing through the tube is small to achieve the purpose of reducing turn-on loss.

Open Snubber Circuit with Saturable Reactance

Figure 6. Open Snubber Circuit with Saturable Reactance

(3) In the figure, Co is a transfer capacitor, and Dc is a feedback diode. These two components feed back energy to the load. When the tube is turned off, the buffer capacitor Cs is charged to the power supply voltage Vcc, and when the tube is turned on next time, the load current is transferred from the freewheeling diode Df to the transistor. At the same time, the voltage on Cs resonates to Co. When the tube is turned off again, the Cs is charged again, the capacitor Co is discharged to the load, and the energy is fed back.

Passive Feedback Shutdown Buffer Circuit

Figure 7. Passive Feedback Shutdown Buffer Circuit

(4) This circuit stores the magnetic field energy and feeds back to the power supply through the transformer. The transformer is wound with two wires, and its primary side has a certain inductance; the polarity of the width side is opposite to that of the primary side, and a reverse diode is connected. When the tube is turned on, the primary side bears all the power supply voltage, and the secondary side has no energized circuit. When the tube is turned off, the polarity of the induced voltage on the secondary side is reversed, and when its voltage is higher than the power supply voltage Vcc, energy is fed to the power supply.

Passive Feedback Opens the Buffer Circuit

Figure 8. Passive Feedback Opens the Buffer Circuit

(5) The turn-on snubber circuit and the turn-off snubber circuit are combined to form a composite snubber circuit, and the composite snubber circuit has a protective effect when the transistor is turned on and off. This kind of circuit is also divided into two types: energy consumption and energy feeding.

a. When the tube is turned on, the snubber capacitor is discharged through the Cs, Rs, and Ls loops, which reduces the current rising rate that the tube bears. In addition, when the tube is turned on, the inductance Ls can also limit the reverse recovery current of the freewheeling diode Df.

Energy-consuming Composite Buffer Circuit

Figure 9. Energy-consuming Composite Buffer Circuit

b. When the transistor is turned off, the capacitor Co and the inductor Ls operate in parallel to feed the stored energy to the load. When the capacitor Co is discharged, the voltage on the inductor Ls gradually decreases to 0, and the load current is conducted through the freewheeling diode Df during this period.

Energy-feeding Compound Snubber Circuit

Figure 10. Energy-feeding Compound Snubber Circuit

The various snubber circuits mentioned above can be divided into two types, namely energy-consuming and energy-feeding. The energy-consuming circuit is simple but relatively consumes more energy, and is suitable for the use of low-power circuits. The energy-feeding circuit is complex, but in a high-power supply, if the energy dissipated by the snubber circuit is dissipated in the form of heat, it is bound to cause a lot of trouble, so the energy-feeding buffer circuit should be used.

Ⅳ FAQ

1. What is a zener breakdown voltage?

A normal p-n junction diode allows electric current only in forward biased condition. ... This sudden rise in electric current causes a junction breakdown called zener or avalanche breakdown. The voltage at which zener breakdown occurs is called zener voltage and the sudden increase in current is called zener current.

2. Which breakdown occurs in Zener diode?
avalanche breakdown
In Zener diodes, avalanche breakdown occurs. When the Vz is greater than 8 volts in a Zener diode, avalanche breakdown occurs because there is an isolation of electrons and holes.

3. What is difference between avalanche and zener breakdown?
The main difference between Zener breakdown and avalanche breakdown is their mechanism of occurrence. Zener breakdown occurs because of the high electric field whereas, the avalanche breakdown occurs because of the collision of free electrons with atoms. Both these breakdowns can occur simultaneously.

4. How do you calculate Zener breakdown voltage?
The reverse current that results after the breakdown, is called Zener current (Iz). At breakdown, increase of VI increases II by large amount, so that V0 = VI– RI II becomes constant. This constant value of V0 which is the reverse breakdown voltage, is called Zener voltage.

5. What is avalanche breakdown of diode?
What is Avalanche Breakdown? The avalanche breakdown occurs when a high reverse voltage is applied across the diode. As we increase the applied reverse voltage, the electric field across the junction increases. This electric field exerts a force on the electrons at the junction and frees them from covalent bonds.

6. How does an avalanche breakdown take place?
Avalanche breakdown usually occurs when a high reverse voltage is applied across the diode. So as we increase the applied reverse voltage, the electric field across the junction will keep increasing. This generated electric field exerts a force on the electrons at the junction and it frees them from covalent bonds.

7. What is avalanche effect of Zener diode?
Avalanche breakdown involves minority carrier electrons in the transition region being accelerated, by the electric field, to energies sufficient for freeing electron-hole pairs via collisions with bound electrons. The Zener and the avalanche effect may occur simultaneously or independently of one another.

8. What do you mean by zener breakdown voltage?
When reverse biased voltage applied to the zener diode reaches zener voltage, it starts allowing large amount of electric current. At this point, a small increase in reverse voltage will rapidly increases the electric current. Because of this sudden rise in electric current, breakdown occurs called zener breakdown.

9. Is Zener voltage the same as breakdown voltage?
The breakdown voltage,commonly called the Zener voltageis the reverse-biased voltage that causes the diode to conduct current. Breakdown voltages usually range from 2.4 V to hundreds of volts.

10. What is meant by Zener effect?
The Zener effect is a type of electrical breakdown that occurs in a reverse-biased PN junction when the electric field enables tunnelling of electrons from the valence to the conduction band of a semiconductor, leading to a large number of free minority carriers which suddenly increase the reverse current.

11. Which factor is responsible for Zener effect?
In effect, electrons from the p-side valence band are able to tunnel across the barrier into the empty states in the n-side conduction band when a small reverse bias is applied. The result is a strong current from n to p in the diode, causing zener breakdown.

12. What is valence breakdown?
Avalanche breakdown (or “the avalanche effect”) is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents within materials which are otherwise good insulators. It is a type of electron avalanche.

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