Zener Diode

Zener Diode

• The zener diode is a silicon p-n junction semiconductor device, which is generally operated in its reverse breakdown region. The zener diodes are fabricated with precise breakdown voltages, by controlling the doping level during manufacturing. The zener diodes have breakdown voltage range from 3 V to 200 V. In 1934, a physicist Carl Zener investigated the breakdown phenomenon in the p-n junction diode.

• The Fig. 1.22.1 (a) shows the symbol of zener diode. The d.c. voltage can be applied to the zener diode so as to make it forward biased or reverse biased. This is shown in the Fig. 1.22.1 (b) and (c). Practically zener diodes are operated in reverse biased mode.

 

1. Characteristics of Zoner Diode

• In the forward biased condition, the normal rectifier diode and the zener diode operate in similar fashion. But the zener diode is designed to be operated in the reverse biased condition. In reverse biased condition, the diode carries reverse saturation current till the reverse voltage applied is less than the reverse breakdown voltage. When the reverse voltage exceeds reverse breakdown voltage, the current through it changes drastically but the voltage across it remains almost constant. Such a breakdown region is a normal operating region for a zener diode. The normal operating regions for a rectifier diode and a zener diode are shown in the Fig. 1.22.2 (a) and (b).

• The breakdown characteristic for a zener diode is significantly important, as it is an operating region for the diode. When the reverse voltage applied to a zener diode is increased, initially the current through it is very small, of the order of few µ.A or less. This is the reverse leakage current of the diode, denoted by I₀. At a certain reverse voltage, current through zener diode increases rapidly. The change from a low value to large value of current is very sharp and well defined. Such a sharp change in the reverse characteristics is called knee or zener knee of the curve. At this knee, a breakdown is said to occur in the device. The reverse bias voltage at which the breakdown occurs is called zener breakdown voltage, denoted as V_z.

• The voltage V_z is set by carefully controlling the doping level during manufacturing process.

• The current corresponding to a knee point is called zener knee current and it is a minimum current zener must carry to operate in reverse breakdown region. It is denoted as I_ZK or I_Zmin-

• From the bottom of the knee, the zener breakdown voltage remains almost constant, though it increases slightly as the zener current Iz, increases. The current at which the nominal zener breakdown voltage is specified is called zener test current, denoted as I_ZT. This value and corresponding zener voltage V_Z are specified on a datasheet of a zener diode. Every zener diode has a capacity. As current increases, the power dissipation P_Z = V_Z I_Z increases. If this dissipation increases beyond certain value, the diode may get damaged. 

• The maximum current a zener diode can carry safely is called zener maximum current and is denoted as IZM or IZmax.

• In practical circuits to limit the zener current between IZmin and IZmax, a current limiting resistor is used in series with the zener diode.

• The complete V-I characteristics of the zener diode is shown in the Fig. 1.22.3.

 

2. Equivalent Circuit of Zener Diode

• When the breakdown occurs then Iz may increase from IZmin to IZmax but voltage across zener remains constant. Hence actually the internal zener impedance decreases as current increases in the zener region. But this impedance is very small. Hence ideally the zener diode is indicated by a battery of voltage Vz, which remains fairly constant in the zener region. This is shown in the Fig. 1.22.4.

• Practically though very small, zener has its internal resistance. In the zener region, this resistance is called dynamic resistance of the zener denoted as Zz. Practically zener region is not exactly vertical. The small change in zener current ΔIZ produces a small change in zener voltage ΔVZ. The ratio of ΔVZ to ΔIz is called zener resistance Zz. This is shown in the Fig. 1.22.5 (a). Hence practically zener equivalent circuit is shown with a battery of Vz along with a series resistance Zz as indicated in the Fig. 1.22.5 (b).

• From the graph the dynamic resistance is defined as,

This value is specified generally at zener test current I_ZT

Key Point : In most of the cases this value is almost constant over the full range of zener region i.e. from I_Zmin to  Z_max'      or<^er of few tens of ohms.

 

3. Breakdown Mechanisms in Zener Diode

• There are two distinct mechanisms due to which breakdown may occur in the zener diode. One is called zener breakdown and the other avalanche breakdown. For most of commercially available silicon diodes, the maximum value of voltage breakdown by the zener mechanism is low, in the region of 5 V to 8 V. Consequently, for devices with breakdown voltages lower than 5 V, the zener  mechanism predominates, between 5 and 8 V, both zener and avalanche mechanism are involved; whereas, above 8 V the avalanche mechanism alone takes over.

• Practically zener breakdown is observed in the zener diodes with breakdown voltages less than 6 V while avalanche breakdown is observed in the zener diodes with breakdown voltages greater than 6 V. Commercially all such diodes are referred as zener diodes whether breakdown mechanism is zener or avalanche.

a. Zener Breakdown

• The zener breakdown mechanism can be described qualitatively as follows :

The application of reverse bias, voltage (6 V or less) causes a field across the depletion region, at p-n junction, of the order of 3 × 10⁵ V/cm. An electric field of such high magnitude exerts a large force on the valence electrons of the atom, tending to separate them from their respective nuclei. Actual breaking of the covalent bonds occurs when the electric field intensity becomes more than 3 x 105 V/cm. Hence, electron-hole pairs are generated in large numbers and a sudden increase of current is observed. If a limiting resistance in the circuit does not prevent the current from increasing to high value, the diode may be destroyed because of excessive heating at the junction. In zener breakdown, the value of breakdown voltage decreases as p-n junction temperature increases, thus such diodes have negative temperature coefficient.

b. Avalanche Breakdown

• As seen earlier, the applied reverse bias causes a small reverse current I₀ to flow in the device. This is due to movement of minority charge particles, viz. electrons from the p-material and holes from the n-material. The polarity of reverse bias voltage is such that only the minority charge particles are able to cross the p-n junction, while the majority charge particles move away from the junction. As the applied reverse bias voltage becomes larger, the minority charge carriers increasingly accelerate. There are collisions between these particles and electrons involved in the covalent bonds of the crystal structure.

• If the applied voltage is such that the travelling electrons do not have high velocity, then the collisions take some energy away from them, altering their velocity. If the applied voltage is increased, the velocity and hence the kinetic energy (K. E. = 1/2 mv² ) of electron increases. If such an electron dashes against an electron involved in covalent bond, then the collision gives bond-valence electron enough energy to enable it to break its covalent bond. Thus, one electron by collision creates an electron-hole pair. These secondary particles are also accelerated and participate in collisions that generate new electron-hole pairs.

This phenomenon is known as carrier multiplication. Electron-hole pairs are generated so quickly and in such large number that there is an apparent avalanche or self-sustained multiplication process. At this stage junction is said to be in breakdown and current starts increasing rapidly. To limit such current below I_Zmax, current limiting resistor is necessary.

• Zener diodes having zener voltage above 6 V exhibit avalanche breakdown and such diodes have positive temperature coefficient. The value of breakdown voltage increases as p-n junction temperature increases. Thus zener diode has positive temperature coefficient.

c. Comparison of Breakdown Mechanisms

Zener breakdown

1. Breaking of covalent bonds is due to intense electric field across the narrow depletion region. This generates large number of free electrons to cause breakdown.        

2. This occurs for zener diodes with Vz less than 6 V. 

3. The temperature coefficient is negative. 

4. The breakdown voltage decreases as junction temperature increases.   

5. The V-I characteristics is very sharp in breakdown region.

Avalanche breakdown

1. Breaking of covalent bonds is due to collision of accelerated charge carriers having large velocities and kinetic energy with adjacent atoms. The process is called carrier multiplication.

2. This occurs for zener diodes with Vz greater than 6 V.

3. The temperature coefficient is positive.

4. The breakdown voltage increases as the junction temperature increases.

5. The V-I characteristics is not as sharp as zener breakdown in breakdown region.

 

4. Comparison of Zener Diode and P-N Junction Diode

 

5. Specifications of Zener Diode

• In the data sheet of zener diode, the important specifications or ratings about a zener diode are specified. The various important specifications of a zener diode are,

1. Nominal Zener Voltage Vz or VZT : It is the voltage across a zener diode when it is operated in its zener region. It remains constant as long as zener diode is operated in the zener region. It is specified at a specific value of zener test current denoted as I7T and has a standard tolerance of ±10%

2. Zener Test Current I_ZT : It is the reverse current through a zener diode at which the nominal zener voltage Vz is specified.

At this current various other parameters such as dynamic zener impedance are specified.

3. Dynamic Zener Impedance Zz or ZZT : This is a.c. zener impedance which is the ratio of A VZ to AIZ. It is the reciprocal of the slope of the zener characteristics in the zener region.

This is generally specified at zener test current I_ZT. Some manufacturers specify this at the zener knee current I_ZK or IZmin, hence denoted as Z_ZK.

4. Maximum Zener Current IZmax : The maximum reverse current which a zener diode can carry safely without damaging it due to excessive heating is called maximum zener current.

It indicates the end of the zener region.

It decides the maximum power dissipation rating of a zener diode.

5. Minimum Zener Current I_Zmin or I_ZK : The minimum reverse current which must flow through a zener diode to operate it in the reverse biased condition is called minimum zener current.

It is nothing but a current corresponding to a knee point. It is also called breakover current or knee current.

It indicates the start of a zener region.

6. Maximum Power Dissipation P_D(max) : It is defined as the product of the zener voltage V_Z and maximum zener current I_Zmax

P_D(max) = V_Z × I_Zmax

• The power dissipation in the zener diode must be less than P_D(max) to avoid the damage of the zener diode.

 

6. Applications of Zener Diode

• The various applications of zener diode are,

1. As a voltage regulating element in voltage regulators.

2. In various protection circuits.

3. In zener limiters i.e. clipping circuits which are used to clipp off the unwanted portion of the voltage waveform.

Review Questions

1. Narrate the operation of a zener diode.

AU : May-08, Marks 6; Dec.-13, Marks 4

2. Explain V-I characteristics of zener diode.

AU : May-08, 10, Marks 8; Dec.-13, Marks 4

3. Draw and explain the V-I characteristics of a zener diode. What are the two breakdown mechanisms in a zener diode ?

4. Differentiate between zener avalanche breakdown.

5. Compare zener diode and conventional p-n junction diode.