Zener diode

Zener diode

• A zener diode is a silicon p‒n junction semiconductor device which is operated in its reverse breakdown region.

• The zener diodes are fabricated with precise breakdown voltages by controlling the doping level during manufacturing.

• When reverse biased, if the reverse current of zener diode is limited using a series resistance then the power dissipation at the junction is limited to such a level which will not damage the diode and the zener diode continues to operate safely in reverse breakdown region.

• Fig. 3.1.1 (a) shows the symbol of zener diode. Fig. 3.1.1 (b) shows the forward biasing of zener diode. It acts similar to the conventional diode in forward biased. Fig. 3.1.1 (c) shows the reverse biasing of zener diode. But in reverse biased, it is operated in reverse breakdown region.

1. Breakdown Mechanisms in Zener Diode

• In zener diode there are two distinct mechanisms due to which breakdown occurs.

1. Zener breakdown: When reverse voltage is less than 6 V, there exists an intense electric field across the p‒n junction. Due to this, high force is exerted on the valence electrons of atoms, tending to separate them from respective nuclei. Thus large electron‒hole pairs are generated due to which there is a reverse breakdown, causing large current to flow.

2. Avalanche breakdown: When reverse voltage is greater than 6 V, the minority charge carriers accelerate. There are collisions between these carriers and electrons involved in the covalent bonds of the crystal. Due to this collision, the valence electron in covalent bond becomes free and accelerate. This further collides with another valence electron to make it free. This is called carrier multiplication. This action is very fast to generate large electron‒hole pairs and breakdown occurs.

Zener breakdown

1. Breakdown is due to intense electric field across the junction.

2. Occurs for zeners with zener voltage 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. Breakdown is due to the collision of accelerated charge carriers with the adjacent atoms and due to carrier multiplication.

2. Occurs for zeners with zener voltage greater than 6 V

3. The temperature coefficient is positive.

4. The breakdown voltage increases as junction temperature increases.

5. The V‒1 characteristics is not as sharp as zener breakdown.

2. V‒I Characteristics of Zener Diode

• In the forward biased condition, the normal diode and the zener diode operate in similar fashion.

• But zener diode is designed to operate in reverse breakdown region hence its reverse V‒I characteristics is important.

• When the reverse voltage is applied to zener diode, initially current is small, which is it's reverse saturation current.

• At a certain reverse voltage, the reverse breakdown occurs and current in the zener diode increases rapidly. The sharp change in the zener current is called knee or zener knee of the reverse characteristics.

• The reverse bias voltage at which the breakdown occurs is called zener breakdown voltage, denoted as Vz. This value is carefully designed by controlling the doping level during manufacturing.

• The V‒I characteristics of zener diode is shown in Fig. 3.1.2.

• For zener diodes, practically two currents are specified. The I_Zmin is minimum current through the zener diode to maintain its reverse breakdown operation.

• The I_Zmax is the maximum current which zener diode can take safely maintaining its reverse breakdown operation, i.e. constant V_Z across it. If reverse current exceeds this value, the diode may get damaged due to excessive power dissipation.

3. Equivalent Circuit of a Zener Diode

• Practically though very small, zener has its internal resistance.

• In the zener region, this resistance is called dynamic resistance of the zener denoted as Z_Z.

• Practically zener region is not exactly vertical. The small change in zener current Δlz produces a small change in zener voltage ΔV_Z. The ratio of ΔV_Z to ΔI_Z is called zener resistance Z_Z. This is shown in Fig. 3.1.3 (a).

• Hence practically zener equivalent circuit is shown with a battery of V_Z alongwith a series resistance Z_Z as indicated in Fig. 3.1.3 (b).

• Mathematically zener resistance is given by,

Z_Z = ΔV₂/ΔΙ_Ζ

= 1 / [ΔI₂/ΔV_Ζ]

= 1/ Slope of the reverse characteristics in zener region

4. Zener Diode Ratings

1. Zener voltage (V_Z): This is the normal working voltage of zener diode which is specified at a specific value of zener test current I_ZT. The standard tolerance is ± 10 % is specified means it can vary within ±10% of its nominal value.

2. Zener test current (I_ZT or I_Z): It is the normal working current of the zener diode at which the nominal zener voltage V_Z is specified.

3. Dynamic resistance (Z_Z): It is the resistance of the zener diode specified at normal working current and voltage but obtained as the ratio of ΔV_Z to Δl_Z.

4. Minimum zener current (I_Zmin): It is minimum current through the zener diode to maintain its reverse breakdown operation.

5. Maximum zener current (I_Zmax): It is the maximum current which zener diode can take safely without exceeding its maximum power dissipation limit.

6. Power dissipation (P_D): It is the product of the normal zener voltage V_Z and the normal working zener current I_Z.

7. Maximum power dissipation (P_D(max)): It is the maximum power which zener diode can dissipate safely. It is the product of the normal zener voltage V_Z and the maximum zener current I_Zmax.

5. Comparison of Zener Diode and Conventional Diode

Zener diode

1. Operated in reverse breakdown condition.

2. The characteristics lies in third quadrant.

3. Dynamic zener resistance is very small in reverse breakdown condition.

4. Zener diode symbol is, 

5. The conduction in zener is opposite to that of arrow in the symbol, as operated in breakdown region.

6. The power dissipation capability is very high.

7. Applications of zener diode are voltage regulator, protection circuits, voltage limiters etc.

P‒N junction diode

1. Operated in forward biased condition and never operated in reverse breakdown condition.

2. The characteristics lies in first quadrant.

3. The diode resistance in reverse biased condition is very high.

4. The p‒n junction diode symbol is, 

5. The conduction when forward biased is in same direction as that of arrow in the symbol, when forward biased.

6. The power dissipation capability is very low compared to zener diodes.

7. Applications of p‒n junction diode are rectifiers, voltage multipliers, clippers, clampers and many electronic devices.

6. Applications of Zener Diode

• The various applications of zener diode are,

1. As a voltage regulator.

2. In voltage clipper circuits.

3. For controlling the output amplitude.

4. As a reference voltage in comparator circuits.

5. As a standard voltage source in calibrating the instruments.

Review Questions

1. Explain the breakdown mechanisms in zener diode.

2. Distinguish between zener and avalanche breakdown.

3. Draw the VI characteristics of zener diode and explain its operation.

4. Explain the dynamic resistance and equivalent circuit of a zener diode.

5. Explain the zener diode parameters.

6. Compare zener diode and the conventional p‒n unction diode.

7. State the applications of zener diode.