Zener Breakdown and Avalanche Breakdown

Figure 1 gives the volt-ampere characteristic of a PN diode including the breakdown region. Thus, when a PN diode is highly reverse biased, the junction may breakdown i.e. it presents extremely small resistance with the result that the current increases abruptly at an almost constant voltage. This current may be limited only by placing a suitable resistor in the external circuit. However, the breakdown is not permanent. The diode returns to its normal condition when this large reverse bias is removed. The breakdown of diode may be of two types: (a) zener breakdown and (b) avalanche breakdown.

Zener Breakdown:

In the diode using heavily doped P and N regions, on application of a large reverse bias, zener breakdown may take place due to direct breaking of covalent bonds due to strong electric field at the junction. The new electron-hole pairs so created greatly increase the reverse current at an almost constant value of reverse bias typically below 6 volts for heavily diodes. Due to heavy doping of P and N regions, the width of depletion region becomes extremely small. Then, for an applied reverse bias of voltage as small as 6 volts or less, the field across the depletion region becomes very high, of the order of 10⁶ volt/cm causing zener breakdown.

Avalanche Breakdown:

For moderate doping in a diode, the depletion layer width is quite large so that the reverse bias needed to produce strong field to cause zener breakdown becomes excessively large. In such moderately or lightly doped diodes, the breakdown does not take place through zener breakdown mechanism but uses the mechanism of avalanche multiplication. Then let a thermally generated carrier fall down the junction barrier and acquire kinetic energy from the applied voltage. This carrier on colliding with a crystal ion with high kinetic energy causes rupture of a covalent bond. Such an ionization is called Impact ionization. Now in addition to the original carrier, a new electron-hole pair gets created.

These new carriers, in turn, may also require enough energy from the applied field, collide with crystal ions and generated new electron-hole pairs. Tis process is cumulative and is known as avalanche multiplication. The net result is a large reverse current at an almost constant high voltage drop and avalanche breakdown is said to take place.

Zener Diode (Breakdown Diode)

A zener diode, also called Breakdown diode is an PN diode specially designed for operation in the breakdown region in reverse bias condition. The diode may use either zener breakdown or avalanche breakdown. Diode using zener breakdown uses heavy doping of the order of 1 impurity atom per 10⁵ silicon atoms and is 10⁶ volt/cm. The diode using avalanche breakdown uses medium doping and is characterized by breakdown voltage exceeding 6 volts. However, the word zener is commonly used for all diodes having breakdown characteristics in the reverse direction.

Figure 1 gives the volt-ampere characteristics of typical silicon and germanium zener diodes. Breakdown diode is operated over the breakdown region and the current is limited by an external resistance. The breakdown voltage V_Z may be suitably selected by proper choice of doping level.

Circuit Symbol and Equivalent Circuit

Figure 2 (a) gives the circuit symbol of zener diode while figure 2(b) gives the equivalent circuit. this equivalent circuit includes a small dynamic resistance and a dc battery of voltage V_Z, equal to the zener voltage. The resistor r_z represents the resistance corresponding to the slight slope of the nominally vertical region of the characteristic during breakdown. However, in most applications, external resistors are much larger than r_z and hence r_z may be neglected in the equivalent circuit.

Zener Diode Specifications

Zener Voltage V_Z Manufacturers specify the value of breakdown voltage V_B, also known as the zener voltage V_Z at specific value of test current, Zener diodes are available for value of V_Z from 2.5 volts to over 500 volts with accuracies between 5% to 20%.

Power Dissipation It is the product of V_Z and I_Z. The maximum power rating varies from 150 mW to 250 watts.

Breakover Current I_ZK There is some curvature of the reverse breakdown characteristic at low values of I_Z. Hence, we specify some value of current near the knee where the voltage across the diode starts to differ appreciably for V_Z. Figure 3 shows this current I_ZK.

Dynamic Impedance This is defined as,

$r_z = \dfrac{\Delta V_Z}{\Delta I_Z}$ ………….(1)

Where $\Delta V_Z$ and $\Delta I_Z$ are the increments in V_Z and I_Z at any pint P on the reverse breakdown characteristic shown in figure 3.

Ideally, the breakdown curve should be a perfect vertical line and r_z has a small value ranging from a few ohms to several hundred ohms.

Effect of Temperature on V_Z Voltage V_Z is influenced by temperature variant. Temperature coefficient of V_Z gives the present change in V_Z with change in temperature and is defined as,

$T_c = \dfrac{\Delta V_Z}{V_Z(T_1-T_0)} = 100%$ ……..(2)

Where $\Delta V_z$ is the change in V_Z due to temperature change (T₁-T₀).

Study of variation of T_C with V_Z revels that

T_C may be both positive and negative and lies in the range +-0.1 percent per degree C.
For V_Z appreciably exceeding 6 volts characterizing avalanche breaking, T_C is positive.
For V_Z below 6 volts characterizing true zener breakdown, T_C is negative.

Zener Diode as a Voltage Regulator

Figure 4 gives the circuit using zener diode for providing a constant voltage V_Z across load positive R_L in-spite of variation in input voltage V_i or load resistance R_L. the source voltage V_i and the load resistor R_L are so selected that the diode operates in the breakdown region. From figure 4 we find that the voltage V_Z across the diode is also the voltage across the load resistor R_L and it undergoes very small change in-spite of variation in source voltage V_i or the load current. In fact the zener diode maintains the output voltage V_Z constant by adjusting the diode current I_Z itself so as to permit a constant load current through R_L.