Zener Diode – Electronic device – Readyzone

Zener Diode 

First we discuss about the Zener region.

Zener region

Even though the scale of Fig. 6 is in tens of volts in the negative region, there is a point where the application of too negative a voltage will result in a sharp change in the characteristics, as shown in Fig. 7. The current increases at a very rapid rate in a direction opposite to that of the positive voltage region. The reverse-bias potential that results in this dramatic change in characteristics is called the Zener potential and is given the symbol V_Z

As the voltage across the diode increases in the reverse-bias region, the velocity of the minority carriers responsible for the reverse saturation current I_s will also increase. Eventually, their velocity and associated kinetic energyW_k = ½ mv² ) will be sufficient to release additional carriers through collisions with otherwise stable atomic structures. That is, an ionization process will result whereby valence electrons absorb sufficient energy to leave the parent atom. These additional carriers can then aid the ionization process to the point where a high avalanche current is established and the avalanche breakdown region determined.

Semiconductor_diode

Fig. 6 – Silicon semiconductor diode characteristics.

The avalanche region (V_Z) can be brought closer to the vertical axis by increasing the doping levels in the p- and n-type materials. However, as V_Z decreases to very low levels, such as -5 V, another mechanism, called Zener breakdown, will contribute to the sharp change in the characteristic. It occurs because there is a strong electric field in the region of the junction that can disrupt the bonding forces within the atom and “generate” carriers. Although the Zener breakdown mechanism is a significant contributor only at lower levels of V_Z, this sharp change in the characteristic at any level is called the Zener region and diodes employing this unique portion of the characteristic of a p-n junction are called Zener diodes.

Zener_diode

Fig. 7 – Zener region.

The Zener region of the semiconductor diode described must be avoided if the response of a system is not to be completely altered by the sharp change in characteristics in this reverse-voltage region.

The maximum reverse-bias potential that can be applied before entering the Zener region is called the peak inverse voltage (referred to simply as the PIV rating) or the peak reverse voltage (denoted by PRV rating).

 Zener diode

The characteristic drops in an almost vertical manner at a reverse-bias potential denoted V_Z. The fact that the curve drops down and away from the horizontal axis rather than up and away for the positive V_D region reveals that the current in the Zener region has a direction opposite to that of a forward-biased diode.

Zener_diode

Fig. 8 – Conduction direction: (a) Zener diode; (b) semiconductor diode.

This region of unique characteristics is employed in the design of Zener diodes, which have the graphic symbol appearing in Fig. 8a. Both the semiconductor diode and zener diode are presented side by side in Fig. 8 to ensure that the direction of conduction of each is clearly understood together with the required polarity of the applied voltage. For the semiconductor diode the “on” state will support a current in the direction of the arrow in the symbol. For the Zener diode the direction of conduction is opposite to that of the arrow in the symbol as pointed out in the introduction to this section. Note also that the polarity of V_D and V_Z are the same as would be obtained if each were a resistive element. 

The location of the Zener region can be controlled by varying the doping levels. An increase in doping, producing an increase in the number of added impurities, will decrease the Zener potential. Zener diodes are available having Zener potentials of 1.8 to 200 V with power ratings from ¼ to 50 W. Because of its higher temperature and current capability, silicon is usually preferred in the manufacture of Zener diodes.

Zener_diode

Fig. 9 – Zener equivalent circuit: (a) complete;
(b) approximate.

The complete equivalent circuit of the Zener diode in the Zener region includes a small dynamic resistance and dc battery equal to the Zener potential, as shown in Fig. 9. For all applications to follow, however, we shall assume as a first approximation that the external resistors are much larger in magnitude than the Zener-equivalent resistor and that the equivalent circuit is simply the one indicated in Fig. 9b.

A larger drawing of the Zener region is provided in Fig. 10 to permit a description of the Zener nameplate data appearing in Table 1 for a 10-V, 500-mW, 20% diode. The term nominal associated with V_Z indicates that it is a typical average value. Since this is a 20% diode, the Zener potential can be expected to vary as 10 V ± 20% or from 8 to 12 V in its range of application. Also available are 10% and 5% diodes with the same specifications. The test current I_ZT is the current defined by the ¼ power level, and Z_ZT is the dynamic impedance at this current level. The maximum knee impedance occurs at the knee current of I_ZK.

The reverse saturation current is provided at a particular potential level, and I_ZM is the maximum current for the 20% unit. 

Zener_diode

Fig. 10 – Zener test
characteristics.

Zener diode

TABLE 1 – Electrical Characteristics (25°C Ambient Temperature Unless Otherwise Noted)

The temperature coefficient reflects the percent change in V_Z with temperature. It is defined by the equation

zener diode

where V_Z is the resulting change in Zener potential due to the temperature variation. Note in Fig. 11a that the temperature coefficient can be positive, negative, or even zero for different Zener levels. A positive value would reflect an increase in V_Z with an increase in temperature, while a negative value would result in a decrease in
value with increase in temperature. The 24-V, 6.8-V, and 3.6-V levels refer to three Zener diodes having these nominal values within the same family of Zeners. The curve for the 10-V Zener would naturally lie between the curves of the 6.8-V and 24-V devices. Returning to Eq. (1), T_0 is the temperature at which V_Z is provided (normally room temperature—25°C), and T_1 is the new level. 

Zener_diode

Fig. 11 – Electrical characteristics for a 10-V, 500-mW Zener diode.

The variation in dynamic impedance (fundamentally, its series resistance) with current appears in Fig. 11b. Again, the 10-V Zener appears between the 6.8-V and 24-V Zeners. Note that the heavier the current, the less the resistance value. Also note that as you drop below the knee of the curve, the resistance increases to significant levels.

The terminal identification and the casing for a variety of Zener diodes appear in Fig. 12. Figure 13 is an actual photograph of a variety of Zener devices. Note that their appearance is very similar to the semiconductor diode.

Zener_diode

Fig. 12 – Zener terminal
identification and symbols.

Zener_diode

Fig. 13 – Zener diodes.
(Courtesy Siemens Corporation.)

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Keywords – Zener diode, zener diodes, zener, zener diode voltage, zener diode application, zener diode working, zener diode circuit, power zener diode, zener diode application.

Reference :- Robert L. Boylestad and Louis Nashelsky, Electronic device and circuit theory, Seventh edition, Prentice Hall

One Comment

  1. Zener diodes are widely used in electronic equipment of all kinds and are one of the basic building blocks of electronic circuits.

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