T HE Z ENER D IODE ◆ 113

3–1 T HE Z ENER D IODE

A major application for zener diodes is as a type of voltage regulator for providing
stable reference voltages for use in power supplies, voltmeters, and other instruments.
In this section, you will see how the zener diode maintains a nearly constant dc voltage
under the proper operating conditions. You will learn the conditions and limitations for
properly using the zener diode and the factors that affect its performance.
After completing this section, you should be able to
❏ Describe the characteristics of a zener diode and analyze its operation
❏ Recognize a zener diode by its schematic symbol
❏ Discuss zener breakdown
◆ Define avalanche breakdown

❏ Explain zener breakdown characteristics

◆ Describe zener regulation

❏ Discuss zener equivalent circuits

❏ Define temperature coefficient
◆ Analyze zener voltage as a function of temperature

❏ Discuss zener power dissipation and derating

◆ Apply power derating to a zener diode

❏ Interpret zener diode datasheets

The symbol for a zener diode is shown in Figure 3–1. Instead of a straight line repre- Cathode (K)
senting the cathode, the zener diode has a bent line that reminds you of the letter Z (for
zener). A zener diode is a silicon pn junction device that is designed for operation in the
reverse-breakdown region. The breakdown voltage of a zener diode is set by carefully con-
trolling the doping level during manufacture. Recall, from the discussion of the diode char-
acteristic curve in Chapter 2, that when a diode reaches reverse breakdown, its voltage Anode (A)
remains almost constant even though the current changes drastically, and this is the key to
䊱 F I G U R E 3– 1
zener diode operation. This volt-ampere characteristic is shown again in Figure 3–2 with
the normal operating region for zener diodes shown as a shaded area. Zener diode symbol.

IF 䊴 F I G U R E 3– 2
General zener diode V-I characteristic.

Breakdown
VZ
VR VF
Reverse-
breakdown
region is
normal
operating
region for
zener
diode IR

Zener Breakdown
Zener diodes are designed to operate in reverse breakdown. Two types of reverse breakdown
in a zener diode are avalanche and zener. The avalanche effect, discussed in Chapter 2, occurs
in both rectifier and zener diodes at a sufficiently high reverse voltage. Zener breakdown
114 ◆ S PECIAL -P URPOSE D IODES

occurs in a zener diode at low reverse voltages. A zener diode is heavily doped to reduce the
HISTORY NOTE
breakdown voltage. This causes a very thin depletion region. As a result, an intense electric
Clarence Melvin Zener, an field exists within the depletion region. Near the zener breakdown voltage (VZ), the field is in-
American physicist, was born in tense enough to pull electrons from their valence bands and create current.
Indianapolis and earned his PhD Zener diodes with breakdown voltages of less than approximately 5 V operate predom-
from Harvard in 1930. He was the inately in zener breakdown. Those with breakdown voltages greater than approximately
first to describe the properties of 5 V operate predominately in avalanche breakdown. Both types, however, are called
reverse breakdown that are zener diodes. Zeners are commercially available with breakdown voltages from less than
exploited by the zener diode. As a 1 V to more than 250 V with specified tolerances from 1% to 20%.
result, Bell Labs, where the device
was developed, named the diode Breakdown Characteristics
after him. He was also involved in
Figure 3–3 shows the reverse portion of a zener diode’s characteristic curve. Notice that as
areas of superconductivity,
the reverse voltage (VR) is increased, the reverse current (IR) remains extremely small up to
metallurgy, and geometric
the “knee” of the curve. The reverse current is also called the zener current, IZ. At this
programming.
point, the breakdown effect begins; the internal zener resistance, also called zener imped-
ance (ZZ), begins to decrease as the reverse current increases rapidly. From the bottom of
the knee, the zener breakdown voltage (VZ) remains essentially constant although it in-
creases slightly as the zener current, IZ, increases.

䊳 FIG UR E 3 – 3
Reverse characteristic of a zener VZ @ IZ
diode. VZ is usually specified at a VR
value of the zener current known as IZK (zener knee current)
the test current.

IZ (zener test current)

IZM (zener maximum current)

IR

Zener Regulation The ability to keep the reverse voltage across its terminals essentially
constant is the key feature of the zener diode. A zener diode operating in breakdown acts
as a voltage regulator because it maintains a nearly constant voltage across its terminals
over a specified range of reverse-current values.
A minimum value of reverse current, IZK, must be maintained in order to keep the diode
in breakdown for voltage regulation. You can see on the curve in Figure 3–3 that when the
reverse current is reduced below the knee of the curve, the voltage decreases drastically
and regulation is lost. Also, there is a maximum current, IZM, above which the diode may
be damaged due to excessive power dissipation. So, basically, the zener diode maintains a
nearly constant voltage across its terminals for values of reverse current ranging from IZK
to IZM. A nominal zener voltage, VZ, is usually specified on a datasheet at a value of reverse
current called the zener test current.

Zener Equivalent Circuits

Figure 3–4 shows the ideal model (first approximation) of a zener diode in reverse break-
down and its ideal characteristic curve. It has a constant voltage drop equal to the nominal
zener voltage. This constant voltage drop across the zener diode produced by reverse
breakdown is represented by a dc voltage symbol even though the zener diode does not
produce a voltage.
 T HE Z ENER D IODE ◆ 115

䊴 F I G U R E 3– 4
Ideal zener diode equivalent circuit
VZ 0
VR model and the characteristic curve.

+
V
– Z

IR
(a) Ideal model (b) Ideal characteristic curve

Figure 3–5(a) represents the practical model (second approximation) of a zener diode,
where the zener impedance (resistance), ZZ, is included. Since the actual voltage curve is
not ideally vertical, a change in zener current (¢IZ) produces a small change in zener volt-
age (¢VZ), as illustrated in Figure 3–5(b). By Ohm’s law, the ratio of ¢VZ to ¢IZ is the
impedance, as expressed in the following equation:
¢VZ
ZZ ⴝ Equation 3–1
¢IZ
Normally, ZZ is specified at the zener test current. In most cases, you can assume that ZZ is
a small constant over the full range of zener current values and is purely resistive. It is best
to avoid operating a zener diode near the knee of the curve because the impedance changes
dramatically in that area.

⌬VZ 䊴 F I G U R E 3– 5

VR 0 Practical zener diode equivalent

circuit and the characteristic curve
IZK
illustrating ZZ.
+

ZZ
VZ
+
⌬VZ
– ZZ = ⌬IZ
⌬IZ
–

IZM

IR
(a) Practical model (b) Characteristic curve. The slope is exaggerated for illustration.

For most circuit analysis and troubleshooting work, the ideal model will give very good
results and is much easier to use than more complicated models. When a zener diode is op-
erating normally, it will be in reverse breakdown and you should observe the nominal
breakdown voltage across it. Most schematics will indicate on the drawing what this volt-
age should be.
116 ◆ S PECIAL -P URPOSE D IODES

EXAMPLE 3–1 A zener diode exhibits a certain change in VZ for a certain change in IZ on a portion of
the linear characteristic curve between IZK and IZM as illustrated in Figure 3–6. What
is the zener impedance?

䊳 F IGURE 3–6
⌬VZ = 50 mV
0
VR
IZK

10 mA

⌬IZ = 5 mA

15 mA

IZM

IR

¢VZ 50 mV
Solution ZZ = = = 10 æ
¢IZ 5 mA
Related Problem* Calculate the zener impedance if the change in zener voltage is 100 mV for a 20 mA
change in zener current on the linear portion of the characteristic curve.

*
Answers can be found at www.pearsonhighered.com/floyd.

Temperature Coefficient
The temperature coefficient specifies the percent change in zener voltage for each degree
Celsius change in temperature. For example, a 12 V zener diode with a positive temper-
ature coefficient of 0.01%/°C will exhibit a 1.2 mV increase in VZ when the junction
temperature increases one degree Celsius. The formula for calculating the change in
zener voltage for a given junction temperature change, for a specified temperature
coefficient, is
Equation 3–2 ¢VZ ⴝ VZ : TC : ¢T
where VZ is the nominal zener voltage at the reference temperature of 25°C, TC is the tem-
perature coefficient, and ¢T is the change in temperature from the reference temperature.
A positive TC means that the zener voltage increases with an increase in temperature or
decreases with a decrease in temperature. A negative TC means that the zener voltage
decreases with an increase in temperature or increases with a decrease in temperature.
In some cases, the temperature coefficient is expressed in mV/°C rather than as %/°C.
For these cases, ¢VZ is calculated as
Equation 3–3 ¢VZ ⴝ TC : ¢T