Zener diode

Zener diode

Zener diode is a heavily doped silicon diode used in reverse biased condition and is named after its inventor C. Zener. It is specially designed to be operated in the breakdown region. The doping level of the Silicon diode can be varied to have a wide range of breakdown voltages from 2 V to over 1000 V.

As explained in the previous section, Zener breakdown occurs due to the breaking of covalent bonds by the strong electric field set up in the depletion region by the reverse voltage. It produces an extremely large number of electrons and holes which constitute the reverse saturation current. The current is limited by both external resistance and power dissipation of the diode. A Zener diodes is shown in Figure 9.19(a) and its circuit symbol of Zener diode is shown in Figure 9.19(b).

It looks like an ordinary p-n junction diode except the cathode lead approximating the shape of a ‘z’ letter. The arrow head points the direction of conventional current. In Figure 9.19(a), black ring indicates the cathode lead.

V-I Characteristics of Zener diode

The circuit to study the forward and reverse characteristics of a Zener diode is shown in Figure 9.20(a) and Figure 9.20 (b). The V-I characteristics of a Zener diode is shown in Figure 9.20(c). The forward characteristic of a Zener diode is similar to that of an ordinary p-n junction diode. It starts conducting approximately around 0.7 V. However, the reverse characteristics is highly significant in Zener diode. The increase in reverse voltage  normally generates very small reverse current. While in Zener diode, when the reverse voltage is increased to the breakdown voltage (V_Z), the increase in current is very sharp. The voltage remains almost constant throughout the breakdown region. In Figure 9.20(c), I_Z(max) represents the maximum reverse current. If the reverse current is increased further, the diode will be damaged. The important parameters on the reverse characteristics are

V_Z →Zener breakdown voltage

I_Z(min) → minimum current to sustain breakdown

I_Z(max) → maximum current limited by maximum power dissipation.

The Zener diode is operated in the reverse bais having the voltage greater than V_Z and current less than I_Z(max). The reverse characteristic is not exactly vertical which means that the diode possesses some small resistance called Zener dynamic impedance. Zener resistance is the inverse of the slope in the breakdown region. It means an increase in the Zener current produces only a very small increase in the reverse voltage. However this can be neglected. The voltage of an ideal Zener diode does not change once it goes into breakdown. It means that V_Z remains almost constant even when I_Z increases considerably.

The maximum reverse bias that can be applied before entering into the Zener region is called the Peak inverse voltage. Commercially referred as PIV rating.

Applications

The zener diode can be used as

• Voltage regulators

• Calibrating voltages

• Provide fixed reference voltage in a network for biasing

• Protection of any gadget against damage from accidental application of excessive voltage.

Zener diode as a voltage regulator

A Zener diode working in the breakdown region can serve as a voltage regulator. It maintains a constant output voltage even when input voltage V_i or load current I_L varies. The circuit used for the same is shown in Figure 9.21. Here in this circuit, the input voltage V_i is regulated at a constant voltage, V_z (Zener voltage) at the output represented as V₀ using a Zener diode. The output voltage is maintained constant as long as the input voltage does not fall below V_z.

When the potential developed across the diode is greater than V_Z , the diode moves into the Zener breakdown region. It conducts and draws relatively large current through the series resistance R_s. The total current I passing through R_s equals the sum of diode current I_Z and load current I_L (I = I_Z + I_L ). It is to be noted that the total current is always less than the maximum Zener diode current.

Under all conditions V_o =V_Z . Thus, output voltage is regulated.

EXAMPLE: 9.3

Find the current through the Zener diode when the load resistance is 1 KΩ. Use diode approximation.

Solution

Voltage across AB is V_Z = 9V

Voltage drop across R = 15 - 9 = 6V

Therefore current through the resistor R,

I = 6 / 1×10³ =6 mA

Voltage across the load resistor = V_AB = 9V

Current through load resistor

I_L = V_AB/R_L = 9 / 2×10³ = 4.5 mA

The current through the Zener diode,

I_Z = I −I_L =6 mA− 4.5mA =1.5 mA