Zener diodes operate in reverse bias at the Zener voltage. When the reverse voltage is greater than or equal to the Zener voltage, the Zener diode is on and conducts a Zener current. When the reverse voltage is less than the Zener voltage, the diode is off. The breakdown characteristics of a Zener diode show that the reverse current remains small until the "knee" where breakdown begins and the internal Zener resistance decreases rapidly. The Zener equivalent circuit models the Zener impedance, so a change in Zener current produces a small change in Zener voltage.

2. Instructor: Muhammad Bilal
Zener DiodesZener Diodes
The Zener is a diode operated
in reverse bias at the Zener
Voltage (Vz).
• When Vi ≥ VZ
– The Zener is on
– Voltage across the Zener is VZ
– Zener current: IZ = IR – IRL
– The Zener Power: PZ = VZIZ
• When Vi < VZ
– The Zener is off
– The Zener acts as an open circuit

3. Instructor: Muhammad Bilal
Breakdown Characteristics
Figure shows the reverse portion of a zener diode’s characteristic
curve. As the reverse voltage (VR) is increased, the reverse current (IR)
remains extremely small up to the “knee” of the curve. The reverse
current is also called the zener current, IZ. At this point, the breakdown
effect begins; the internal zener resistance, also called zener impedance
(ZZ), begins to decrease as reverse current increases rapidly.

4. Instructor: Muhammad Bilal
Zener Equivalent Circuit
Figure (b) represents the practical model of a zener diode, where the
zener impedance (ZZ) is included. Since the actual voltage curve is not
ideally vertical, a change in zener current (ΔIZ) produces a small change
in zener voltage (ΔVZ), as illustrated in Figure (c).
Z
Z
Z
I
V
Z
∆
∆
=

5. Instructor: Muhammad Bilal
Varactor Diodes
A varactor diode is best explained as a variable capacitor. Think of the
depletion region a variable dielectric. The diode is placed in reverse bias.
The dielectric is “adjusted” by bias changes.

6. Instructor: Muhammad Bilal
Varactor Diodes
The varactor diode can be useful in filter
circuits as the adjustable component.

7. Instructor: Muhammad Bilal
Optical Diodes
The light-emitting diode (LED) emits photons as visible
light. It’s purpose is for indication and other intelligible displays.
Various impurities are added during the doping process to vary
the color output.

8. Instructor: Muhammad Bilal

9. Instructor: Muhammad Bilal
Optical Diodes
The seven segment display is an example of LEDs use for
display of decimal digits.

10. Instructor: Muhammad Bilal
Optical Diodes
The seven segment display is an example of LEDs use for
display of decimal digits.