1. ZENER DIODE / BREAKDOWN DIODE
Zener diode permits the current to flow in forward direction as well as in reverse direction when the voltage exceeds breakdown voltage i.e. zener voltage (VZ)
Operates in breakdown region in reverse bias condition.
Zener diode may use avalanche breakdown or zener breakdown mechanism,
Fig 1: Zener diode symbol

2. Zener diodes are like ordinary PN junction diode except that it is properly doped so as to have a sharp breakdown voltage.
Generally, Si is preferred over Ge because:
- Higher operating temperature
- Current capability
- Knee point is more sharp in case of Si diode.
Effect of temperature on Vz
The effect of temperature on zener voltage is expressed as temperature coefficient (TC). Temperature coefficient is the relative change of a physical property when temperature is changed by 1 K.
It is defined by the equation,
𝑇𝐶= ∆ 𝑉 𝑍 𝑉 𝑍 𝑇 1 − 𝑇 0 ×100
Here, ∆ 𝑉 𝑍 = Resulting change in zener potential due to variation in temperature
𝑉 𝑍 = Original zener breakdown voltage at temperature 𝑇 0
𝑇 1 = New temperature
Positive value of TC indicates increase in VZ with increase in temperature while when TC is negative VZ decreases with increase in temperature.

3. V-I CHARACTERISTIC OF ZENER DIODE
(In mA)
Leakage current
(In V)
Zener current

4. The forward characteristic is similar to normal PN Junction diode.
On gradually increasing the reverse biasing , the junction breaks down at a specific well defined voltage known as zener voltage (VZ) and current increases abruptly known as zener current (IZ).
VZ depends on
- Doping concentration
-Thickness of depletion layer
During the operation in breakdown region, the current through the diode is limited by the external circuit within permissible values, so it does not burn out.
Applications of Zener diode
- Switching operation (Zener diode can produce a sudden change from low current to high current. It makes possible an extremely fast performance in computer applications)
-Peak clipper
- Voltage regulator/ shunt regulator ( Circuits which maintain constant output voltage even when either input voltage or load current varies).
-Zener diode as reference element (Refer from Boylstead Page 88)

5. LIGHT EMITTING DIODE
LED emits light in response to sufficient forward current (IF).
Radiated light(in mW) IF
I F (in mA)
Figure 3: Forward biased LED
Figure 4: Light output versus forward current
Few examples of semiconductor material used for manufacturing of LED are GaAsP, GaAs, GaP, SiC.

6. STRUCTURE OF LED
Electron hole recombination takes place in P region, hence it is kept uppermost.
On forward biasing PN junction, electrons and holes recombine. Free electrons, found in conduction band, have greater energy than holes (found in valence band). After recombination, the energy of electron decreases and falls from CB to VB, thus releasing energy in the form of heat or light. This process of emitting photon is known as electroluminescence.
The whole unit is enclosed in a hermetically sealed enclosure, which is also having a lens to focus the light out of LED structure.
Metal film anode connection
Gold film cathode connection

7. Semiconductor material
Choice of material depends on
The value of forbidden energy gap (Eg). This value of Eg determines the wavelength of light emitted due to release of energy during recombination process.
NOTE:
How does an incandescent light bulb work?
A wire filament is heated by passing an electric current through it. It works on the principle that any material heated to a high enough temperature will glow. The color of light given off depends on the temperature of the object being heated.
LEDs are solid-state devices, they can be extremely small and durable and provide much longer lamp life than normal light sources
Semiconductor material
Wavelength emitted
Color
GaAs nm
Infra-red
GaAsP nm
Red
GaInN
450 nm
White
AlGaP nm
Green
SiC nm
Blue

8. Applications ADVANTAGES
LED’s are used in visual displays of calculators, watches and many other display devices.
A very popular use of LED is in construction of “seven segment display”.
Lighting applications.
Remote controls, such as for TVs and VCRs, often use infrared LEDs.
In optical fiber and Free Space Optic communications.
ADVANTAGES
Fast response time (nanoseconds)
Offer good contrast ratios for visibility.
In general, LEDs operate at voltage levels from 1.7 to 3.3 V, which makes them completely compatible with solid-state circuits.