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

• 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,

Here, = Resulting change in zener potential due to variation in temperature = Original zener breakdown voltage at temperature = 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.

V-I CHARACTERISTIC OF ZENER DIODE

(In V)Leakage current

Zener current

(In mA)

• 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)

LIGHT EMITTING DIODE

• LED emits light in response to sufficient forward current (IF).

IF

Figure 3: Forward biased LED

Radiated light(in mW)

I F (in mA)

Figure 4: Light output versus forward current

• Few examples of semiconductor material used for manufacturing of LED are GaAsP, GaAs, GaP, SiC.

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

• 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 850-940 nm Infra-red

GaAsP 630-660 nm Red

GaInN 450 nm White

AlGaP 550-570 nm Green

SiC 430-505 nm Blue

Applications

• 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.