Engineering

1. College Name : Shree Swami Atmanand Saraswati Institute Of
Technology(SSASIT)(076)
Year : 2nd year(3rd sem) EC-2015
Subject Name : Electronic Devices &Circuits(EDC) (2131006)
Topic Name : SPECIAL PURPOSE DIODE
Prepared By :
Bhadani Reshma D. (140760111002)
Guided By : Prof.Foram N. Dharsandiya

2. THE ZENER DIODE
•Zener diodes are commonly used for voltage
regulation, that is, they maintain a
•constant voltage at the output.
•It is designed to work in the reverse breakdown
region.
• The breakdown voltage is adjusted by
controlling the level of doping.
•The V-I characteristic and zener diode symbol
are shown in Figure 1.

3. • There are two types of breakdowns in zener diodes.
•Zener Breakdown
•Avalanche Breakdown
• Avalanche breakdown occurs at high voltages typically
more than 5 V.
• Zener breakdown occurs at low voltages typically less
than 5 V.
Zener Breakdown

5. Breakdown Characteristics
•As the reverse voltage VR increases from 0V, the reverse
current IR remains extremely small.
•The reverse current starts to increase rapidly when the
VR reaches the knee point.
•The current at this point is called the zener knee
current IZK.
•The breakdown effect starts at this point.
• After this, the current starts to increase as the zener
impedance ZZ decreases.
• From the knee, the zener voltage VZ remains almost
constant.

7. Zener Regulation
• Keeping the voltage constant across its terminal is
the main advantage of zener diode.
•The minimum zener current required maintaining
voltage regulation is IZK.
•The maximum zener current allowed before it is damaged
is IZM.
• The zener voltage VZ, specified in the datasheets is the
voltage at the zener test current IZT.

8. Temperature Coefficient
•Temperature coefficient (𝑇𝐶) describes the percent
change in zener voltage for each degree Celsius change
in temperature.
• The formula for calculating the change in zener
voltage Δ𝑉𝑍 is given by
•Δ𝑉𝑍=𝑉𝑍×𝑇𝐶×Δ𝑇
• A positive temperature coefficient means the zener
voltage will increase with increase in temperature or
decrease with decrease in temperature.
• A negative temperature coefficient means the zener
voltage will decrease with increase in temperature or
increase with decrease in temperature.

9. Zener Power Dissipation and Derating
• Zener diode are specified to operate at the maximum
DC power dissipation PD(max).
•This is given in datasheet of the diode.
•The power dissipated by a zener diode at any zener
current IZ is given as 𝑃𝐷=𝑉𝑍𝐼𝑍.

10. ZENER DIODE APPLICATIONS
Zener Regulation with a Varying
Input Voltage (No Load Condition)

11. • Zener diode regulators are not veryefficient so they are
limited to applications that require low current to the load.
•Figure 5 illustrates the concept of zener voltage regulation.
•The zener diode will regulate the output voltage provided 𝑉𝐼
𝑁>𝑉𝑍 and 𝐼𝑍𝐾<𝐼𝑍<𝐼𝑍𝑀.
•As the input voltage VIN increases, IZ will increase and
there will be very small change in the output voltage
𝑉𝑍+Δ𝑉𝑍.
• As the input voltage VIN decreases, IZ will decrease and
there will be very small change in the output

12. THE VARACTOR DIODE
•The junction capacitance of diodes change with
reverse bias.
•Diodes designed to be used as voltage controlled
capacitors are called varactors.
• They always work in reverse bias.
•The depletion region acts as dielectric and p and n-
types act
•like capacitor plates

13. Basic Operation
• The capacitance of a material can be determined by
plate area A, dielectric constant 𝜖 and plate separation d
and is expressed as
𝐶=𝐴𝜖/𝑑
• As the reverse bias increases, the depletion region
becomes wide and increases the plate separation d. This
decreases the capacitance.
• As the reverse bias decreases, the depletion region
becomes narrow and decreases the plate separation d.
This increases the capacitance.

14. OPTICAL DIODES
•Two types of optical diodes are discussed in this
section.
• Light-Emitting Diode (LED) – These are light
emitters.
• Photodiodes – These are light detectors.

15. The Light-Emitting Diodes (LED)
Basic Operation
• When the LED is forward biased, electrons cross the
pn junction and Recombines with the holes in the p-
type.
•When these high energy electrons recombine with
holes, they release energy in the form of photons.
•The emission of these photons is called
electroluminescence.

16. •The doping determines the wavelength of the emitted
photons.

17. • The forward voltage VF across an LED is higher than
silicon rectifier diodes
• (typically between 1.2V to 3.2V).
• Reverse breakdown is lower than silicon rectifier
diodes (3V to 10V).
LED Biasing
• Figure 11(a) shows a typical LED circuit.
• The graph in Figure 11(b) shows that power of light
output is directly proportional to the forward current
IF.

19. Applications of LED
1. Seven – Segment Display
• LED’s are widely used in various applications
like indicator lights, read out displays etc.
• A very common application of LED’s is the
seven-segment display shown in Figure 12.
• Each segment is an LED.
• By forward biasing selected combination of
segments, any decimal digits can be formed.

21. 2. Remote Controls
• Infrared LED’s are commonly used in remote
controls.
• Infrared LED emits beam of invisible light.
• Each button corresponds to an electrical code
which is converted to a light code and
transmitted through the LED.
• The receiver recognizes the code and takes the
required action.

22. OTHER TYPES OF DIODES
• There are other types of diodes used for special
purpose and applications. They will be discussed in
• this section. Some of these are
• The Laser Diode
• The Schottky Diode
• The PIN Diode
• The Tunnel Diode
• Current Regulator Diode

23. The Laser Diode
• Laser stands for light
• amplification by stimulated emission of
radiation.
• Laser light is monochromatic meaning it
consists of single light.
Construction
• The pn junction is formed by two layers of
doped gallium arsenide .
• The length of the pn junction is related to the
wavelength of the laser.

25. The Schottky Diode
• Schottky diodes (Figure 21) are high current diodes
used in high frequency and fast switching
applications.
• A schottky diode is formed by joining a doped n-type
with a metal such as gold, silver of platinum.

26. • The forward voltage drop is 0.3V.
• There are only majority carriers with no reverse
leakage current.
• The metal conductor has many conduction band
electrons and n-type is also heavily doped.
• When forward biased, the n-type electrons move
across to the metal region and rapidly loss energy.
• The process is very fast which makes schottky
diodes ideal for fast switching application.

27. The PIN Diode
• The pin diode consists of
heavily doped p and n
region separated by
intrinsic (i) region
• In reverse bias, the pin
diode acts like a constant
capacitance
• When forward biased, it
acts like a current-
controlled variable
resistance .

28. The Tunnel Diode
• The tunnel diode exhibit a
characteristic known as
negative resistance.
• This makes it useful in
oscillator applications.
• The p and n region of the
tunnel diode are heavily
doped

29. Current Regulator Diode
• The current regulator diode keeps a constant current rather
than constant voltage as in the case of zener diode.
• The current regulator works in the forward bias and the
forward current remains constant for forward voltage range
from 1.5V to 6V.
• The constant forward current in called the regulator
current IP.
• This device should never be operated in reverse bias.