1. Special Purpose Diodes
BHAVIN V KAKANI
Electronics & Communication Engineering Department
IT-NU

2. Application Specific diodes
Zener Diode
Varactor Diode
Schottky Diode
Light Emitting Diode (LED)
Laser Diode

3. (For Voltage Regulation)
Zener Diode
(For Voltage Regulation)

4. Introduction
A zener diode is a Si pn junction diode that is designed to operate in the reverse breakdown region.
Normal diode destroys in this region due to excessive current.
A zener diode is heavily doped to reduce the reverse breakdown voltage.
This causes a very thin depletion layer. As a result, a zener diode has a sharp reverse breakdown voltage VZ.

5. Introduction The curve reveals, two things happen when VZ is reached:
(i) The diode current increases rapidly. (ii) The reverse voltage VZ across the diode remains almost constant.
So, The zener diode operated in this region will have a relatively constant voltage across it, regardless of the value of current through the device.
This permits the zener diode to be used as a voltage regulator.

6. Introduction
The ability to keep the reverse voltage across its terminals essentially constant is the key feature of the zener diode.
A minimum value of reverse current, IZK, must be maintained in order to keep the diode in breakdown for voltage regulation.
when the reverse current is reduced below the knee of the curve, the voltage decreases drastically and regulation is lost.
There is a maximum current, IZM, above which the diode may be damaged due to excessive power dissipation.
So, The zener diode maintains a nearly constant voltage across its terminals for values of reverse current ranging from IZK to IZM.

7. Zener Equivalent Circuit
Ideal Zener exhibits a constant voltage, regardless of current draw.
Ideal Zener exhibits no resistance characteristics.

8. Principle of Zener breakdown
Zener breakdown occurs due to 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 large saturation current whose value is only limited by the external resistance in the circuit.
It is independent of the applied voltage.

9. Zener voltages
Zener diodes are available having Zener voltages of 2.4V to 200V.
This voltages is temperature dependent.
Their power dissipation is given by the relation VzIz.
Maximum ratings may vary from 150mW to 50W.

10. Zener Biasing
For proper working of a Zener diode in any circuit, it is essential that it must
Be reverse biased
Have voltage across it greater than Vz.
Be in a circuit where current is less than Izmax.

11. Diode Identification
IN750 (10W power) IN4000 (50W power)

12. Analysis of Zener circuit:

13. Circuit-1

14. Circuit-2

15. Circuit-3

18. Voltage Regulation
It is a measure of a circuits ability to maintain a constant output voltage even when either input voltage or load current varies.
Zener diode serve this purpose in breakdown region.
Vin is the input voltage whose variations are to be regulated

19. The Zener diode is reverse connected across Vin.
When p.d. across the diode is greater than Vz, it conducts and draw relatively large current through series resistance R.
The total current passing through R is
I = Iz + IL
If Vout = Vz, then
Vin = IR + Vz = IR + Vout

20. Case-1 (RL fixed, Vin varies)
Vin is increased slightly
It will increase I
This increase in I will be absorbed by the Zener diode without affecting IL.
The increase in Vin is dropped across R thereby keeping Vout =Constant
If Vin falls, drop across R reduced and thus keeping Vout =constant

21. Vin IZ Vout
Vin IZ Vout

22. Case-2 (Vin fixed, Iz changed)
When IL increases, diode current Iz decreases thereby keeping I and IR drop constant.
So Vout remains unaffected
Should IL decrease, Iz increases and keep I and IR drop constant, because
Vout = Vin-IR= Vin-(IL+Iz)R
R = (Vin-Vout)/(Iz+IL)

28. (Variable Capacitance)
Varactor Diode
(Variable Capacitance)

29. Introduction
The junction capacitance of diodes varies with the amount of reverse bias.
Varactor diodes are specially designed to take advantage of this characteristic and are used as voltage-controlled capacitors .
These devices are commonly used in communication systems.
Varactor diodes are also referred to as varicaps or tuning diodes.

30. A varactor diode is best explained as a variable capacitor
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.

31. In a varactor diode, these capacitance parameters are controlled by the method of doping near the pn junction and the size and geometry of the diode’s construction.
Nominal varactor capacitances are typically available from a few picofarads to several hundred picofarads.
A major application of varactors is in tuning circuits.

33. Light Emitting Diode
(Light Emitter)

34. Introduction
LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

35. The light-emitting diode (LED) emits photons as visible light
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.

36. The difference in energy between the electrons and the holes corresponds to the energy of visible light. When recombination takes place, the recombining electrons release energy in the form of photons.
The emitted light tends to be monochromatic (one color) that depends on the band gap (and other factors).
A large exposed surface area on one layer of the semiconductive material permits the photons to be emitted as visible light. This process, called electroluminescence
Various impurities are added during the doping process to establish the wavelength of the emitted light.

37. How does it work? Recombination produces light!! Electrical Contacts
P-n junction
Electrical Contacts
A typical LED needs a p-n junction
There are a lot of electrons and holes at the junction due to excitations
Electrons from n need to be injected to p to promote recombination
Recombination produces light!!
Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

38. Blue uses silicon carbide or gallium nitride
Todays LEDs (green,red, yellow) are based on indium, gallium, aluminum phosphide
Blue uses silicon carbide or gallium nitride
IR (infrared) – GaAs (gallium arsenide)
LED Biasing: 1.2V to 3.2V is typical.
Note: Some newer LED’s run at higher voltages and emit immense light energy. Applications:
Traffic signals
Outdoor video screens
Runway markers

39. Excitation E
Electron (excited by the biased forward voltage) is in the conduction band k
Normally the recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band (band exterma).
The emission of light is therefore;
hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition)
Hole is in valance band

40. Getting to know LED Advantages of Light Emitting Diodes (LEDs)
Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diode’s life in comparison to an incandescent bulb ( hours life time compared to ~1000 hours for incandescence light bulb)
Efficiency: Diodes emit almost no heat and run at very low amperes.
Greater Light Intensity: Since each diode emits its own light
Cost:
Not too bad
Robustness:
Solid state component, not as fragile as incandescence light bulb

41. Applications of LEDs
The seven segment display is an example of LEDs use for display of decimal digits.

42. (Monochromatic Emission)
LASER diode
(Monochromatic Emission)

43. Introduction
The term laser stands for light amplification by stimulated emission of radiation.
Laser light is monochromatic, which means that it consists of a single color and not a mixture of colors.
Laser light is also called coherent light, a single wavelength.
The laser diode normally emits coherent light, whereas the LED emits incoherent light.

44. Forward bias the diode and electrons move thru the junction, recombination occurs (as ordinary). Recombinations result in photon release, causing a chain reaction of releases and avalanching photons which form an intense laser beam.
Each photon produced in this process is identical to the other photons in energy level, phase relationship, and frequency.
Laser diodes have a threshold level of current above which the laser action occurs and below which the diode behaves essentially as an LED, emitting incoherent light.

45. Applications
Laser diodes and photodiodes are used in the pick-up system of compact disk (CD) players.
Narrow beam communication
Satellite communication
Defence services
Industrial production

46. (High Frequency, High switching device)
Schottky Diode
(High Frequency, High switching device)

47. Introduction
Schottky diodes are high-current diodes used primarily in high-frequency and fast- switching applications.
They are also known as hot-carrier diodes.
The term hot-carrier is derived from the higher energy level of electrons in the n region compared to those in the metal region.
A Schottky diode is formed by joining a doped semiconductor region (usually n- type) with a metal such as gold, silver, or platinum.

48. The forward voltage drop is typically around 0
The forward voltage drop is typically around 0.3 V because there is no depletion region as in a pn junction diode.
The Schottky diode operates only with majority carriers. There are no minority carriers and thus no reverse leakage current as in other types of diodes.
The metal region is heavily occupied with conduction-band electrons, and the n- type semiconductor region is lightly doped.
When forward-biased, the higher energy electrons in the n region are injected into the metal region where they give up their excess energy very rapidly.
Since there are no minority carriers, as in a conventional rectifier diode, there is a very rapid response to a change in bias.

49. The Schottky is a fast-switching diode, and most of its applications make use of this property.
It can be used in high-frequency applications and in many digital circuits to decrease switching times.