PART 4

PART 2DIODES and APPLICATIONS I. DIODES1. VACUUM TUBE DIODE Also known as Fleming Valve

Components that conduct current in one direction only

Developed by J.A Fleming

2. SEMICONDUCTOR DIODE

An electronic device that results from the fusion of P-type material and an N-type material

Formation of the Depletion Region

a. Diffusion

Movement of electrons from a region of high concentration to lower concentration b. At the junction, the electrons from then n-type and holes from the p- type attract each other, combine and cancel their net charges c. Due to the cancellation of electrons and holes at the junction, the junction will have no more carriers so it is called a depleted region or depletion regionIdeal diode is a nonlinear device with a current versus voltage

Biasing the PN Junction a. Forward Biased P-type is more positive than the N-type

A condition that allows current to flow through the pn junction

The pn junction narrows

b. Reverse Biased

P-type is more negative than the N-type A condition that prevents current to flow through the pn junction The pn junction widens

Diode Parameters a. Bulk Resistance (Rb)

Rb = Rp + Rn =

Where: Vf forward voltage drop

Vb barrier potential

b. Junction Resistance (Rj)

; Id forward dc current

c. Dynamic or AC Resistance (Rac)

Rac = Rj + Rb

d. Forward Voltage Drop

Forward Voltage Drop = Power dissipated / forward DC current e. Forward current

The magnitude of the current the diode can handle without being destroyed under forward biased condition f. Reverse Saturation Current It is the magnitude of the current that will leak when the iode is reverse biased

Leakage current

g. Reverse Breakdown Voltage

Peak inverse voltage(PIV) Maximum reverse voltage hat can be applied before current surges Voltage across the inductor when the diode is open

h. Reverse DC Resistance (Rr)

Rr = Reverse voltage/ reverse current i. Linear Power Derating Factor

The reduction of power handling capability of diode due to the increase in ambient temperature from the room temperature

Review Question:

A silicon diode has a forward voltage drop of 1.2V for a forward DC current of 100 mA. It has a reverse current of 1 uA for a reverse voltage of 10 V. Calculate the:

a. bulk resistance

Rb = = = 5 ohms

b. reverse resistance

Rr = = = 10 Mega ohms c. AC resistance at forward DC current of 2.5 mA

Rac = Rj + Rb

But Rj = = 10.4 ohms

SoRac = 10.4 + 5 = 15.4 ohms

II. SPECIAL PURPOSE DIODES 1. ZENER DIODE Diode designed to operate in the reverse breakdown region

a. Zener Breakdown When the breakdown voltage is below 5 voltsb. Avalanche Breakdown When the breakdown voltage is above 5 volts

Typical breakdown voltages of 1.8 Volts to 200 Volts with specified tolerances from 1 % to 20%

With very stable voltage drop

Useful as voltage regulator

Symbol :

2. POINT CONTACT DIODE Semiconductor diode having fine wire whose point is permanent contact with the surface of a wafer of a semiconductor material such as Silicon, Germanium or Gallium Arsenide

The fine wire is called cat-whisker

For signal mixing and detection

3. SCHOTTKY DIODE Also known as Surface Barrier Diode Also known as hot-carrier diodes This type of diode has no depletion layer which eliminates the stored charges in the junction

A rectifying metal semiconductor junction such as gold, silver and platinum. Typical forward voltage drop is typically around 0.25 V to 0.3 V Switch off faster than an ordinary diode

Can rectify frequencies up to 300 MHz

ESBAR(Epitaxial Schottky Barrier)

Symbol: 4. VARACTOR (varicap)

Voltage-variable capacitor

When this diode is reversed bias, the width of the depletion layer increases with the reverse voltage

The key idea is that the capacitances is controlled by voltage

Used foe electronic tuning, harmonic generator and parametric amplifier

Symbol: Figure of Merit =

where: ffrequency(Hz); Rs Series resistance

Ct total diode capacitance(F)

5. TUNNEL DIODE Also known as Esaki diode

Type of diode that exhibit the phenomenon known as negative resistance

Negative resistance implies that an increase in forward voltage produces a decrease in forward current for a certain part. Utilizes a heavily doped material and therefore have so many electrons. Has a very thin depletion region The extremely narrow depletion region emits electrons to tunnel through the pn junction at very low forward bias voltage Used for oscillators and amplifiersSymbol:

6. BACK DIODE Conducts better in the reverse (-0.1 V) than in the forward (+0.7 V) direction

Designed such that its high current flow takes place when the junction is reversed bias

Symbol:

7. PIN Diode

Positive Intrinsic-Negative Diode

The intrinsic material between the P and N layer offers impedance at microwave frequencies being controlled by low frequency signals

Used in microwave switches 8. LIGHT EMITTING DIODE In a forward biased LED, free electrons cross the junction and fall into holes. As these electrons fall from higher to a lower energy level, they radiate energy goes off in the form of heat. But in a LED, the energy is being radiated as a light

Commonly used Gallium Arsenide, Gallium Arsenide Phosphide and Gallium Phosphide GaAs LEDs emit infrared (IR) radiation which is non invisible, GaAsP produces either red or yellow visible light and GaP emits red or green visible light.

Red is the most common color of LEDs Electroluminescense is the process involved when large surface area on one layer of one semiconductive material permits the photons to be emitted as visible light Irradiance is the power per unit area at a given distance from a LED source expressed in mW/ cm2.Symbol: Typical voltage Drop:

1.5 V to 2.5 V for currents between 10 mA and 50 mA

Nominal Voltage Drop: 2V

Reverse breakdown: 3V 10V

9. LASER DIODE Term laser stands for light amplification by stimulated emission of radiation

Laser light is monochromatic meaning it consists of a single color and not a mixture of colors

Laser light is also a coherent light meaning single wavelength

Laser diode pn junction is formed by two layers of doped gallium arsenide

It is forward biased

Laser diodes and photodiodes are used in the pick-up system of compact disk (CD) layers. Audio information is digitally recorded in stereo on the surface of a compact disk in the microscopic pits and flats.10. PHOTODIODE Operated in reverse bias condition

Is one that is optimized for its sensitivity to light

A window let light to pass through the package of the junction. The incoming light produces free electrons and holes producing larger reverse current Dark current is the reverse current flowing through the photodiode when there is no incident lightSymbol:

III. APPLICATIONS

1. DIODE RECTIFIER Type of diode circuit that converts alternating current into unidirectional current(DC)

Typically seen in power supplies

Half Wave Rectifier

A rectifier circuit with a single diode that conduct current during positive or negative half cycles of input AC signal at a rate determined by the input frequency.

Conversion Factor = 40.6 %

Freqoutput = Freqinput

Full Wave Rectifier

A rectifier circuit that conducts current during positive and negative half cycles of input AC signal.

Conversion factor = 81.2 %

Freqoutput = 2 x Freqinput

Comparison of RectifiersHalf-waveCenter-TappedBridge-Type

Vrms

Vave

Irms

Iave

PIVVpk2VpkVpk

2. CLIPPERS Electronic circuits that have the ability to clip-off a portion of the input signal with out distorting the remaining part of the alternating waveform

Other names, limiters, amplitude selectors and slicersExamples of Clipper Circuits

3. CLAMPERS Circuits that shift the waveform of the input signal either all above or below the reference voltage Add or restore a DC level to an electrical signal Also known as DC restorer Clamping circuit is often used in television receivers as a dc restorer. Incoming composite video signal is normally processed through capacitively coupled amplifiers that eliminate the dc component losing the reference levels which must be restored using clamping circuits before applying to the picture tube.Example of a Clamper Circuit:

4. VOLTAGE MULTIPLIER Circuit which produces a greater DC output voltage than AC input voltage to the rectifiers Ss clamping action to increase peak rectified voltages without the necessity of increasing the inputs transformers voltage rating Used in high-voltage, low current applications such as TV receiversVoltage Doubler A voltage multiplier with a multiplication factor of 2Example:

At the first negative half cycle: At the first positive half cycle:

D1 = Forward Bias D1 = Reverse Bias

D2 = Reverse Bias D2 = Forward Bias

VC1= Vp VC1= Vp

VC2= 2Vp Vo = VC2 = 2Vp

Voltage Tripler Addition of another diode-capacitor section to the half wave voltage doubler creates voltage tripler. The PIV of each diode is 2Vp Example

Voltage Quadrupler The addition of still another diode-capacitor section in a voltage tripler circuit

The PIV of each diode is 2Vp

Example:

5. VOLTAGE REGULATION Means maintaining the output voltage at any load

The type of diode used for regulation is Zener diodeZener Regulation

Rs = =

Power Rating (Pz) = Vz Izmax

Take Note: as a rule of thumb, Izmin = 0.1IzmaxLoad regulation =

EMBED Equation.3 TEST YOURSELF 2Review Questions

1. The vacuum tube diode was developed by J. Fleming and is also known as ________.a. Fleming diodeb. Fleming valvec. audiond. dioneAnswer b. Fleming valve

2. In case a PN junction is forward biased

a. Holes and electrons seized to move

b. Electrons and holes moves away the junction

c. Electrons and holes move toward the junction

d. depletion region decreases

Answer d. depletion region decreases

3. The bulk resistance of a diode is_____

a. the resistance of N-material only

b. the resistance of P material only

c. the resistance of the junction only

d. the resistance of the P and the N material

Answer d. the resistance of the P and the N material

4. The depletion layer of the PN junction diode has

a. only free mobile electrons

b. only free mobile holes

c. both free mobile holes and mobile electrons

d.neither free mobile electrons nor holes

Answer d. neither free mobile electrons nor holes

5. A zener diode is invariably used with

a. forward biased

b. reverse biased

c. Zero bias

d. no biasAnswer b. reversed biased6. The light emitting diode produces light when

a. Unbiased

b. forward bias

c. reverse bias

d. zero biasAnswer b. forward bias7. The PN junction offers

a. a high resistance in forward as well as reverse direction

b. Low resistance in forward as well as in reverse

c. conducts in forward direction only

d. conducts in the reverse direction onlyAnswer c. conducts in forward direction only8. The DC resistance of a diode is ________than its AC resistance.a. same as

b. more than

c. less than

d. all of the aboveAnswer c. less than9. The PIV rating of a crystal diode is ________that of equivalent vacuum diode.a. lower than

b. more than

c. the same as

d. all of the aboveAnswer a. lower than10. Crystal diode is a _____device.

a. non-linear

b. linear

c. amplifying

d. oscillatingAnswer a. non-linear11. A zener diode has______.

a. one PN junction

b. two PN junction

c. three PN junction

d. 1.5 PN junctionAnswer a. one PN junction12 The doping level in a zener diode is ______that of a crystal diode.a. more than

b. less than

c. the same as

d. none of theseAnswer a. more than13. The most widely used rectifier circuit is ______.a. half wave

b. center tap

c. bridge type

d. book typeAnswer c. bridge type14. If PIV rating of a diode is exceeded, the diode ________.a. stops conduction

b. is destroyed

c. conducts heavily in the forward direction

d. is not destroyedAnswer b. is destroyed15. The _______filter circuit results in the best voltage regulation

a. choke input

b. capacitor input

c. resistant input

d. transformer inputAnswer a. choke input16. A 60 Hz sine voltage is applied to the input of a half wave rectifier, what is the output frequency?

a. 60 Hz

b. 120 Hz

c. 30 Hz

d. 180 HzAnswer a. 60 Hz17. If the load resistance of a capacitor filtered - full wave rectifier is reduced, the ripple voltage

a. increases

b. decreases

c, is not affected

d. has different frequencyAnswer a. increases18. Load regulation is determined by

a. changes in load current and input voltage

b. changes in load current and output voltage

c. changes in load resistance and input voltage

d. changes in zener current and load currentAnswer b. changes in load current and output voltage

19. If you are checking a 60 Hz full-wave bridge rectifier and observe that the output has a 60 Hz ripple,

a. the circuit is working properly

b. the transformer secondary is shorted

c. there is an open diode

d. the filter capacitor is leakyAnswer c. there is an open diode20. The cathode of a zener diode in a voltage regulator is normally

a. more positive than the anode

b. more negative than the anode

c. at +0.7 V

d. groundedAnswer a. more positive than the anode21. A no load condition means that

a. the load has infinite resistance

b. the load has zero resistance

c. the output terminals are pen

d. answers a and cAnswer d. answers a and c22. The internal resistance of a photodiode

a. increases with light intensity when reversed bias

b. decreases with light intensity when reversed bias

c. increases with light intensity when forward bias

d. decreases with light intensity when forward biasAnswer b. decreases with light intensity when reversed bias 23. A diode that has a negative resistance is the ________.

a. Schottky diode

b. Tunnel diode

c. Laser diode

d. hot-carrier diode

Answer b. Tunnel diode

24. The small value of direct current that flows when a semiconductor device has a reverse bias (November, 1999)

a. surge current

b. bias current

c. reverse current

d. current limitAnswer c. reverse current

25. How does a junction diode rated? (November, 1996)

a. maximum reverse current and PIV

b. maximum forward current and PIV

c. maximum forward current and capacitance

d. maximum reverse current and capacitanceAnswer b. maximum forward current and PIV

26. Where do you measure the positive direct current output voltage from a half wave rectifier circuit? (November, 2003)

a. anode terminal

b. any terminal

c. cathode terminal

d. ground terminalAnswer c. cathode terminal 27. The main disadvantage of a conventional full-wave rectifier is that the peak voltage is_____that of a half wave rectifier. (November, 2003)

a. triple

b. half

c. double

d. quadrupleAnswer b. half28. What is the condition of the diode in a series limiter when the output is developed? (November, 2003)

a. conducting

b. cut-off

c. shorted

d. shuntedAnswer a. conducting29. As a rule of thumb, what is an acceptable ratio of back-to-forward resistance for a diode? (November, 2003)

a. 2 to 1

b. less than 10 to 1

c greater than 10 to 1

d. 5 to 1Answer c. greater than 10 to 130. What do you call the tiny crystal of semiconductor material that glows when an electric current passes through it? (November, 2003)

a. LED

b. laser beam

c. photon

d. liquid crystalAnswer d. liquid crystal31. The percentage change in output voltage for a given change in load current is called________ (November, 2003)

a. load regulation

b. line regulation

c. output regulation

d. voltage regulationAnswer a. load regulation32. How many diodes does a bridge circuits require? (April, 2004)

a. two diodes

b. three diodes

c. five diodes

d. four diodesAnswer d. four diodes33. What type of circuit would a zener diode is most likely used in? (April, 2004)

a. voltage regulator

b. amplifier

c. oscillator

d. rectifier Answer a. voltage regulator34. What is the correct operating voltage for a typical LED? (April, 2004)

a. +3 to +6 V

b. -6 to +5 V

c. +12 to +18 V

d. 0 to +120 VAnswer a. +3 to +6 V

35. The ratio of a varactors capacitance at specified minimum voltage to the capacitance at a specified maximum voltage.

a. tuning ratio

b. percentage ratio

c. capacity ratio

d. efficiency ratioAnswer a. tuning ratio

EMBED Visio.Drawing.5

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Power Supply

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Cathode Band

Cathode

Anode

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cathode

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Metal silicon junction

cathode

Anode

Construction of Schottky Diode

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