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UNIT II

SPECIAL SEMICONDUCTOR DEVICES

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A photodiode is a semiconductor device that converts light into an electrical current. The current

is generated when photons are absorbed in the photodiode.

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Applications of Photo Diode:

Photodiodes are used in

• Light detectors

• Demodulators

• Encoders

• Optical communication systems

• High speed counting and switching systems

• Computer card punching and tapes

• Light operated switches

• Sound track films

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Module 1 Questions:

1. Compare Zener Breakdown and Avalanche Breakdown in detail.

2. Discuss the applications of Zener Diode.

3. Draw and explain the VI characteristics of a Zener Diode.

4. Compare and contrast Zener diode and conventional PN Junction Diode.

5. Draw and explain the basic structure of LED. Mention the applications of LED.

6. Write notes on Liquid Crystal Display.

7. Describe the characteristics and applications of a photodiode.

8. What is Varactor Diode? Mention the applications of Varactor Diode.

9. Draw and explain VI characteristics of Tunnel Diode.

10. Discuss the energy band structure of a Tunnel Diode.

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Silicon Controlled Rectifier:

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Normally SCRs have high switching speed and can handle heavy current flow. This makes them

ideal for many applications like

1. Power switching circuits (for both AC and DC)

2. Zero-voltage switching circuits

3. Over voltage protection circuits

4. Controlled Rectifiers

5. Inverters

6. AC Power Control (including lights, motors, etc.)

7. Pulse Circuits

8. Battery Charging Regulator

9. Latching Relays

10. Computer Logic Circuits

11. Remote Switching Units

12. Phase Angle Triggered Controllers

13. Timing Circuits

14. IC Triggering Circuits

15. Welding Machine Control

16. Temperature Control Systems

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TRIAC (TRIODE AC SWITCH):

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DIAC (DIODE AC SWITCH):

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UNIJUNCTION TRANSISTOR (UJT):

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A phototransistor is a device that converts light energy into electric energy. A Phototransistor is

an electronic switching and current amplification component which relies on exposure to light to

operate. When light falls on the junction, reverse current flows which is proportional to the

luminance. Phototransistors are used extensively to detect light pulses and convert them into

digital electrical signals. These are operated by light rather than electric current. Providing large

amount of gain, low cost and these phototransistors might be used in numerous applications.

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Advantages: Phototransistors have several important advantages that separate them from other optical sensors.

They produce a higher current than photodiodes and also produce a voltage, something that

photoresistors cannot do. Phototransistors are very fast and their output is practically

instantaneous. They are relatively inexpensive, simple, and so small that several of them can fit

onto a single integrated computer chip.

Disadvantages: While phototransistors can be advantageous, they also have several disadvantages.

Phototransistors made of silicon cannot handle voltages over 1,000 Volts. They do not allow

electrons to move as freely as other devices, such as electron tubes, do. Also, phototransistors are

also more vulnerable to electrical surges/spikes and electromagnetic energy.

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Applications of Photo Transistor:

The various phototransistor applications are punch-card readers, computer logic circuitry,

lighting control, level indicators and relays. Phototransistors are often used in smoke detectors,

infrared receivers, and CD players. Phototransistors can also be used in astronomy, night vision,

and laser range-finding.

An infrared emitter, or IR emitter, is a source of light energy in the infrared spectrum. It is a light

emitting diode (LED) that is used in order to transmit infrared signals from a remote control.

In general, the more they are in quantity and the better the emitters are, the stronger and wider

the resulting signal is. A remote with strong emitters can often be used without directly pointing

at the desired device. Infrared emitters are also partly responsible for limits on the range of

frequencies that can be controlled. An IR emitter generates infrared light that transmits

information and commands from one device to another. Typically one device receives the signal

then passes the infrared (IR) signal through the emitter to another device.

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Module 2 Questions:

1. Discuss the basic structure and characteristics of TRIAC.

2. Write notes on DIAC. Mention the applications of DIAC.

3. Draw the basic structure of an SCR. Explain its characteristics and list the applications.

4. Define Holding Current and Latching Current of SCR.

5. With neat diagram, describe the working principle and characteristics of UJT.

6. Write notes on Photo Transistor.

7. Discuss about IR Emitters and mention the applications of IR Emitters.

8. Explain the construction and applications of Solar Cell.

9. Explain the construction and working principle of Schottky Barrier Diode.

10. Compare the V – I characteristics of Schottky Barrier Diode with PN Junction diode.