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SPECIAL DIODES P. SYAM SUNDAR ASSOCIATE PROFESSOR DEPT. OF ECE K L UNIVERSITY

SPECIAL DIODES

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SPECIAL DIODES. P. SYAM SUNDAR ASSOCIATE PROFESSOR DEPT. OF ECE K L UNIVERSITY. SPECIAL DIODES. ZENER DIODE SCHOTTKY – BARRIER DIODE (SBD) VARACTOR DIODE PHOTO DIODE LIGHT EMITTING DIODE (LED). ZENER DIODE. i. -V Z0. -V Z. -V ZK. 0. SYMBOL. V. -I ZK. -. +. Q. Slope = 1/ r Z. - PowerPoint PPT Presentation

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Page 1: SPECIAL DIODES

SPECIAL DIODES

P. SYAM SUNDARASSOCIATE

PROFESSORDEPT. OF ECE

K L UNIVERSITY

Page 2: SPECIAL DIODES

• ZENER DIODE

• SCHOTTKY – BARRIER DIODE (SBD)

• VARACTOR DIODE

• PHOTO DIODE

• LIGHT EMITTING DIODE (LED)

SPECIAL DIODES

Page 3: SPECIAL DIODES

ZENER DIODE

+-VZ

IZ

SYMBOL-VZK

-VZ0-VZ

-IZK

-IZT

ΔIΔV

Slope = 1/rZ

0

ΔV = ΔI rZ

V

i

Q

IZK – Knee Current (from data sheets)

IZT – Test Current

VZ – Zener Voltage

rZ – Incremental (Dynamic) Resistance

Page 4: SPECIAL DIODES

• Used in designing Voltage Regulators

• Operate in Break down region

• Current flows in to cathode

• Cathode positive w.r.t. anode

• Can operate safely up to 70 mA

• VZO and VZK are approximately equal

• The dependence of Zener Voltage on temperature is given

by Temperature coefficient TC or temco.

VZ = VZO + rZIZ

For IZ > IZK andVZ > VZO

Page 5: SPECIAL DIODES

SCHOTTKY BARRIER DIODE (SBD)• It is a metal-semiconductor (MS) diode. (These are the oldest

diodes).

• Metal contact with moderately doped n type material.

• The general shape of the Schottky diode and I-V characteristics are similar to PN junction diodes, but the details of current flow are different.

• In a PN junction diodes, current is due to– Recombination in the depletion layer under small forward bias.– Hole injection from p+ side under larger forward bias.

• In a Schottky diodes current is due to– Electron injection from the semiconductor to the metal.

Page 6: SPECIAL DIODES

SCHOTTKY BARRIER DIODE (SBD)

Page 7: SPECIAL DIODES

kTkTqV

TAIIIBA

ewhere1e 2*ss

A

where B is Schottky barrier height, VA is applied voltage, A is area, A* is Richardson’s constant.

V – I cHARECTERISTICS

Page 8: SPECIAL DIODES

• Current is conducted by majority carrier (electrons).

• Switching speed of the SBD is much higher.

• The forward voltage of SBD is lower than that of PN junction

diode.

SBDForward Voltage Drop

PN diodeForward Voltage Drop

Silicon 0.3V – 0.5V 0.6V – 0.8V

V – I cHARECTERISTICS

Page 9: SPECIAL DIODES

VARACTOR DIODE• Variable Capacitors

• Transition capacitance under reverse bias

• Diffusion capacitance under forward bias

• Used in automatic tuning of radio receivers

Page 10: SPECIAL DIODES

VARACTOR DIODE

Page 11: SPECIAL DIODES

PHOTO DIODE

• Used to convert light to electric signal

• Reverse biased PN diode is exposed to light

• Photons liberated causes breakage of covalent bonds

• Liberation of electron – hole pairs

• Results in flow of reverse current across the junction

called photo current

• Photo current is proportional to intensity of light

Page 12: SPECIAL DIODES

PHOTO DIODE

Page 13: SPECIAL DIODES

LIGHT EMITTING DIODE (LED)

• The operation is inverse to that of a photo diode

• It converts forward current in to light

• Minority carriers are injected across the junction and diffuse

in to P & N regions

• Minority carriers recombine with majority carriers emitting photons

• Use direct band gap materials like Gallium Arsenide

• Light emitted proportional to the no. of re-combinations

• Wide range of applications in different types of displays

Page 14: SPECIAL DIODES

LED applicationsDisplay instruments like DVMs

Colourful lights

Produce coherent light with narrow band width (Laser Diode –

used in CD Players & Optical communications)

Opto-isolator – combination of LED and Photo diode used to

reduce electrical interference on signal transmission in a system

and used in digital system design and design of medical

instruments to reduce risk of electric shock to patients

Page 15: SPECIAL DIODES

LIGHT EMITTING DIODE (LED)

Page 16: SPECIAL DIODES

Direct band gap semiconductors used for LEDs: Galium Arsenide (Ga As)

Gallium Antimony (Ga Sb)

Arsenic, Antimony, Phosphorous

Impurities added: Group – II materials like Zinc (Zn), Magnesium (Mg), Cadmium (Cd)

Donors: Group – VI materials like Tellicum (Te), Sulphur (S) etc…

Impurity Concentration: 1017 – 1018 /cm3 for donor atoms and

1017 – 1019 /cm3 for Acceptor atoms

Colours: Gallium Phosphide – Zinc Oxide RED

Gallium Phosphide – N GREEN

Silicon Carbide – SiC YELLOW

Gallium Phosphide, P, N AMBER

Page 17: SPECIAL DIODES

LIGHT EMITTING DIODE (LED)

Page 18: SPECIAL DIODES

SEMICONDUCTORS SYMBOLS