SPECIAL DIODES

P. SYAM SUNDARASSOCIATE

PROFESSORDEPT. OF ECE

K L UNIVERSITY

• ZENER DIODE

• SCHOTTKY – BARRIER DIODE (SBD)

• VARACTOR DIODE

• PHOTO DIODE

• LIGHT EMITTING DIODE (LED)

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

• 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

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.

SCHOTTKY BARRIER DIODE (SBD)

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

• 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

VARACTOR DIODE• Variable Capacitors

• Transition capacitance under reverse bias

• Diffusion capacitance under forward bias

• Used in automatic tuning of radio receivers

VARACTOR DIODE

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

PHOTO DIODE

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

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

LIGHT EMITTING DIODE (LED)

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

LIGHT EMITTING DIODE (LED)

SEMICONDUCTORS SYMBOLS