UnitI Diode

7/30/2019 UnitI Diode

1/31

Electronics Devices & Circuits

Unit I:Semiconductor Diodesand Applications

Department of Electronics & Telecommunication

5/2/2013 1EDC/III SEM


2/31

Syllabus

Semiconductor Diode and Power Supplies: P-Njunction diode, open circuited junction

Forward and reverse bias ,V-I characteristics

photodiode, LEDs, Tunnel diode

Half Wave ,ripple factor

Half Wave ,ripple factor

Filters, Voltage doublers

Zener and emitter follower type series

regulators

5/2/2013 EDC/III SEM 2


3/31

OutlineThe p-n junction (forward bias and

reverse bias)Ideal Diode

V-I Characteristics

Application of P-N jun.diodeAvalanche Breakdown

Zener Break Down

Dynamic Characteristics

Types of Diode



4/31

I ntroduction to Semiconductor Devices

Semiconductor p-n junction diodes

p

n



5/31

5

Ideal Diodes

Forward bias

(on)

Reverse bias

(off)

5/2/2013 EDC/III SEM


6/31

The I-V characteristic of the

diode

IS

1

kT

qVII S exp



7/31

The experimental I-V

characteristic of a Si diode



8/31

p- n diode circuit notation

p

n

1p

kT

qV

IS

1p

kT

qV

IS

When plus is applied to the p-side, the current is high.

This voltage polarity is called FORWARD.

When plus is applied to the n-side, the current is

nearly zero. This voltage polarity is called REVERSE.



9/31

p- n diode applications: Light emitter

P-n junction can emit thelight when forward biased

p-type n-type

+

-

+

-

Electrons drift into p-material and find plenty of holes there. They

RECOMBINE by filling up the empty positions.Holes drift into n-material and find plenty of electrons there. They also

RECOMBINE by filling up the empty positions.

The energy released in the process of annihilation produces

PHOTONSthe particles of light5/2/2013 9EDC/III SEM


10/31

+

-

p- n diode applications:Photodetectors

P-n junction can detect lightwhen reverse biased

p-type n-type

When the light illuminates the p-n junction, the photons energy RELEASES free

electrons and holes.

They are referred to as PHOTO-ELECTRONS and PHOTO-HOLES

The applied voltage separates the photo-carriers attracting electrons toward

plus and holes toward minus

As long as the light is ON, there is a current flowing through the p-n junction5/2/2013 10EDC/III SEM

l h kd


11/31

Avalanche Breakdown

11/34

Impact ionization mechanism

Predominant breakdown

mechanismTotal current during

avalanche multiplication

In(w) = M * Ino



12/31

Zener Breakdown

12/34

Zener effect

Doping level > 1018/Cm3

In case of Ge, Si

E (field) > 106 V/m

Highly doped junction ( narrow W)

Mechanism is termed tunnelling or zener breakdown



13/31

Dynamic Resistance

Kristin Ackerson, Virginia Tech EE

Spring 2002

Inverse of the slope of the transconductance curve

The equation for dynamic resistance is:

rF = VT

IDIt is used in determining the voltage drop across the diode.

The ac component of the diode voltage is found using the

following equation:

vF = vac rF

rF + RSThe voltage drop through the diode is a combination of the acand dc components and is equal to:

VD = V + vF



14/31

Negative Resistance Device It is a device which exhibits a negative

incremental resistance over a limited range ofV-I characteristic.

It is of two types :-

1. Current controllable type : V-I curve is amulti valued function of voltage and single

valued function of current .eg:- UJT, p-n-p-ndiode

2. Voltage controllable type : V-I curve is amulti valued function of current and singlevalued function of voltage. eg:- SCS, Tunneldiode -



15/31

TUNNEL DIODE (Esaki Diode)

It was introduced by Leo Esaki in 1958.

Heavily-doped p-n junction

Impurity concentration is 1 part in 10^3 as compared to

1 part in 10^8 in p-n junction diode

Width of the depletion layer is very small

(about 100 A).

It is generally made up of Ge and GaAs.

It shows tunneling phenomenon.

Circuit symbol of tunnel diode is :

EV



16/31

- Ve Resistance

Region

VfVp

Ip

VvForward Voltage

Reversevoltage

Iv

Reverse

Curren

t

Forward

Curren

t

Ip:- Peak Current

Iv :- Valley Current

Vp:- Peak Voltage

Vv:- Valley Voltage

Vf:- Peak Forward

Voltage

CHARACTERISTIC OF TUNNEL DIODE



17/31

A Zener is a diode operated in reverse bias at the Peak

Inverse Voltage (PIV) called the Zener Voltage (VZ).

Common Zener Voltages: 1.8V to 200V

Zener Diode

17175/2/2013 17EDC/III SEM


18/31

Types of Diodes and Their Uses

Schottky Diodes: Very fast switching time Great diode for digital circuit applications.

They are very common in computers because of their abilityto be switched on and off so quickly.

A K

Schematic Symbol for a

Schottky Diode

Shockley Diodes: The Shockley diode is a four-layer diode while other diodesare normally made with only two layers. These types of

diodes are generally used to control the average power

delivered to a load.

A K

Schematic Symbol for a four-

layer Shockley Diode


Li ht E itti Di d


19/31

It is designed with a very large band gap.

Lower band gap LEDs (Light-Emitting Diodes) emit infrared

radiation.

While LEDs with higher bandgap energy emit visible light.

LEDs are extremely bright and last longer than regular bulbs for

relatively low cost.

AK

Schematic Symbol for aLight-Emitting Diode

The arrows in the LED

representation indicateemitted light.

Light-Emitting Diodes:



20/31

Photodiodes: Photodiodes are sensitive to received light. They are

constructed so their p n junction can be exposed to the outside

through a clear window or lens.

In Photoconductive mode the saturation current increases in

proportion to the intensity of the received light - used in CD

players.

In Photovoltaic mode, when the pn junction is exposed to a

certain wavelength of light, the diode generates voltage and can

be used as an energy source - used in the production of solar

power.A K

A K

Schematic Symbols for

Photodiodes



21/31

21

Rectifier circuits

Block diagram of a dc power supply



22/31

22

Half-wave rectifier

Simple

Wastes half the input



23/31

23

Full-wave rectifierVS > 0

VS < 0

Current goes through load in same direction for + VS.

VO is positive for + VS.

Requires center-tap transformer



24/31

24

Contd

Entire input waveform is used5/2/2013 EDC/III SEM

B id ifi


25/31

25

Bridge rectifier

A type of full-wave rectifier

Center-tap not needed

Most popular rectifier

VS > 0 D1, D2 on; D3, D4 off

VS < 0 D3, D4 on; D1, D2 off


B id tifi


26/31

26

Bridge rectifier

VO is 2VD less than VS



27/31

27

Filter

Capacitor acts as a filter.Vi charges capacitor as Vi increases.

As Vi decreases, capacitor supplies current to load.


Filt


28/31

28

Filter

Diode on

Diode off

When the diode is off, the capacitor discharges.

Vo = Vpexp(-t/RC)Assuming t T, and T=1/f

VP - Vr = Vpexp(-1/fRC) half-wave rectifier (t T)

VP - Vr = Vpexp(-1/2fRC) full-wave rectifier (t T/2)5/2/2013 EDC/III SEM


29/31

29

Basic Zener Characteristics

Zener diodes are

operated in their

reverse breakdown

mode to provide

voltage regulation in a

circuit.

The point where the

reverse current begins

to increase is called

the knee voltage. The

current at this point is

the knee current.



30/31

30

Zener Voltage Regulator



31/31

Summary

The p-n junction (forward bias and reverse bias)

Ideal Diode

V-I Characteristics & applications

Avalanche Breakdown

Zener Break Down

Dynamic Characteristics

Types of Diode

Types of Rectifier

Voltage Regulator

5/2/2013 EDC/III SEM 31

UnitI Diode

Documents

Transcript of UnitI Diode