Semi conductor and rectifier

Unit-1Semiconductor Devices

PREPARED BY,

A.JOHNY RENOALD M.E.,(Ph.D.,)

13.7.2017 Semiconductor Devices 2

Introduction

Atom structure

Types of Semiconductor Devices

Diode-Forward & Reverse Bias

Rectifier & types

Transistor

Types of transistors

Queries


Introduction

● Semiconductor:

● Unit which consists lagely or wholly,of semiconductor materials which can perform useful function in electronic apparatus & solid state research.

● Ex:p-n jun diode,transistros,IC's and etc.

Introduction

• Electronic devices such as diodes, transistors, andintegrated circuits are made of a semiconductormaterial. To properly understand how thesedevices work, a basic knowledge of the structureof atoms and the interaction of atomic particles isrequired.

• An important concept introduced in this chapter isthat of the pn junction that is formed when twodifferent types of semiconductive material arejoined. The pn junction is fundamental to theoperation of devices such as the diode and certaintypes of transistors.

ATOM STRUCTURE

• All matter is made of atoms; and all atoms consist or electrons, protons, and neutrons.

• According to the classical Bohr model, atomshave planetary type of structure that consists of acentral nucleus surrounded by orbiting electrons.The nucleus consists of positively chargedparticles called protons and uncharged particlescalled neutrons. The basic particles of negaticecharge are called electrons.

For example,

the simplest atom is that ofhydrogen, which has one protonand one electron. As anotherexample,

the helium atom has two protonsand two neutrons in the nucleusand two electrons orbiting thenucleus.


Types of Semiconductors

There are two types of Semiconductors

1)Intrisic Semiconductors

2)Extrinsic Semiconductors

Intrinsic Semiconductors(PURE)

Ex:Si,Ge etc

Extrinsic Semiconductors:(IMPURE)Ex:p-type and n-type

N-TYPE SEMICONDUCTOR

• To increase the number of conduction-bandelectrons in intrinsic silicon, pentavalentimpurity atoms are added. These are atomswith five valence electrons such as arsenic(As), phosphorous (P), bismuth (Bi), andantimony (Sb).

P-TYPE SEMICONDUCTOR

• To increase the number of holes in intrinsicsilicon, trivalent impurity atoms are added.These are atoms with three valenceelectrons such as boron (B), indium (In), andgallium (Ga).


DIODE

● Diode is device with two electrodes.

● A p-n juncion diode is a two terminal device made up of semiconductor crystal.

● P-N junction diode

+ -

P-n junction diode ckt symbol


Forward Bias

● Resistance is very small

+ - mADepletion Layer

B

p n

P-n junction-Forward Bias


Reverse Bias

● Resistance is very high

● Conduction is due to thermal agitation.

Depletion Layer

B

+-

p n

MA

P-n junction-Reverse Bias

Rectifiers

Contents

Design of Rectifier Circuits

Half Wave Rectification

Full Wave Rectifier

Block diagram of Power Supply

Rectifier

• A circuit that converts ac voltage of main supply into pulsating dc voltage using one or more PN junction diodes.

• Half Wave Rectifier

• Full Wave Rectifier

• Center Tap Rectifier

• Bridge Rectifier

Half Wave Rectifier

Circuit Diagram

Operation of Half Wave Rectifier

Waveform of Half Wave Rectifier

Average DC load Current (IDC )

Average DC voltage (Edc)

RMS Load Current (Irms)

RMS Load Voltage (Erms)

DC Power Delivered to the load

How effectively a rectifier converts ac into dc:

• Rectifier Efficiency (η)

• Ripple Factor (r)

Rectifier Efficiency (η)

Tells us the percentage of total input ac power

that is converted into useful dc output power.

η = 40.6 %

Under best conditions (no diode loss) only 40.6% of the

ac input power is converted into dc power.

The rest remains as the ac power in the load

Ripple Factor

Measure of purity of the dc output of a rectifier

Defined as the ratio of ac component of the output

wave to the dc component in the wave

Ripple Factor

This indicates that the ripple content in the output are 1.211 times the dc component.

i.e. 121.1 % of dc component.

The ripple factor is very high.

Therefore a half wave rectifier is a poor converter of ac to dc.

The ripple factor is minimized using filter circuits along with the rectifier.

Disadvantage of HWR

•The ripple factor of half wave rectifier is 1.21, which is quite high.•The output contains lot of ripples

•The maximum theoretical efficiency is 40%. •The practical value will be quite less than this.•This indicates that HWR is quite inefficient.

Center Tap Rectifier

Full Wave Rectifier

Working of Center Tap RectifierCurrent Flow during the positive half of the input cycle

Current Flow during the negative half of the input cycle

Waveforms

Average DC current

Average (DC) Voltage

RMS Load Current (Irms)

RMS Load Voltage

DC Output Power

AC input power (Pac)

Rectifier Efficiency (η)

Ripple Factor

This indicates that the ripple contents in the output are 48%of the dc component which is much less than that for thehalf wave rectifier.

Advantages of Full Wave Rectifier

• Efficiency is higher.

• The large dc power output

• The ripple factor is less

Disadvantages of Full Wave Rectifier

• PIV rating of diode is higher.

• Higher PIV diodes are larger in size and costlier.

• The cost of center tap transformer is high.

Bridge Rectifier

Working of Bridge Rectifier

Waveforms of Bridge Rectifier

Parameters :

Advantages of Bridge Rectifier

• It does not need center tap transformer secondary.• The transformer secondary voltage of CT rectifier is

2Vm, where as in Bridge the transformer secondarymust have a peak voltage of Vm. That is thetransformer secondary of CT rectifier must havedouble the number of turns. Such transformers arecostlier.

• If stepping up or stepping down of voltage is notneeded, we may even do away without transformer.

• Each diode in center tap has a PIV rating of 2Vm,whereas diodes in bridge rectifier needs a PIV ratingof Vm. Hence the diodes for use in center taprectifier are costlier than meant for bridge rectifier.

Disadvantages of Bridge Rectifier

• It requires four diodes, two of which conductin alternate half cycles. This creates a totalvoltage drop of 1.4V (if Si diodes are used).

• Therefore this creates a problem if low dcvoltage is required.

• The secondary voltage is low and two diodevoltage drop of 1.4V becomes significant.

Semi conductor and rectifier

Engineering

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