# In-05-01 Bas. Semiconductor Theory

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```					Abu Dhabi National Oil Co. ADNOC Technical Institute

INSTRUMENTATION
INDUSTRIAL ELECTRONICS II

UNIT 1 BASIC SEMICONDUCTOR THEORY
BASIC SEMICONDUCTOR THEORY DATE OF ISSUE 8-DEC-09 PAGE 1 OF 15 IN-05-01

UNITS IN THIS COURSE

UNIT 1

BASIC SEMICONDUCTOR THEORY

UNIT 2

DIODE APPLICATION

UNIT 3

THE CONTROLLED DIODE

UNIT 4

TRANSISTORS

UNIT 5

BASIC SEMICONDUCTOR THEORY DATE OF ISSUE 8-DEC-09

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Paragraph 1.0 UNIT OBJECTIVES 1.1 INTRODUCTION 1.2 BASIC ATOMIC THEORY 1.2.1 Electrical Conduction 1.3 THE INTRINSIC SEMICONDUCTOR 1.3.1 Doping an Intrinsic Semiconductor 1.3.2 The PN Junction 1.3.3 The PN Junction Diode 1.3.4 Diode Symbols Page 4 5 5 8 9 10 12 13 14

BASIC SEMICONDUCTOR THEORY DATE OF ISSUE 8-DEC-09

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ADNOC TECHNICAL INSTITUTE 1.0 UNIT OBJECTIVES The student will be able to :  Explain an electric current as a movement of electrons or holes. Explain the difference between a conductor, insulator and semiconductor by electron shell theory.    Explain the terms ‘P’ type and ‘N’ type material. Describe the action of a PN junction. With the aid of a sketch explain the action of a PN junction diode.

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ADNOC TECHNICAL INSTITUTE 1.1 INTRODUCTION The aim of this unit is to explain, in basic terms, the physics of the conductor, insulator, semiconductor and the PN junction.

1.2 BASIC ATOMIC THEORY The basic unit of all matter is the atom. simplified diagram of an atom. Figure 1-1 shows a very

VERY SMALL NUCLEUS

+ + + PROTONS NEUTRONS ELECTRON ‘SHELLS’ OR ‘ORBITALS +

Figure 1-1 The Simple Atom
The atom contains a central group of particles called the NUCLEUS. This nucleus is made up of PROTONS with a POSITIVE CHARGE and NEUTRONS with ZERO CHARGE. The ELECTRON clouds, with a NEGATIVE CHARGE, move around the nucleus in a spherical space at a fixed distance from the nucleus called a SHELL. The number of electrons and protons in an atom are equal, so an atom has no charge. (element). The number of electrons or protons in an atom depends on the substance

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ADNOC TECHNICAL INSTITUTE For example: Protons 1 8 14 29 92 Electrons 1 8 14 29 92

Hydrogen Oxygen Silicon Copper Uranium

There are about 100 elements. Every element has an atomic number. The atomic number tells you how many protons/electrons are in the element. So, the atomic number of Hydrogen is 1, Oxygen is 8 and so on. Everything is made of atoms. If all the atoms in a substance are of one type that substance is an element. If the atoms are of two or more types the substance is a compound. A typical compound is water, which is made of Hydrogen and Oxygen. It's basic unit (molecule) is shown as H2O. The electrical properties of an element depend on how many electrons are in the outside shell. Good conductors like copper only have 1 electron in the outside shell. Good insulators like neon have a full outside shell (8 electrons). Semiconductors are half-full; eg, silicon. Elements are classified according to the number of electrons in the outside shell from group 1 (good conductors) to group 8 (good insulators). Group 3, 4 and 5 materials are used to make semiconductor devices in use today, e.g. diode, transistor, integrated circuit etc.(see Figure 1-2)

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ADNOC TECHNICAL INSTITUTE  Group 1 example Lithium

VERY SMALL NUCLEUS

3 PROTONS 4 NEUTRONS

One electron in the outside shell



Group 4 example Silicon

VERY SMALL NUCLEUS

14 PROTONS 14 NEUTRONS

Four electrons in the outside shell  Group 8 example Neon

VERY SMALL NUCLEUS

10 PROTONS 10 NEUTRONS

Eight electrons in the outside shell

Figure 1-2 Atomic Shell Structure examples
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ADNOC TECHNICAL INSTITUTE 1.2.1 Electrical Conduction

ELECTRON MOTION ELECTRON HOLE ELECTRON APPLIED EMF NEGATIVE CONNECTION HOLE MOTION APPLIED EMF POSITIVE CONNECTION CONVENTIONAL CURRENT

Figure 1-3 Simple Electrical Conduction
Figure 1-3 shows a few atoms in a conductor. Each atom has a single (free) electron in the outside shell. When an EMF is applied, the free electron (which has a negative charge) is pulled towards the positive charge. electron. The ‘hole’ left by the electron is filled by the next moving So, the hole (the positive charge) appears to move to the

negative. The electron flow of an electric current is from negative to positive. Hole flow is from positive to negative. It is hole flow that is used to show the direction of an electric current, (conventional flow). Note : The charge of an electron is very small. A current of 1 Amp

means that 1.6 x 1019 electrons are moving per second.

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ADNOC TECHNICAL INSTITUTE 1.3 THE INTRINSIC SEMICONDUCTOR The intrinsic semiconductor is a group 4 material with 4 electrons in the outside shell. This means there are four (4) empty places in the outside shell into which an electron can move. The most common intrinsic They are produced as semiconductors are silicon and germanium. regular pattern (see Figure 1-4).
ATOMS

crystals. Crystals are an extremely pure form in which the atoms have a

BINDING FORCES HOLDING ATOMS AS A SOLID

Figure 1-4 Arrangement of Atoms in a Crystal
The atoms hold together as a solid, by sharing the electrons between them to keep the outside shell full some of the time. This arrangement in which atoms share electrons is called a covalent bond.

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ADNOC TECHNICAL INSTITUTE A simple diagram of this is shown in Figure 1-5.

B

-eA -eE -eB
E

-eA

-eD -eA

A

C

-eC -eA
D

Figure 1-5 Covalent Bonding
Note :- A good quality crystal has less than 1 part in 10,000,000,000 impurities. 1.3.1 Doping an Intrinsic Semiconductor Doping an intrinsic semiconductor is the trick which makes all semiconductor devices work. There are only two types of doping that can be done. N Type A small amount of a group 5 material; eg, arsenic, which has 5 electrons in its outer shell, is added to an intrinsic semiconductor. This causes the shared area around a covalent bond to have a 'spare' electron which is free to move; 4 from silicon 5 from arsenic = 9 electrons. There is one spare electron because 8 is a full shell.

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ADNOC TECHNICAL INSTITUTE Figure 1-6 shows the action of an N type material which has free electrons to give away: doNate.

Si FREE ELECTRON Si

AS

Si

COVALENT BONDS Si

Figure 1-6 ‘N’ Type Material
P Type A small quantity of a group 3 material, eg, aluminium, which has 3 electrons in the outer shell, is added to an intrinsic semiconductor. This causes the shared area around a covalent bond to be one electron short. There are 4 electrons from silicon and 3 from aluminium which makes 7 electrons in total. This is one short of a full shell so a ‘hole’ is produced where an electron can go. The P type material will take in electrons, accePtor. Figure 1-7 shows the action of a ‘P’ type material.

Si ELECTRON MISSING ‘HOLE’ Si

Al

Si

COVALENT BONDS Si

Figure 1-7 ‘P’ Type Material

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ADNOC TECHNICAL INSTITUTE Note: The doping rates are very small, about one part in a million. However, doping rates vary depending on the amount of excess holes or electrons required.

1.3.2 The PN Junction
CONTACT POTENTIAL (BARRIER VOLTAGE)

+
P
DEPLETION LAYER

N

Figure 1-8 The PN Junction
Figure 1-8 shows what happens when a piece of ‘P’ type material and a piece of ‘N’ type material are joined together. The excess holes from the P side and excess electrons from the N side cross the barrier and cancel each other out. The ‘P’ area gains electrons and goes NEGATIVE, the ‘N’ area gains holes and goes POSITIVE. CONTACT POTENTIAL is formed. A The electric field which it produces

stops any more hole-electron connections. The area where these holes and electrons cancel each other is called the DEPLETION LAYER.

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ADNOC TECHNICAL INSTITUTE 1.3.3 The PN Junction Diode

P
DEPLETION LAYER

N

+

REVERSE BIAS

+

HOLES

N
FORWARD BIAS

P

ELECTRONS

+I

FORWARD BIAS

N POSITIVE TO P

0.7v
ZENER BREAKDOWN VOLTAGE

P POSITIVE TO N

-I
REVERSE BIAS

Figure 1-9 The PN Junction Diode
Figure 1-9 shows what happens when an external voltage is applied to a PN Junction. If the ‘N’ region is made positive to the ‘P’ region the depletion layer gets stronger and no current flows. If the ‘P’ region is made positive to the ‘N’ region, the contact potential and depletion layer are broken down and current will flow. The PN junction acts like a non-

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ADNOC TECHNICAL INSTITUTE return valve. It allows current to flow in one direction only. Therefore, it is a diode. The current depends on the size of the voltage applied Note :- The current will only flow in the forward direction after the contact potential (barrier voltages) has been overcome. This is about 0.7v with silicon diodes and about 0.2v with germanium diodes. If enough voltage is applied in the reverse direction, the diode will break down and high current will flow. The amount of voltage needed to do this is called the ZENER breakdown voltage. 1.3.4 Diode Symbols
ANODE

OR
CATHODE

FORWARD CURRENT FLOW
(CONVENTIONAL - HOLE FLOW)

(a)

(b)

BAND INDICATES THE CATHODE

P (REGION) A

N (REGION) C
SYMBOL PRINTED ON DIODE

FORWARD CURRENT FLOW
(CONVENTIONAL - HOLE FLOW)

(c)

(d)

Figure 1-10 Diode Symbols

Figure 1-10 shows the common symbols for a diode. Figures (a) and (b) show the symbol used on an electronic diagram. The ‘P’ region is called the ‘ANODE’ and the ‘N’ region the “CATHODE’. Forward current flow is

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ADNOC TECHNICAL INSTITUTE from anode to cathode. Figures (c) and (d) show the markings on the diode itself. Two examples of industrial semiconductor diodes are shown below. STUD MOUNTING

8551

30 mm

High forward current rating maximum 240 A. Zener breakdown voltage at least 1000 V.

WIRE CONNECTIONS

5 mm

Miniature plastic encapsulated (covered) diodes with a maximum forward current rating of 1A. Zener
IN 4000 SERIES

breakdown voltage up to 1000 V.

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