McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-1 Electronics Principles & Applications Eighth Edition Chapter 2 Semiconductors (student version) Charles A. Schuler ©2013

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-2 Conductors and Insulators Semiconductors N-type Semiconductors P-type Semiconductors Majority and Minority Carriers Other Materials Band Gaps INTRODUCTION

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-3 Dear Student: This presentation is arranged in segments. Each segment is preceded by a Concept Preview slide and is followed by a Concept Review slide. When you reach a Concept Review slide, you can return to the beginning of that segment by clicking on the Repeat Segment button. This will allow you to view that segment again, if you want to.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-4 Concept Preview The nucleus of any atom is positively charged. Negatively charged electrons orbit the nucleus. The net charge on any atom is zero because the protons and electrons are equal in number. The valence orbit is the outermost orbit. Copper has only one valence electron and is an excellent conductor. Materials with a full valence orbit act as insulators. Materials with 8 electrons in the valence orbit act as insulators.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-5 N N N N The center of an atom is called the nucleus. Most atoms have neutrons which have no charge. A nucleus also has protons and they have a positive charge. Negative electrons orbit the nucleus.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-6 N N N N This is a copper atom. It has 29 protons. It has 29 electrons. Its net charge = 0. Valence electron

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-7 The valence electron is the important feature. N N N N Valence electron Its attraction to the nucleus is relatively weak.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-8 The valence electron The nucleus plus the inner electron orbits A simple model of the copper atom looks like this:

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved. 2-9 Copper wire is used to conduct electricity because the valence electrons move freely through its structure. Remember, the valence electrons are weakly attracted to the nuclei.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved So far, we know that copper’s single valence electron makes it a good conductor. Other valence arrangements are different. Such a material acts as an electrical insulator. The rule of eight states that a material as shown below would be stable since its valence orbit is full. No Vacancy The valence is 8 and the orbit is full.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Atomic quiz The dense and central part of any atom is called the ________. nucleus Atom net charge is 0 since the number of protons equals the number of ______. electrons The outermost orbit of all atoms is called the ________ orbit. valence Good electrical conductors have free valence ________. electrons The magic number for valence stability is ________. eight

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Concept Review The nucleus of any atom is positively charged. Negatively charged electrons orbit the nucleus. The net charge on any atom is zero because the protons and electrons are equal in number. The valence orbit is the outermost orbit. Copper has only one valence electron and is an excellent conductor. Materials with a full valence orbit act as insulators. Materials with 8 electrons in the valence orbit act as insulators. Repeat Segment

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Concept Preview Silicon has 4 valence electrons. Silicon atoms can form covalent bonds with each other. Covalent silicon satisfies the rule of 8 and acts as an insulator at room temperature. Donor impurities have 5 valence electrons. N-type silicon has been doped with a donor impurity to make it semiconduct. Acceptor impurities have 3 valence electrons. P-type silicon has been doped with an acceptor impurity to make it semiconduct.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Atoms of the same type can join together and form covalent bonds. This is an electron sharing process. Silicon atoms have four valence electrons.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved The covalent sharing satisfies the rule of eight. In this structure, one bond is formed with each neighbor.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This is a silicon crystal. It does not conduct because its valence electrons are captured by covalent bonds.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Thermal carriers Covalent bonds can be broken by heating a silicon crystal. Free electron Hole

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved The thermal carriers support the flow of current.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Heating silicon crystals to make them conduct is not practical!

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This is an arsenic atom. A silicon crystal can be doped with a donor impurity Each donor atom that enters the crystal adds a free electron. Free electron

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Silicon that has been doped with arsenic is called N-type. The free electrons in N-type silicon support the flow of current.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This is a boron atom. A silicon crystal can be doped with an acceptor impurity Each acceptor atom that enters the crystal creates a hole. Hole

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Silicon that has been doped with boron is called P-type. The holes in P-type silicon support the flow of current.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved What are two practical methods of making silicon semi conduct? Hole Free electron Add a pentavalent impurity. Add a trivalent impurity. (N-type) (P-type)

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This is an N-type crystal. Due to heat, it could have a few free holes. These are called minority carriers.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This is a P-type crystal. Due to heat, it could have a few free electrons. These are called minority carriers.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Semiconductor quiz A pure silicon crystal, at room temperature, acts as an electrical ________. insulator The current carriers, in N-type silicon, are called ________. electrons The current carriers, in P-type silicon, are called ________. holes The minority carriers, in N-type silicon, are called ________. holes The minority carriers, in P-type silicon, are called ________. electrons

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Concept Review Silicon has 4 valence electrons. Silicon atoms can form covalent bonds with each other. Covalent silicon satisfies the rule of 8 and acts as an insulator at room temperature. Donor impurities have 5 valence electrons. N-type silicon has been doped with a donor impurity to make it semiconduct. Acceptor impurities have 3 valence electrons. P-type silicon has been doped with an acceptor impurity to make it semiconduct. Repeat Segment

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved Silicon is the workhorse of the semiconductor industry but compound semiconductors help out in key areas. Gallium arsenide Indium phosphide Mercury cadmium telluride Silicon carbide Cadmium sulfide Cadmium telluride Other Materials Some researchers are investigating carbon nanotubes and graphene for possible future applications in electronics.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved This Kodak display uses organic light-emitting diodes.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved The energy band diagrams below show a gap between the conduction band and the valence band, but not for the conductor. In doped silicon, the dopant band effectively narrows the gap and makes it easier for electrons to enter the conduction band and support the flow of current. No gap (in fact, the valence and conduction bands overlap.

McGraw-Hill © 2013 The McGraw-Hill Companies, Inc. All rights reserved REVIEW Conductors and Insulators Semiconductors N-type Semiconductors P-type Semiconductors Majority and Minority Carriers Other Materials Band Gaps