Electronic Circuits Autumn-2016

Course Books Textbook: Electronic Devices by Thomas L. Floyd Conventional Current Version, 9th Edition Reference Book: Microelectronic Circuits by Adel Sedra/Smith, 6th Edition

Policies Assignments will be handwritten, submitted in proper folder. Grading Policy Assignments 10% Quizzes 10% Mid-Semester Exam 30% Final Exam 50% Assignments: Assignments will be handwritten, submitted in proper folder. No plagiarism and copying as per HEC Policy. Quizzes: Quizzes may be announced / unannounced. Attendances: 75% (min) (Lab & Theory both) Lab is a mandatory Component and will have its own Grade distribution & Policy.

Course Outline Semiconductor Materials Diode BJT FET Internal Structure Operation Biasing Applications

Chapter 1 INTRODUCTION

Objectives Discuss basic operation of a diode Discuss the basic structure of atoms Discuss properties of insulators, conductors, and semiconductors Discuss covalent bonding Describe the properties of both p and n type materials Discuss both forward and reverse biasing of a p-n junction

Introduction The basic function of a diode is to restrict current flow to one direction. Fig. 1-32a & b forward and reverse bias schematic diagram Forward bias Current flows Reverse Bias No current flow

Bohr model of an atom As seen in this model, electrons circle the nucleus. Atomic structure of a material determines it’s ability to conduct or insulate. Figure 1-1 Bohr Model

Electrons & Shells Electrons orbits the nucleus at certain distances Electrons near the nucleus has less energy as compared to the electrons in distant orbits Discrete values of electrons energies exist so distance of electrons from nucleus is also fixed Orbits of same energies are grouped as shells Each shell has a defined number of electrons it will hold. This is a fact of nature and can be determined by the formula, 2n2. The outermost shell is called valence shell Number of electrons in valence shell determine the chemical & electrical properties of the element If an atom loses a valence electron, positive ion is formed If an extra electron is added to valence shell, negative ion is formed

The Quantum Model Unlike Bohr’s Model, electrons do not circulate in fixed circular orbits Wave-particle duality Uncertainly principle Each shell comprises orbitals s(2),p(6),d(10),f(14) Quantum representation of Si, atomic number:14

Conductors, Insulators, and Semiconductors The ability of a material to conduct current is based on its atomic structure. Each shell has a defined number of electrons it will hold. This is a fact of nature and can be determined by the formula, 2n2. The outer shell is called the valence shell. The less complete a shell is filled to capacity the more conductive the material is.

Conductors, Insulators, and Semiconductors The valence shell determines the ability of material to conduct current. A Copper atom has only 1 electron in it’s valence ring. This makes it a good conductor. It takes 2n2 electrons or in this case 32 electrons to fill the valence shell. A Silicon atom has 4 electrons in it’s valence ring. This makes it a semiconductor. It takes 2n2 electrons or in this case or 18 electrons to fill the valence shell. Fig. 1-6 Copper and Silicon atoms

Comparison of Silicon & Germanium

Covalent Bonding Covalent bonding is a bonding of two or more atoms by the interaction of their valence electrons. Fig. 1-8 Covalent bonding

Covalent Bonding Certain atoms will combine in this way to form a crystal structure. Silicon and Germanium atoms combine in this way in their intrinsic or pure state. Fig. 1-9 Intrinsic Silicon

Current in Semiconductor @ 0 (Zero) kelvin @ room temperature

N-type and P-type Semiconductors The process of creating N and P type materials is called doping. Other atoms with 5 electrons such as Antimony are added to Silicon to increase the free electrons. Other atoms with 3 electrons such as Boron are added to Silicon to create a deficiency of electrons or hole charges. N-type P-type Fig. 1-15 & 16 Pentavalent and Trivalent

The Depletion Region p region n region p region n region With the formation of the p and n materials combination of electrons and holes at the junction takes place. Fig 1-18 a & b depletion region This creates the depletion region and has a barrier potential. This potential cannot be measured with a voltmeter but it will cause a small voltage drop.

Forward and Reverse Bias Forward Bias Reverse Bias Voltage source or bias connections are + to the p material and – to the n material Bias must be greater than .3 V for Germanium or .7 V for Silicon diodes. The depletion region narrows. Voltage source or bias connections are – to the p material and + to the n material Bias must be less than the break down voltage. Current flow is negligible in most cases. The depletion region widens. Fig. 1-22b depletion region forward & Fig. 1-23 depletion region reverse

Forward Bias Measurements With Small Voltage Applied In this case with the voltage applied is less than the barrier potential so the diode for all practical purposes is still in a non-conducting state. Current is very small. Fig 1-26a measurements with meters

Forward Bias Measurements With Applied Voltage Greater Than the Barrier Voltage. With the applied voltage exceeding the barrier potential the now fully forward biased diode conducts. Note that the only practical loss is the .7 Volts dropped across the diode. Fig. 1-26b measurements with meters

Ideal Diode Characteristic Curve In this characteristic curve we do not consider the voltage drop or the resistive properties. Current flow proportionally increases with voltage. Fig. 1-33 ideal diode curve

Practical Diode Characteristic Curve In most cases we consider only the forward bias voltage drop of a diode. Once this voltage is overcome the current increases proportionally with voltage.This drop is particularly important to consider in low voltage applications. Fig. 1-34c practical curve

Complex Characteristic Curve of a Diode The voltage drop is not the only loss of a diode. In some cases we must take into account other factors such as the resistive effects as well as reverse breakdown. Fig. 1-35 complex

Troubleshooting Diodes Testing a diode is quite simple, particularly if the multimeter used has a diode check function. With the diode check function a specific known voltage is applied from the meter across the diode. With the diode check function a good diode will show approximately .7 V or .3 V when forward biased. Fig 1-38 DMM check w/electrode labels When checking in reverse bias the full applied testing voltage will be seen on the display. K A A K

Troubleshooting Diodes An ohmmeter can be used to check the forward and reverse resistance of a diode if the ohmmeter has enough voltage to force the diode into conduction. Of course, in forward biased connection low resistance will be seen and in reverse biased connection high resistance will be seen.

Troubleshooting Diodes Open Diode In the case of an open diode no current flows in either direction which is indicated by the full checking voltage with the diode check function or high resistance using an ohmmeter in both forward and reverse connections. Shorted Diode In the case of a shorted diode maximum current flows indicated by a 0 V with the diode check function or low resistance with an ohmmeter in both forward and reverse connections.

Diode Packages Diodes come in a variety of sizes and shapes. The design and structure is determined by what type of circuit they will be used in. Add diode photo from Intro. Page

Summary Diodes, transistors, and integrated circuits are all made of semiconductor material. P-materials are doped with trivalent impurities N-materials are doped with pentavalent impurities P and N type materials are joined together to form a PN junction. A diode is nothing more than a PN junction. At the junction a depletion region is formed. This creates barrier which requires approximately .3 V for a Germanium and .7 V for Silicon for conduction to take place.

Summary A diode conducts when forward biased and does not conduct when reverse biased When reversed biased a diode can only withstand so much applied voltage. The voltage at which avalanche current occurs is called reverse breakdown voltage. There are three ways of analyzing a diode. These are ideal, practical, and complex. Typically we use a practical diode model.