Chapter 2 Introduction to integrated circuit devices 2 0 Introduction to integrated circuit devices 2 1 Introduction In this chapter the basics of the devices that make up integrated circuits will be presented The objective of this chapter is to give the reader enough of an understanding of the operation of devices fabricated on integrated circuits ICs to be able to understand the process trade offs and challenges discussed later in the book For readers who already have a good working familiarity with device physics this chapter may be skipped 2 2 Basic electronic concepts Electronic circuits regulate and control the flow of electric current Electric current is the flow of electrons the tiny subatomic particles that surround the nucleus of atoms Electrons carry a fixed negative electric charge and the movement of electrons carries charge from one location to another the flow of electrons is referred to as electric current Electric current is driven by a difference in potential from one location to another location measured in volts Electric current flows easily through materials that are conductors and is blocked by materials that are insulators The amount of resistance that a material presents to the flow of electric current is logically called resistance Conductors have low resistance to the flow of electric current and insulators have extremely high resistance essentially infinite until the voltage is so high that the material breaks down For a given voltage the higher the resistance the lower the current that will flow and the lower the resistance the higher the current that will flow Conversely for a given resistance the higher the voltage the higher the current that will flow and the lower the voltage the lower the current that will flow 2 3 Electronic circuit elements Electronic circuits are made up of a number of elements used to control current flow There are a wide variety of different circuit elements but for the purpose of this discussion the circuit elements will be restricted to the four most commonly used in ICs resistors capacitors diodes and transistors Resistors provide a fixed amount of resistance to current flow Capacitors store electronic charge until discharged somewhat similar to a battery Diodes allow current to flow in one direction but not in the opposite direction a one way valve Transistors provides two major modes of action one a switch turning current flow on and off or two act as an amplifier whereby an input current produces a larger output current An IC is nothing more than a number of these components connected together as a circuit all formed on the same substrate 2 4 Atomic structure and band theory Atoms are made up of positively charged protons neutral neutrons and negatively charged electrons Protons and neutrons are relatively heavy particles with similar masses and electrons are relatively light particles with much lower mass The nucleus of an atom is made up of protons and neutrons with electrons arrayed around it The electrons occupy specific allowed energy levels with each level accommodating two electrons Outer levels may also contain sub levels each capable of holding two electrons For silicon the element of most interest to this publication the outermost energy level has 3 sub levels with a total capacity of 6 electrons two per sub Copyright 2000 2002 IC Knowledge all rights reserved Basic concepts Current the flow of electrons carrying electric charge Voltage the force driving the flow of current Resistance a materials resistance to the flow of electric current Conductor a material that readily supports the flow of electric current Insulator a material that blocks the flow of electric current Circuit elements Resistors resists current flow Capacitors stores charge Diodes allows current to flow in only one direction Transistor switches and or amplifies current Sub atomic particles Proton relatively heavy positively charge particle present in the nucleus Neutron relatively heavy neutral particle present in the nucleus Electron relatively light negatively charged particle 6 Chapter 2 Introduction to integrated circuit devices level The actual number of electrons occupying the outer most levels is 2 so there are 4 unfilled levels In solid silicon the outer most levels are filled by the four nearest silicon neighbor atoms sharing electrons see figure 7 Silicon atom Shared electron covalent bonds Figure 7 Silicon covalent bonds When atoms exist in a solid the proximity of electrons on neighboring atoms leads to a further splitting of allowed energy levels The outer most energy levels split into two bands of closely spaced energy levels known as the conduction and the valence bands The two bands are separated from each other by a gap in the allowed energy levels The width of the energy gap determines the electrical conduction of the material A large gap of several electron volts eV results in an insulating material a gap of a few tenths of an eV to a few eV is a semiconductor and a material with overlapping bands is a conductor The reason that the energy gap determines the conduction properties of the material is as follows At zero degrees kelvin minus 273 centigrade the valence band is completely filled with electrons and the conduction band is empty At room temperature the electrons in the material have gained thermal energy If the energy gap is large very few if any electrons will have sufficient thermal energy to jump to the conduction band where the electrons are free to move and participate in conduction If the gap is small or non existent many electrons will jump to the conduction band and be free to move 2 5 Intrinsic semiconductors Materials with energy gaps of a few tenths of an eV to a few eV at room temperature will contain electrons that have jumped from the valence band to the conduction band due to thermal energy The electrons in the conduction band will be free to move about in the material participating in conduction In addition when the electrons jump from the valence band they leave behind a missing electron or hole in the valence band Electrons from neighboring atoms may move into the hole left behind by the escaping electron The hole will then effectively have moved to a neighboring atom where it may move once again by another electron from a neighboring atom filling it Since atoms with an associated hole in the valence band have one more proton than electron the hole has an effective positive charge equal and
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