2.0. Introduction to integrated circuit devices2.1. Introduction2.2. Basic electronic concepts2.3. Electronic circuit elements2.4. Atomic structure and band theory2.5. Intrinsic semiconductors2.6. Extrinsic semiconductors2.7. PN junction2.8. Bipolar transistors2.9. MOS Capacitor2.10. The MOSFET2.10.1. MOSFET off-state2.10.2. MOSFET on-state2.11. MOSFET scaling2.12. ConclusionChapter 2 - Introduction to integrated circuit devicesCopyright © 2000-2002 IC Knowledge, all rights reserved. 62.0. Introduction to integrated circuit devices2.1. IntroductionIn 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 phys-ics, this chapter may be skipped.2.2. Basic electronic conceptsElectronic 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 move-ment of electrons carries charge from one location to another - the flow of elec-trons 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 (essen-tially 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 elementsElectronic circuits are made up of a number of elements used to control cur-rent 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 com-monly 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, pro-vides 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 theoryAtoms 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 Basic concepts• Current - the flow of electrons carrying electric charge.• Voltage - the force driving the flow of current.• Resistance - a materi-als resistance to the flow of electric cur-rent. • 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 cur-rent flow.• Capacitors - stores charge.• Diodes - allows cur-rent to flow in only one direction.• Transistor - switches and or amplifies cur-rent.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 particleChapter 2 - Introduction to integrated circuit devicesCopyright © 2000-2002 IC Knowledge, all rights reserved. 7level. 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.Figure 7. Silicon covalent bondsWhen 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 con-duction 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 semiconductorsMaterials with energy gaps of a few tenths of an eV to a few eV at room tem-perature will contain electrons that have jumped from the valence band to the con-duction band due to thermal energy. The electrons in the conduction band
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