7 Semiconductor Devices and Circuits 7.1 Semiconductors....................................... 530 Introduction • Diodes 7.2 Bipolar Junction and Junction Field-Effect Transistors .. 533 Bipolar Junction Transistors • Amplifier Configurations • Junction Field-Effect Transistors 7.3 Metal-Oxide-Semiconductor Field-Effect Transistor .... 545 Introduction • Current-Voltage Characteristics • Important Device Parameters • Limitations on Miniaturization 7.4 Image Capture Devices ................................ 558 Image Capture • Point Operations • Image Enhancement 7.5 Image Display Devices ................................ 565 Introduction • LCD Projection Systems 7.6 Solid-State Amplifiers ................................. 577 Sidney Soclof Introduction • Linear Amplifiers and Characterizing Distortion • • California State University Nonlinear Amplifiers and Characterizing Distortion Linear Amplifier Classes of Operation • Nonlinear Amplifier Classes of John R. Brews Operation University of Arizona 7.7 Operational Amplifiers ................................ 611 Introduction • The Ideal Op-Amp • A Collection of Functional Edward J. Delp, III Circuits • Op-Amp Limitations Purdue University 7.8 Applications of Operational Amplifiers ................ 641 Jerry C. Whitaker Instrumentation Amplifiers • Active Filter Circuits • Other Editor-in-Chief Operational Elements 7.9 Switched-Capacitor Circuits ........................... 677 Timothy P. Hulick Introduction • Switched-Capacitor Simulation of a Resistor Acrodyne Industries, Inc. • Switched-Capacitor Integrators • Parasitic Capacitors • Peter Aronhime Parasitic-Insensitive Switched-Capacitor Integrators • Switched-Capacitor Biquad University of Louisville 7.10 Semiconductor Failure Modes ......................... 687 Ezz I. El-Masry Discrete Semiconductor Failure Modes • Integrated Circuit • • Technical Institute of Nova Scotia Failure Modes Hybrid Microcircuits and Failures Memory IC Failure Modes • IC Packages and Failures • Lead Finish Victor Meeldijk • Screening and Rescreening Tests • Electrostatic Discharge Intel Corporation Effects 529 Copyright 2005 by Taylor & Francis Group 530 Electronics Handbook 7.1 Semiconductors Sidney Soclof 7.1.1 Introduction Transistors form the basis of all modern electronic devices and systems, including the integrated circuits used in systems ranging from radio and television to computers. Transistors are solid-state electron devices made out of a category of materials called semiconductors. The mostly widely used semiconductor for transistors, by far, is silicon, although gallium arsenide, which is a compound semiconductor, is used for some very high-speed applications. Semiconductors Semiconductorsareacategoryofmaterialswithanelectricalconductivitythatisbetweenthatofconductors and insulators. Good conductors, which are all metals, have electrical resistivities down in the range of 10−6 -cm. Insulators have electrical resistivities that are up in the range from 106 to as much as about 1012 -cm. Semiconductors have resistivities that are generally in the range of 10−4–104 -cm. The resistivity of a semiconductor is strongly influenced by impurities, called dopants, that are purposely added to the material to change the electronic characteristics. We will first consider the case of the pure, or intrinsic semiconductor. As a result of the thermal energy present in the material, electrons can break loose from covalent bonds and become free electrons able to move through the solid and contribute to the electrical conductivity. The covalent bonds left behind have an electron vacancy called a hole. Electrons from neighboring covalent bonds can easily move into an adjacent bond with an electron vacancy, or hole, and thus the hold can move from one covalent bond to an adjacent bond. As this process continues, we can say that the hole is moving through the material. These holes act as if they have a positive charge equal in magnitude to the electron charge, and they can also contribute to the electrical conductivity. Thus, in a semiconductor there are two types of mobile electrical charge carriers that can contribute to the electrical conductivity, the free electrons and the holes. Since the electrons and holes are generated in equal numbers, and recombine in equal numbers, the free electron and hole populations are equal. In the extrinsic or doped semiconductor, impurities are purposely added to modify the electronic characteristics. In the case of silicon, every silicon atom shares its four valence electrons with each of its four nearest neighbors in covalent bonds. If an impurity or dopant atom with a valency of five, such as phosphorus, is substituted for silicon, four of the five valence electrons of the dopant atom will be held in covalent bonds. The extra, or fifth electron will not be in a covalent bond, and is loosely held. At room temperature, almost all of these extra electrons will have broken loose from their parent atoms, and become free electrons. These pentavalent dopants thus donate free electrons to the semiconductor and are called donors. These donated electrons upset the balance between the electron and hole populations, so there are now more electrons than holes. This is now called an N-type semiconductor, in which the electrons are the majority carriers, and holes are the minority carriers. In an N-type semiconductor the free electron concentration is generally many orders of magnitude larger than the hole concentration. If an impurity or dopant atom with a valency of three, such as boron, is substituted for silicon, three of the four valence electrons of the dopant atom will be held in covalent bonds. One of the covalent bonds will be missing an electron. An electron from a neighboring silicon-to-silicon covalent bond, however, can easily jump into this electron vacancy, thereby creating a vacancy, or hole, in the silicon-to-silicon covalent bond. Thus, these trivalent dopants accept free electrons, thereby generating holes, and are called acceptors. These additional holes upset the balance between the electron and hole populations, and so there are now more holes than electrons. This is called a P-type semiconductor, in which the holes are the majority carriers, and the electrons are the minority carriers. In a P-type semiconductor the hole concentration is generally many orders of magnitude larger than the electron concentration. Figure 7.1 shows a single crystal chip of silicon that is doped with acceptors to make it P-type on one side, and doped with donors to make it N-type on the other side. The transition between the two sides is Copyright 2005 by Taylor & Francis Group Semiconductor Devices and Circuits 531 called the PN junction. As a result of the concentra- P N tion difference of the free electrons and holes there will be an initial flow of these charge carriers across the junction, which will result in the N-type side FIGURE 7.1 PN junction. attaining a net positive charge with respect to the P-type side. This results in the formation of an electric potential hill or barrier at the junction. Under equilibrium conditions the height of this potential hill, called the contact potential is such that the flow of the majority carrier holes from the P-type side up the hill to the N-type side is reduced to the extent that it becomes equal to the flow of the minority carrier holes from the N-type side down the hill to the P-type side. Similarly, the flow of the majority carrier free electrons from the N-type side is reduced to the extent that it becomes equal to the flow of the minority carrier electrons from the P-type side. Thus, the net current flow across the junction under equilibrium conditions is zero. 7.1.2 Diodes In Fig. 7.2 the silicon chip is connected as a diode P N P N or two-terminal electron device. The situation in whichabiasvoltageisappliedisshown.InFig.7.2(a) +− −+ the bias voltage is a forward bias,whichproduces (a) (b) a reduction in the height of the potential hill at the junction. This allows for a large increase in the flow FIGURE 7.2 Biasing of a diode: (a) forward bias, (b) of electrons and holes across the junction. As the reverse bias. forward bias voltage increases, the forward current will increase at approximately an exponential rate, and can become very large. The variation of forward current flow with forward bias voltage is given by the diode equation as I = I0(exp(qV/nkT) − 1) where I0 = reverse saturation current, constant q = electron charge n = dimensionless factor between 1 and 2 k = Boltzmann’s constant T = absolute temperature, K If we define the thermal voltage as VT = kT/q, the diode equation can be written as I = I0(exp(V/nVT ) − 1) ∼ At room temperature VT = 26 mV, and n is typically around 1.5 for silicon diodes. In Fig. 7.2(b) the bias voltage is a reverse bias, which produces an increase in the height of the potential hill at the junction. This essentially chokes off the flow of electrons from the N-type side to the P-type side, and holes from the P-type side to the N-type side. The only thing left is the very small trickle of electrons from the P-type side and holes from the N-type side. Thus the reverse current of the diode will be very small. In Fig. 7.3 the circuit schematic symbol for the diode is shown, and in Fig. 7.4 a graph of the current vs. voltage curve for the diode is presented. The P-type side of the diode is called the anode, and the CATHODE ANODE N-type side is the cathode of the diode. The forward N-TYPE P-TYPE current of diodes can be very large, in the case of large power diodes, up into the range of 10–100 A. FIGURE 7.3 Diode symbol. Copyright 2005 by Taylor & Francis Group 532 Electronics Handbook The reverse current is generally very small, often down in the low nanoampere, or even picoampere range. The diode is basically a one-way voltage- I controlled current switch. It allows current to flow in the forward direction when a forward bias volt- age is applied, but when a reverse bias is applied the FORWARD BIAS current flow becomes extremely small.