Last Revision: February 2008 Page 1 of 21 © 2008 by the Regents of the University of California. All rights reserved. University of California at Berkeley Physics 111 Laboratory Basic Semiconductor Circuits (BSC) Lab 3 Semiconductor Diodes © 2008 Copyrighted by the Regents of the University of California. All rights reserved. References: Hayes & Horowitz Pages 65–69, 71–74, 76–78 Horowitz & Hill Chapters 1.06, 1.25–1.31, 4.15, Appendix F Lab notes Sections on Small Signal Input and Output Impedance in the Linear Circuit I lab This is the first of three labs on basic semiconductor components. You will study semiconductor char-acteristics and some of their applications, leading up to the design and construction of a differential amplifier. This lab studies diodes. You will find the relation between the voltage and current in a diode, and study temperature effects, rectification, nonlinear phenomena, and frequency doubling. Before coming to class complete this list of tasks: • Completely read the Lab Write-up • Answer the pre-lab questions utilizing the references and the write-up • Perform any circuit calculations or anything that can be done outside of lab. • Plan out how to perform Lab tasks. Pre-lab questions: 1. In a few sentences, explain what diodes are and how they are useful. 2. Show that the –1 term in Eq. (1) may be neglected for typical operating parameters (V > 0.1 V, kT ≈ 1/40 eV, Isat = 10−9 A). 3. Why is there a ripple on top of the DC voltage output by the circuit in 3.8? 4. What is a load line used for? What are the relative advantages of graphical and iterative analy-sis? Disclaimer: Please keep in mind that this lab is one of the most analysis & plotting intensive labs. Do not leave your analysis section to the last minute. Also, using a computer to do the analysis is much easier & quicker; so, bring a storage device to class or alternatively e-mail yourself the data files from the diode characteristic traces found by the Curve Tracer so you will have access to them both at your lab station and home. Excel is a powerful timesaver when graphing similar data sets mulitiple times. A final safety tip, Do not touch components while power is on only after they have had a chance to cool off . This is especially true if the circuit has been build wrong. The LEDs will burn out if you hook them directly to the 5V power supplies without a current limiting resistor. NOTE: This lab uses liquid nitrogen.Physics 111 BSC Laboratory Lab 3 Semiconductor Diodes Background Diodes and pn Junctions Diodes and transistors are made from semiconducting materials: typically crystalline silicon. Pure silicon has few free electrons and is quite resistive. To increase its conductivity, the silicon is nor-mally “doped”1 with other elements. Some dopants, like phosphor, arsenic and antimony, easily give up one of their electrons to the now impure silicon.2 As these donated electrons are free to move about the silicon, its conductivity increases dramatically.3 Other dopants, like boron, indium and aluminum, grab electrons from the surrounding silicon at-oms, leaving positively charged silicon ions behind. In turn, these now positive silicon ions try to lessen their charge by grabbing electrons from their neighbors…the net result is that there are re-gions of “positiveness” floating around the crystal lattice. Such “absences of electrons” are called holes. Amazingly, holes behave almost exactly like positively charged electrons; they move, respond to electric fields, and appear to have a mass close to the electron mass. A doped semiconductor with more mobile electrons than holes is called an “n-type” semiconductor; conversely, a doped semiconductor with more holes than mobile electrons is called a “p-type” semi-conductor. If doping’s only effect was to increase semiconductor conductivity, semiconductors would be obscure, little-used materials. The utility of semiconductors comes from the remarkable effects of placing p and n-type materials next to each other. Such juxtapositions are called “pn” junctions. An isolated pn junction makes a semiconductor diode. Other semiconductor components are made from more com-plicated arrangements; bipolar npn transistors, for example, are made by sandwiching a p layer in between two n layers, hence the name npn. The current through an ideal pn junction is given by the diode equation. iV ieVnkT() exp=⎛⎝⎜⎞⎠⎟−⎡⎣⎢⎤⎦⎥sat1, (1) where V is the voltage drop across the junction, isat is a constant called the saturation current and depends on the temperature and the particular geometry and material of the junction, e = 1.6 × 10−19 C is the charge of an electron, k = 1.38 × 10−23 J/K is Boltzmann’s constant, and T is the temperature in Kelvin.4 The constant n varies between 1 and 2 depending on the particular di-ode, but is typically equal to 2 for discrete diodes. Notice that the diode’s response is directional and highly nonlinear. When forward biased, (V positive) enormous currents can flow through the diode because of the exponential dependence of I on V. When reverse biased, (V negative), the current ap-proaches −isat. Since isat is typically very small (picoamps are not uncommon), very little current flows.5 Thus the diode acts like a one-way valve; current can only flow in one direction. When for-ward biased, the positive end of the diode is called the anode, and the negative end is called the cathode.6 1 Deliberately contaminated. 2 Of course, dopants that have given up an electron become positively charged. The net charge re-mains zero. 3 Only a few dopant atoms will significantly increase silicon’s conductivity. For example, one dopant atom per 100 million silicon atoms will increase pure silicon’s conductivity by approximately 105. 4 At room temperature, kT/e ≈ 1/40. 5 Junction imperfections in real diodes often cause the reverse biased current to be much bigger than −isat. However, Eq. (1), with the ideal value of isat, is still valid in the forward region. Last Revision: February 2008 Page 2 of 21 6 The terms anode and cathode date from the days of vacuum tube diodes. © 2008 by the Regents of the University of California. All rights reserved.Physics 111 BSC Laboratory Lab 3 Semiconductor Diodes AnodeCathodeDiode+-Forward BiasCurrent flow Nonlinear Circuit Equilibrium Unlike purely linear circuits,