1.1 Set up an oscilloscope, power supply (0-12vdc OK, also a 12Vac transformer), load resistors, 12-v auto lamp; diodes to show rectifier diode characteristics

Use above setup for LED demo. With scope, show effect of forward voltqge overload; reverse voltage overload.

Demo of capacitor and lamp, showing storage.

1.4 Set out a cutaway transistor 2N697 or equiv.; gate valve; variable resistor

1.5 Write on the whiteboard: Edison; Fleming; deForest; thermionic; crystal detector; filament; k, M; m, u, n, p; E or V, I or A, W

Before starting on this week's discussion, let's look at what we talked about last week.

2.1 Gate Valve Why is this gate valve on display [it's a three-terminal controller; very slow in operation, but it has an input, an output, and a control terminal]

2.2 How many of you could wire the experiment just by looking at the schematic? [ask how each student managed to do that]. Can you explain the variable resistor operation?[show variable resistor sample]

2.3 Did Edison invent the first light bulb? [No -- but he did invent the first lighting system] The first vacuum tube?? [Probably so, but he didn't know it]

2.4 The first semiconductor was used in radio receivers about 100 years ago. What did we call this device?

2.5 When we talk about "conductors; semiconductors; insulators", why do we relate these? Is it because they are all yellow in color? They smell like gingerbread/ They stick between your teeth when you chew on them?? What is the relation? Hint: conductors are good carriers of electrical current.

2.6 Review Atomic Structures for Conductors, Semiconductors and Insulators:

2.6.1 Conductors, such as metals, have electrons that move around in a "soup" of ions. Very little energy is required to move these seemingly unattached electrons.

2.6.2 Semiconductors have a very tight, strong electron bond among the atoms, and the electrons are quite difficult to move. At absolute zero, no electron motion occurs; conduction increases markedly with increases in temperature.

2.6.3 Insulators have much tighter, stronger electron bonds among the atoms than semiconductors. Typically, no electron movement occurs until the temperature is raised up to the point of destruction of the material. Any impurities typically will degrade the insulation.

3.1 Introduction: As always, I have to keep my eyes on my notes. There's a story about President Cleveland that will explain why. [5-minute speech preparation vs 2-hour speech] Today, we're going to talk about certain particular semiconductor devices: diodes and transistors, and learn how they work. Are there many of these devices in our daily lives? Where? [computers, light dimmers; CD players; watches and clocks; refrigerator controls; clothes washer controls; ..]

3.2 Diodes: "di" means "twice", "double" or "two." DeForest invented a three-element vacuum tube. What was that called? [a triode].

3.3 A diode has two elements: a cathode, which means "negatively charged", and an anode, which means "positively charged." [ demo of scope and diodes]

3.4 The first diodes: The science -- and the art, because some of it is very tricky! -- of making diodes took a long time. Our first diodes were the crystal detectors, exactly the same as those used in the crystal sets that some of you made. But we needed more -- particularly, we needed battery charges, both for automobile batteries and for the home radio batteries. You see, at this time, in the 1920s, back when I was born, we didn't yet know how to run radios from the socket on the wall. So we made diodes using copper oxide and selenium among other things. But…why didn't we use vacuum tube diodes, because those had been developed about 1904? [they were too fragile, and could not deliver enough current]. Do you know what I mean by the term "current." [a stream of electrons]. [Point out the copper oxide diodes in the semiconductor shadow box].

3.5 Later diodes: We studied semiconductors extensively during the 1930s, but very little changed in the practical machines that we made. Then came World War II, and we were desperate to build good radar sets to use against submarines and airplanes sinking our ships. Our engineers turned back to the old reliable crystal detector, put it in a new package, and it turned out to make a wonderful detector for microwave radar circuits.

3.6 Early Search for the Transistor: In the 1930s, AT&T, (American Telephone and Telegraph) was the "big gorilla" in telephones. It had around 90% of all telephone business. There were no satellites, there were no microwave repeater stations, there were no cable competitors, and there was no fiber-optics. All of the telephone business was carried on wire lines -- telephone wires and cables -- and it all used vacuum tubes for amplifiers. In 1938, the research director of Bell Labs (Bell Telephone Laboratories), which was the most prestigious research lab in the country and possibly in the world, decreed that they had to find something more reliable and less of a power hog than the vacuum tube. Researching the possibilities in semiconductor devices were just two men: William Shockley, and Walter Brattain.

3.7 The Transistor is Born: WWII came along and interrupted the search. About the time that the war ended in 1945, John Bardeen joined Shockley and Brattain. Their studies had narrowed down to examining the surface of a small piece of germanium. Just before Christmas, 1947, they had made a point-contact diode, and were probing the surface with a modified "catswhisker". Suddenly, they realized that they were seeing more current coming out of the device than they were putting in via the probe. They had succeeded in their search -- the transistor worked!! But it didn't work very well. The first production models [show in demo board] were point-contact units, made like the microwave diodes. They were electrically noisy, unreliable, and not very good as amplifiers. The company, AT&T, now had a big problem: should they keep the transistor which looks like the greatest invention yet in electronics, a secret? AT&T talked it over with the US Government, which had paid for some of this research. The two organizations decided that it was too good an invention to keep secret. Six months after that startling day in December, the Company announced the transistor to the world.

3.8 Design of water valve, vacuum tube, and transistor. These three have a few things in common: each one has an input; each one has an output; each one has a control element. The usual, but not the only, way of connecting these is [draw schematics of all three, along with the energy sources]. All three need a source of energy, which they will control. Remember: the transistor doesn't manufacture energy - it merely controls it.

3.9 Transistor construction: We know that transistors are made from semiconductor material, and that allows only a very small current flow. We call pure semiconductor material "intrinsic", because there's nothing added to it However -- and here's the first secret of transistors -- we add a pinch of an impurity, about one sprinkle of a big salt shaker to a railroad car full. This is called "doping" the intrinsic material. The impurity is either "N" type material, which has a few free electrons, or "P" type material, which is lacking a few free electrons. These free electrons can wander through the atomic stew pot because they're not bound tightly as the intrinsic electrons are.

3.10 Movement of charges. There are two forces that cause the electrons to move: one is the random motion that all electrons go through from the presence of heat. This is called "diffusion". The other motion is caused by one that you know well: it's the repulsion force from two like bodies getting near to each other. That is called "drift". [draw a circuit with P and N impurities, and show free charges. Connect a battery to show energy source]

3.11 Transistor operation: you'll see pictures of transistors like this [draw rectangle, with two dividing lines in it, and label it "emitter", "base", and "collector"; connect a battery and a resistive load.] The energy source is applied to the emitter; the base is the control element; the controlled energy flows out of the collector to the load. You might well wonder why more current doesn't flow just between the emitter and the base. Here's the best reason: [Draw image of transistor (with wide emitter, collector; narrow base)

3.12 How to build PN junctions: Everything I've told you so far has skipped over the actual junctions in a transistor. The very first transistors were made with point contact junctions. They were PN junctions, but they were very poor. About 1950, William Shockley of Bell Laboratories invented a second type of transistor, the grown-junction transistor It was much more dependable, but it had failings such as not working at radio frequencies such as the FM band and higher. Following this grown-junction invention, AT&T held symposia [what are those? -meetings to discuss something] with any interested corporations in order to give them more information on how to build the transistors. This began a tremendous burgeoning [growing] of research and development in semiconductors, among many companies such as Texas Instruments, Motorola, RCA, GE, Fairchild Instruments.

3.13 The next invention, an alloy junction solved the high-frequency problem, but was difficult to manufacture. The next step, though, solved just about all problems: this was the vapor-deposited junction, and is still the major production method. [describe process]. The materials used for semiconductors are pretty much the same since the early days: silicon is primary, germanium still has some uses. A few more exotic materials are sapphire and gallium arsenide (?)

3.13 Protecting the junction. The very early junction transistors were packaged in epoxy. These transistors were unreliable. Research showed that the junction itself was attacked by air, water, and any other contaminants that crept along the wires, up through the epoxy. To solve this problem, the transistor was mounted inside a metal or glass case, and then vacuum-sealed, just like a light bulb. But the transistors were still unreliable; good ones were found by testing the transistor and tapping it at the same time. If it showed variations, the transistor was rejected. This problem was caused by minute particles that were present inside the case, and that attacked the junction. All of the above problems were solved by a massive leap forward for transistor production: an engineer working in Fairchild invented a process that put a glass coating around the transistor die [the little block of semiconductor] and sealed the junction against all kinds of attack. The transistor no longer required any other protection [show Fairchild package in semiconductor display box]

4.0 Workshop

4.1 Group 2 of the experimenter's kit. These involve capacitors and diodes (Exp 7-12). .

4.2 Students who do all of the experiments -- and study the text along with the experiments -- may go ahead with any experiments that they wish to do.

Page last updated on January 10, 2002