The Future of Transistors The Future of Transistors The first announcement of the invention of the transistor met with almost no fanfare. The integrated circuit was originally thought to be useful only in military applications. The microprocessor's investors pulled out before it was built, thinking it was a waste of money. The transistor and its offspring have consistently been undervalued -- yet turned out to do more than anyone predicted. Today's predictions also say that there is a limit to just how much the transistor can do. This time around, the predictions are that transistors can't get substantially smaller than they currently are. Then again, in 1961, scientists predicted that no transistor on a chip could ever be smaller than 10 millionths of a meter -- and on a modern Intel Pentium chip they are 100 times smaller than that. With hindsight, such predictions seem ridiculous, and it's easy to think that current predictions will sound just as silly thirty years from now. But modern predictions of the size limit are based on some very fundamental physics -- the size of the atom and the electron. Since transistors run on electric current, they must always, no matter what, be at least big enough to allow electrons through.

On the other hand, all that's really needed is a single electron at a time. A transistor small enough to operate with only one electron would be phenomenally small, yet it is theoretically possible. The transistors of the future could make modern chips seem as big and bulky as vacuum tubes seem to us today. The problem is that once devices become that tiny, everything moves according to the laws of quantum mechanics -- and quantum mechanics allows electrons to do some weird things. In a transistor that small, the electron would act more like a wave than a single particle. As a wave it would smear out in space, and could even tunnel its way through the transistor without truly acting on it. Researchers are nevertheless currently working on innovative ways to build such tiny devices -- abandoning silicon, abandoning all of today's manufacturing methods. Such transistors are known, not surprisingly, as single electron transistors, and they'd be considered "on" or "off" depending on whether they were holding an electron. (Transistors at this level would be solely used as switches for binary coding, not as amplifiers.) In fact, such a tiny device might make use of the quantum weirdness of the ultra-small.

The electron could be coded to have three positions -- instead of simply "on" or "off" it could also have "somewhere between on and off." This would open up doors for entirely new kinds of computers. At the moment, however, there are no effective single electron transistors. Even without new technologies, there's room for miniaturization. By improving on current building techniques, it's likely that current transistors will be at least twice as small by 2010. With nearly a billion transistors on Intel's latest processor that would mean four times as many transistors on a chip are theoretically possible. Chips like this would allow computers to be much "smarter" than they currently are.

Introducing the Vacuum Transistor: A Device Made of Nothing - IEEE Spectrum IEEE Spectrum Introducing the Vacuum Transistor: A Device Made of Nothing This curious mash-up of vacuum tube and MOSFET could one day replace traditional silicon By Jin-Woo Han and Meyya Meyyappan Posted 23 Jun 2014 | 16:14 GMT Advertisement Illustration: Chad Hagen In September 1976, in the midst of the Cold War, Victor Ivanovich Belenko, a disgruntled Soviet pilot, veered off course from a training flight over Siberia in his MiG-25 Foxbat, flew low and fast across the Sea of Japan, and landed the plane at a civilian airport in Hokkaido with just 30 seconds of fuel remaining. His dramatic defection was a boon for U.S. military analysts, who for the first time had an opportunity to examine up close this high-speed Soviet fighter, which they had thought to be one of the world’s most capable aircraft. What they discovered astonished them. For one thing, the airframe was more crudely built than those of contemporary U.S. fighters, being made mostly of steel rather than titanium.

What’s more, they found the plane’s avionics bays to be filled with equipment based on vacuum tubes rather than transistors. The obvious conclusion, previous fears aside, was that even the Soviet Union’s most cutting-edge technology lagged laughably behind the West’s. After all, in the United States vacuum tubes had given way to smaller and less power-hungry solid-state devices two decades earlier, not long after William Shockley, John Bardeen, and Walter Brattain cobbled together the first transistor at Bell Laboratories in 1947. By the mid-1970s, the only vacuum tubes you could find in Western electronics were hidden away in certain kinds of specialized equipment—not counting the ubiquitous picture tubes of television sets. Today even those are gone, and outside of a few niches, vacuum tubes are an extinct technology. So it might come as a surprise to learn that some very modest changes to the fabrication techniques now used to build integrated circuits could yet breathe vacuum electronics back to life. At the NASA Ames Research Center , we’ve been working for the past few years to develop vacuum-channel transistors.

Our research is still at an early stage, but the prototypes we’ve constructed show that this novel device holds extraordinary promise. Vacuum-channel transistors could work 10 times as fast as ordinary silicon transistors and may eventually be able to operate at terahertz frequencies, which have long been beyond the reach of any solid-state device. And they are considerably more tolerant of heat and radiation. To understand why, it helps to know a bit about the construction and functioning of good old-fashioned vacuum tubes. Photo: Gregory Maxwell Lightbulb Descendant: Vacuum tubes were an outgrowth of ordinary lightbulbs, a development spurred on by Thomas Edison’s investigations into the ability of heated filaments to emit electrons. This 1906 example, an early Audion tube, shows the close resemblance to a lightbulb, although the filament in this particular tube is not visible, having long ago burned out. That filament once acted as the cathode from which electrons flew toward the anode or plate, which is located in the center of the glass tube. Current flow from cathode to anode could be controlled by varying the voltage applied to the grid, the zigzag wire seen below the plate.

The thumb-size vacuum tubes that amplified signals in countless radio and television sets during the first half of the 20th century might seem nothing like the metal-oxide semiconductor field-effect transistors (MOSFETs) that regularly dazzle us with their capabilities in today’s digital electronics. But in many ways, they are quite similar. For one, they both are three-terminal devices. The voltage applied to one terminal—the grid for a simple triode vacuum tube and the gate for a MOSFET—controls the amount of current flowing between the other two: from cathode to anode in a vacuum tube and from source to drain in a MOSFET. This ability is what allows each of these devices to function as an amplifier or, if driven hard enough, as a switch. How electric current flows in a vacuum tube is very different from how it flows in a transistor, though. Vacuum tubes rely on a process called thermionic emission : Heating the cathode causes it to shed electrons into the surrounding vacuum.

The current in transistors, on the other hand, comes from the drift and diffusion of electrons (or of “holes,” spots where electrons are missing) between the source and the drain through the solid semiconducting material that separates them. Why did vacuum tubes give way to solid-state electronics so many decades ago? The advantages of semiconductors include lower costs, much smaller size, superior lifetimes, efficiency, ruggedness, reliability, and consistency. Notwithstanding these advantages, when considered purely as a medium for transporting charge, vacuum wins over semiconductors. Electrons propagate freely through the nothingness of a vacuum, whereas they suffer from collisions with the atoms in a solid (a process called crystal-lattice scattering). What’s more, a vacuum isn’t prone to the kind of radiation damage that plagues semiconductors, and it produces less noise and distortion than solid-state materials. The drawbacks of tubes weren’t so vexing when you just needed a handful of them to run your radio or television set. But they proved really troublesome with more complicated circuits.

For example, the 1946 ENIAC computer , which used 17,468 vacuum tubes, consumed 150 kilowatts of power, weighed more than 27 metric tons, and took up almost 200 square meters of floor space. And it kept breaking down all the time, with a tube failing every day or two. Illustration: James Provost Chip in a Bottle: The simplest vacuum tube capable of amplification is the triode, so named because it contains three electrodes: a cathode, an anode, and a grid. Typically, the structure is cylindrically symmetrical, with the cathode surrounded by the grid and the grid surrounded by the anode. Operation is similar to that of a field-effect transistor, here with the voltage applied to the grid controlling the current flow between the other two electrodes. (Triode tubes often have five pins to accommodate two additional electrical connections for the heated filament.) The transistor revolution put an end to such frustrations.

But the ensuing sea change in electronics came about not so much because of the intrinsic advantages of semiconductors but because engineers gained the ability to mass-produce and combine transistors in integrated circuits by chemically engraving, or etching, a silicon wafer with the appropriate pattern. As the technology of integrated-circuit fabrication progressed, more and more transistors could be squeezed onto microchips, allowing the circuitry to become more elaborate from one generation to the next. The electronics also became faster without costing any more. That speed benefit stemmed from the fact that as the transistors became smaller, electrons moving through them had to travel increasingly shorter distances between the source and the drain, allowing each transistor to be turned on and off more quickly. Vacuum tubes, on the other hand, were big and bulky and had to be fabricated individually by mechanical machining. While they were improved over the years, tubes never benefited from anything remotely resembling Moore’s Law. But after four decades of shrinking transistor dimensions, the oxide layer that insulates the gate electrode of a typical MOSFET is now only a few nanometers thick , and just a few tens of nanometers separate its source and drain. Conventional transistors really can’t get much smaller.

Still, the quest for faster and more energy-efficient chips continues. What will the next transistor technology be? Nanowires , carbon nanotubes , and graphene are all being developed intensively. Perhaps one of these approaches will revamp the electronics industry. Or maybe they’ll all fizzle. We’ve been working to develop yet another candidate to replace the MOSFET, one that researchers have been dabbling with off and on for many years: the vacuum-channel transistor. It’s the result of a marriage between traditional vacuum-tube technology and modern semiconductor-fabrication techniques. This curious hybrid combines the best aspects of vacuum tubes and transistors and can be made as small and as cheap as any solid-state device. Indeed, making them small is what eliminates the well-known drawbacks of vacuum tubes. Illustration: James Provost; Inset Image: NASA Ames Research Center Transistorizing the Vacuum Tube: A vacuum-channel transistor closely resembles an ordinary metal-oxide semiconductor field-effect transistor or MOSFET [left]. In a MOSFET, voltage applied to the gate sets up an electric field in the semiconductor material below. This field in turn draws charge carriers into the channel between the source and drain regions, allowing current to flow.

No current flows into the gate, which is insulated from the substrate below it by a thin oxide layer. The vacuum-channel transistor the authors developed [right] similarly uses a thin layer of oxide to insulate the gate from the cathode and anode, which are sharply pointed to intensify the electric field at the tips. In a vacuum tube, an electric filament, similar to the filament in an incandescent lightbulb, is used to heat the cathode sufficiently for it to emit electrons. This is why vacuum tubes need time to warm up and why they consume so much power. It’s also why they frequently burn out (often as a result of a minuscule leak in the tube’s glass envelope). But vacuum-channel transistors don’t need a filament or hot cathode. If the device is made small enough, the electric field across it is sufficient to draw electrons from the source by a process known as field emission . Eliminating the power-sapping heating element reduces the area each device takes up on a chip and makes this new kind of transistor energy efficient. Another weak point of tubes is that they must maintain a high vacuum, typically a thousandth or so of atmospheric pressure, to avoid collisions between electrons and gas molecules.

Under such low pressure, the electric field causes positive ions generated from the residual gas in a tube to accelerate and bombard the cathode, creating sharp, nanometer-scale protrusions, which degrade and, ultimately, destroy it. These long-standing problems of vacuum electronics aren’t insurmountable. What if the distance between cathode and anode were less than the average distance an electron travels before hitting a gas molecule, a distance known as the mean free path ? Then you wouldn’t have to worry about collisions between electrons and gas molecules. For example, the mean free path of electrons in air under normal atmospheric pressure is about 200 nanometers, which on the scale of today’s transistors is pretty large. Use helium instead of air and the mean free path goes up to about 1 micrometer. That means an electron traveling across, say, a 100-nm gap bathed in helium would have only about a 10 percent probability of colliding with the gas. Make the gap smaller still and the chance of collision diminishes further. But even with a low probability of hitting, many electrons are still going to collide with gas molecules.

If the impact knocks a bound electron from the gas molecule, it will become a positively charged ion, which means that the electric field will send it flying toward the cathode. Under the bombardment of all those positive ions, cathodes degrade. So you really want to avoid this as much as possible. Fortunately, if you keep the voltage low, the electrons will never acquire enough energy to ionize helium. So if the dimensions of the vacuum transistor are substantially smaller than the mean free path of electrons (which is not hard to arrange), and the working voltage is low enough (not difficult either), the device can operate just fine at atmospheric pressure. That is, you don’t, in fact, need to maintain any sort of vacuum at all for what is nominally a miniaturized piece of “vacuum” electronics! But how do you turn this new kind of transistor on and off? With a triode vacuum tube, you control the current flowing through it by varying the voltage applied to the grid—a meshlike electrode situated between the cathode and the anode. Positioning the grid close to the cathode enhances the grid’s electrostatic control, although that close positioning tends to increase the amount of current flowing into the grid.

Ideally, no current would ever flow into the grid, because it wastes energy and can even cause the tube to malfunction. But in practice there’s always a little grid current. To avoid such problems, we control current flow in our vacuum-channel transistor just as it’s done in ordinary MOSFETs, using a gate electrode that has an insulating dielectric material (silicon dioxide) separating it from the current channel. The dielectric insulator transfers the electric field where it’s needed while preventing the flow of current into the gate. So you see, the vacuum-channel transistor isn’t at all complicated. Indeed, it operates much more simply than any of the transistor varieties that came before it. Although we are still at an early stage with our research, we believe the recent improvements we’ve made to the vacuum-channel transistor could one day have a huge influence on the electronics industry, particularly for applications where speed is paramount. Our very first effort to fashion a prototype produced a device that could operate at 460 gigahertz—roughly 10 times as fast as the best silicon transistor can manage.

This makes the vacuum-channel transistor very promising for operating in what is sometimes known as the terahertz gap , the portion of the electromagnetic spectrum above microwaves and below infrared. Filling the Gap: Vacuum-channel transistors hold the promise of being able to operate at frequencies above microwaves and below infrared—a region of the spectrum sometimes known as the terahertz gap because of the difficulty that most semiconductor devices have operating at those frequencies. Promising applications for terahertz equipment include directional high-speed communications and hazardous-materials sensing. Such frequencies, which run from about 0.1 to 10 terahertz, are useful for sensing hazardous materials and for secure high-speed telecommunications, to give just a couple of possible applications. But terahertz waves are difficult to take advantage of because conventional semiconductors aren’t capable of generating or detecting this radiation. Vacuum transistors could—pardon the expression—fill that void. These transistors might also find their way into future microprocessors, their method of manufacture being completely compatible with conventional CMOS fabrication. But several problems will need to be solved before that can happen. Our prototype vacuum transistor operates at 10 volts, an order of magnitude higher than modern CMOS chips use.

But researchers at the University of Pittsburgh have been able to build vacuum transistors that operate at just 1 or 2 V , albeit with significant compromises in design flexibility. We’re confident we can reduce the voltage requirements of our device to similar levels by shrinking the distance between its anode and cathode. Also, the sharpness of these electrodes determines how much they concentrate the electric field, and the makeup of the cathode material governs how large a field is needed to extract electrons from it. So we might also be able to reduce the voltage needed by designing electrodes with sharper points or a more advantageous chemical composition that lowers the barrier for the electron escaping from the cathode. This will no doubt be something of a balancing act, because changes made to reduce operating voltage could compromise the long-term stability of the electrodes and the resultant lifetime of the transistor. The next big step for us is to build a large number of vacuum-channel transistors into an integrated circuit. For that, we should be able to use many of the existing computer-aided design tools and simulation software developed for constructing CMOS ICs.

Before we attempt this, however, we’ll need to refine our computer models for this new transistor and to work out suitable design rules for wiring lots of them together. And we’ll have to devise proper packaging methods for these 1-atmosphere, helium-filled devices. Most likely, the techniques currently used to package various microelectromechanical sensors, such as accelerometers and gyroscopes, can be applied to vacuum-channel transistors without too much fuss. Admittedly, a great deal of work remains to be done before we can begin to envision commercial products emerging. But when they eventually do, this new generation of vacuum electronics will surely boast some surprising capabilities. Expect that. Otherwise you might end up feeling a bit like those military analysts who examined that Soviet MiG-25 in Japan back in 1976: Later they realized that its vacuum-based avionics could withstand the electromagnetic pulse from a nuclear blast better than anything the West had in its planes. Only then did they begin to appreciate the value of a little nothingness.

This article originally appeared in print as “The Device Made Of Nothing.” About the Authors Jin-Woo Han and Meyya Meyyappan work at NASA Ames Research Center in Moffett Field, Calif., where Han is a research scientist and Meyyappan is chief scientist for exploration technology. The vacuum-channel transistors they describe grew out of an unrelated attempt to oxidize a single thin nanowire. “It ended up as two separate electrodes,” says Han, who then realized that the botched experiment could be turned into a new kind of transistor. Advertisement 1950s: Transistors Fill The Vacuum: The Digital Age Begins | Defense content from Electronic Design Home > Markets > Defense > 1950s: Transistors Fill The Vacuum: The Digital Age Begins 1950s: Transistors Fill The Vacuum: The Digital Age Begins Oct 21, 2002 Lisa Maliniak | Electronic Design Comments 0 A decade of contradictions: infinite hope for the future coupled with fear of powerful enemies. That is the 1950s, a time of war and then of post-war prosperity.

Rock and roll was evolving from rhythm and blues, soon to be heard blaring from transistor radios from Spokane to Baltimore. Crew-cut kids watched across TV dinners as tales of space travel and futuristic dreams flickered across the screens of RCA consoles. Gas was cheap, tailfins were large, and Americans were consummating their love affair with the open road. The future seemed limitless, and as the decade dawned, few even realized why. But as the 1940s drew to a close, a handful of engineers had made a breakthrough that ultimately would change the world. The infant born in 1947 to Bardeen, Brittain, and Shockley—the point-contact transistor—came of age in the 1950s. It matured into Shockley's junction transistor, which found a home in countless military and consumer applications. The solid-state age had begun, pushing the electronics industry toward modern digital computers and communications. Transistors ran cooler and demanded far less power than the vacuum tubes they would begin replacing, producing smaller, faster, and more powerful electronics.

Initially costly to produce, transistors in the fifties began the trend that the electronics industry has continued ever since: ever-lower cost coupled with greater functionality and integration. Transistor process technology was refined throughout the decade, which culminated in the development of the first integrated circuit. Necessity is indeed the mother of invention, and never was necessity greater than during the wars of the 1940s and early fifties. The massive efforts stemming from World War II, the Korean conflict, and the ensuing Cold War resulted in the mobilization of America's greatest scientific minds. The tense geopolitical faceoff between east and west found the electronics industry being thrust to the battle's front lines, fervently employing the new solid-state technology in increasingly sophisticated defense and weapons systems. Out of urgent military need came technological marvels that kept the United States in the forefront of science, and chief among these was the advent of digital computers. It was the digital computer advances in the 1950s that laid the groundwork for the successful commercial mainframe and mini computers that would emerge in the 1960s, and later evolve into the personal computers of the 1970s and 1980s.

Digital computer technology that had begun as part of the war effort in the forties was refined and then marketed as a commercial product. In 1957, J.M. Bridges, the Defense Department's Director of Electronics, told a group of computer manufacturers that digital computers were destined to replace less reliable analog machines in complex military weapons systems of the future. A hallmark of the development effort, and a sure means of achieving reliability, would be to manufacture standard, modular digital functional blocks that could be combined with little or no change to build complex computing systems. Indeed, even as the Korean War began at the outset of the decade, the concept of the plug-in circuit module had taken form and begun to speed the production of electronic equipment. It permitted circuits to be assembled in different areas and then simply connected together. Modularization, combined with a growing reliance on printed circuits, also brought enhanced reliability, repeatability, and easier servicing when things did go awry. The earliest digital computers, beginning in the forties and continuing into the late 1950s, were based on vacuum tubes. They were unreliable and difficult to program, used lots of power, required very large rooms, and were constantly in need of maintenance.

Storing information was difficult, and the machines could only solve one problem at a time. A breakthrough in digital computing came in 1951, when the Eckert and Mauchly Computer Company of Philadelphia sold the first commercial computer, the UNIVAC 1, to the U.S. Census Bureau. The massive machine retrieved data from memory by transmitting sonic pulses through tubes of mercury. An additional 45 UNIVAC 1 machines would eventually be sold. Other computer advancements came in memory. For example, the invention of ferrite core memories would lead to the Massachusetts Institute of Technology's development of random access memory (RAM), which made retrieving information quick and efficient. Also, the RAMAC disc operating system, introduced by IBM in 1957, was the first data processing system to use record-like discs to store digital data. Each disc had a storage capacity of about 100,000 characters and could be randomly accessed. Although development of the transistor gave electronic design engineers an important building block for the future, it also spurred development of the tools and infrastructure the industry would require for growth.

The 1950s saw a broad array of test and measurement equipment emerge as engineers clamored for ways to quantify the performance of their circuits. As the need for improved test equipment grew, advances in cathode-ray tube (CRT) technology were essential for the equipment to evolve. Improved CRTs were put to good use both in test gear and in television sets. The 1953 development of the digital voltmeter (DVM) by Non-Linear Systems as a separate instrument was a key innovation in test equipment. The DVM, an instrument that was part of many analog computers, increased the accuracy of typical laboratory measurements by more than an order of magnitude over the vacuum-tube analog voltmeters that had preceded it. The industry also began looking ahead to how it would produce components in the future. To that end, Bell Labs developed an experimental transistor-making robot that presaged later advances in automated semiconductor manufacturing. The machine, dubbed "Mr. Meticulous," accurately positioned and welded gold collector and emitter wires to their respective device layers and then automatically tested the finished devices. Click here for several examples of the special photos in this picture album. Automated semiconductor production was pivotal as the technology infiltrated more than military and computer applications.

Space represented a new frontier to be conquered, and there was great political and ideological capital at stake for the victor. In 1957, the Soviet Union startled the world when it launched Sputnik, the first orbital satellite. The United States was caught short with its space program barely off the ground. But the U.S. made a comeback, launching its own first orbital satellite, Explorer I, in 1958. The space race was on, fueled by an explosion in engineering ingenuity from the electronics world. More ambitious probing of the moon and solar system quickly followed the early orbital satellites. The combined efforts of many companies to make the necessary equipment lighter, more precise, and more efficient made these probes possible. Meanwhile, the newborn that was solid-state electronics was making its presence felt on terra firma in a big way. The transistor radio, introduced in 1954, became the fastest selling consumer product of the time. By 1955, car-crazed Americans, fueled by post-war prosperity and cheap gas, were more than ready for a little driving music. RCA's Sarnoff Research Center answered the call with an experimental nine-transistor car radio.

Transistor radios came onto the market in droves, giving America its first taste of its own future: electronics everywhere, packaged to travel and ready for deployment at will whether on the street, in the car, or at home. One of the earliest commercial applications of junction transistors was in hearing aids. As early as January, 1953, NPN transistors saw use in a hearing-aid circuit that ran for six months on a tiny B battery. Tellingly, it foretold of the advances to come in transistorized circuits by being 25% to 30% smaller than earlier models with twice the output power. Through the 1940s, radio was king and the theater of the mind still ruled in post-war America. But television's golden age was just beginning in the early fifties and again, electronics led the way. RCA's experimental solid-state (except for the picture tube) TV receiver of 1952 used 37 semiconductor devices, had a five-inch screen, and weighed about 27 pounds. An inauspicious start, perhaps.

But as with everything else the electronics industry touched in ensuing decades, the challenge of miniaturizing and improving television sets was ultimately met. Throughout the industry's history, many technologies found their genesis in the military before trickling down into consumer products. Modularization and miniaturization were an example of this trend. In the beginning, miniaturization was driven by the Soviets' success with the Sputnik program and was a major objective of government-funded electronics research programs. But through work like RCA's solid-state TV, the drive to minimize quickly made its mark on consumer products. As always, the basic building block of the transistor formed the underpinnings for all of the advances in consumer goods. For most of the 1950s, germanium was used to make transistors. But after Texas Instruments introduced the first silicon transistors in 1954, silicon began to replace germanium as a semiconductor material, thereby extending operating temperatures to military ranges. As the decade wound down, the pieces were in place for the crucial next step along the path of integration.

Engineers at both Fairchild Semiconductor and Texas Instruments sought to produce a single substrate of silicon carrying not only transistors and diodes, but also resistors and capacitors—and then join all the components to form a complete circuit. In 1958, Jack Kilby of Texas Instruments and Robert Noyce of Fairchild Semiconductor independently developed just such a device—the integrated circuit (IC). The first monolithic IC was built in 1958 at Texas Instruments when Kilby constructed a phase-shift oscillator from a single silicon bar. The device required no interconnections between one component and another: The electrical path was through the silicon. TI was also the first company to announce a product line of ICs. By the end of the decade, the transistor had rightfully earned its place in the forefront of technology and, in fact, had begun to fulfill its promise to take the industry much further. Government got into the act in 1958, when President Dwight D. Eisenhower created the Advanced Research Projects Agency (ARPA) to keep the U.S. at the forefront of technology. ARPA would, in the next decade, plant the seeds for one of the greatest technology advances of the century—the Internet.

A decade of contradictions, social and political, managed by its close to coalesce into one of great technological advances. The 1950s set the stage for the computer advances that would be seen in the sixties and on into the seventies. And it was the beginning of rapid change in electronics, changes that would heavily impact the way Americans lived their everyday lives. What Is a Transistor & What Effect Did Its Invention Have on Computers? | The Classroom | Synonym What Is a Transistor & What Effect Did Its Invention Have on Computers? by Andrew May Early transistors were a fraction of the size of the vacuum tubes they replaced. Related Articles What Device Connects the CPU to a Computer System's Other Hardware Devices? A transistor is a miniature electronic component made from a semiconductor material such as silicon. Before transistors, the only way to control the flow of current in an electronic circuit was using large, energy-hungry vacuum tubes, which limited the size and power of computers that could be built. The invention of transistors revolutionized computer design: A modern microprocessor typically contains hundreds of millions of transistors on a single silicon chip.

Invention of the Transistor The transistor was invented in 1947 by John Bardeen, Walter Brattain and William Shockley at Bell Laboratories in New Jersey. The invention was the culmination of a long-running effort to develop a viable alternative to the vacuum tube using semiconductor technology. It earned Bardeen, Brattain and Shockley the Nobel Prize for Physics in 1956. Compared to vacuum tubes, transistors have numerous advantages: They are smaller and less fragile, they require less power to operate and they can function at lower voltages. How a Transistor Works If an electric current is passed between two of a transistor's three terminals, the magnitude of this current can be controlled by a voltage applied to the third terminal. This has two basic applications -- either as a switch or as an amplifier. The first widespread application of the transistor was as an amplifier in portable radios. However, its use in computers is as a switch rather than an amplifier. Computers using vacuum tubes as switches were limited by size constraints to a few thousand such devices. With the invention of the transistor, the situation changed dramatically.

Effect on Computers Transistor-based computers were not only smaller than their vacuum-tube predecessors but also more powerful because more components could be packed inside. Transistorization also made computers faster because the more compact dimensions meant that electrical signals didn't have to travel as far. As time went on, so the number of transistors that could be fabricated on a single chip of silicon increased. This effect was quantified in the 1960s by Gordon Moore, in the mathematical relationship known as Moore's Law. Transistors in the Digital Age Transistors have become one of the basic building blocks of modern technological life. As the essential switching device in all types of microprocessors, they are found in smart phones, digital cameras, electronic games and satellite navigation systems as well as in desktop and laptop computers. Typically a modern microprocessor will contain hundreds of millions of transistors, although the most advanced designs have transistor counts in excess of 2 billion.

References From Tubes To Transistors - Computer History 101: The Development Of The PC Computer History 101: The Development Of The PC Page 1:The First Electronic Computers Page 2:From Tubes To Transistors Page 3:Birth Of The Personal Computer Page 4:The IBM Personal Computer Page 5:Mechanical To Modern Page 5:Mechanical To Modern From Tubes To Transistors From UNIVAC to the latest desktop PCs, computer evolution has moved very rapidly. The first-generation computers were known for using vacuum tubes in their construction. The generation to follow would use the much smaller and more efficient transistor. From Tubes... Any modern digital computer is largely a collection of electronic switches. These switches are used to represent and control the routing of data elements called binary digits (or bits).Because of the on-or-off nature of the binary information and signal routing the computer uses, an efficient electronic switch was required. The first electronic computers used vacuum tubes as switches, and although the tubes worked, they had many problems. The three main components of a basic triode vacuum tube. The type of tube used in early computers was called a triode and was invented by Lee De Forest in 1906.

It consists of a cathode and a plate, separated by a control grid, suspended in a glass vacuum tube. The cathode is heated by a red-hot electric filament, which causes it to emit electrons that are attracted to the plate. The control grid in the middle can control this flow of electrons. By making it negative, you cause the electrons to be repelled back to the cathode; by making it positive, you cause them to be attracted toward the plate. Thus, by controlling the grid current, you can control the on/off output of the plate. Unfortunately, the tube was inefficient as a switch. It consumed a great deal of electrical power and gave off enormous heat—a significant problem in the earlier systems. Primarily because of the heat they generated, tubes were notoriously unreliable—in larger systems, one failed every couple of hours or so. ...To Transistors The invention of the transistor was one of the most important developments leading to the personal computer revolution.The transistor was invented in 1947 and announced in 1948 by Bell Laboratory engineers John Bardeen and Walter Brattain.

Bell associate William Shockley invented the junction transistor a few months later, and all three jointly shared the Nobel Prize in Physics in 1956 for inventing the transistor. The transistor, which essentially functions as a solid-state electronic switch, replaced the less-suitable vacuum tube. Because the transistor was so much smaller and consumed significantly less power, a computer system built with transistors was also much smaller, faster, and more efficient than a computer system built with vacuum tubes. The conversion from tubes to transistors began the trend toward miniaturization that continues to this day. Today’s small laptop PC (or netbook, if you prefer) and even Tablet PC systems, which run on batteries, have more computing power than many earlier systems that filled rooms and consumed huge amounts of electrical power. Although there have been many designs for transistors over the years, the transistors used in modern computers are normally Metal Oxide Semiconductor Field Effect Transistors (MOSFETs). MOSFETs are made from layers of materials deposited on a silicon substrate.

Some of the layers contain silicon with certain impurities added by a process called doping or ion bombardment, whereas other layers include silicon dioxide (which acts as an insulator), polysilicon (which acts as an electrode), and metal to act as the wires to connect the transistor to other components. The composition and arrangement of the different types of doped silicon allow them to act both as a conductor or an insulator, which is why silicon is called a semiconductor. MOSFETs can be constructed as either NMOS or PMOS types, based on the arrangement of doped silicon used. Silicon doped with boron is called P-type (positive) because it lacks electrons, whereas silicon doped with phosphorus is called N-type (negative) because it has an excess of free electrons. MOSFETs have three connections, called the source, gate, and drain. An NMOS transistor is made by using N-type silicon for the source and drain, with P-type silicon placed in between. The gate is positioned above the P-type silicon, separating the source and drain, and is separated from the P-type silicon by an insulating layer of silicon dioxide.

Normally there is no current flow between N-type and P-type silicon, thus preventing electron flow between the source and drain. When a positive voltage is placed on the gate, the gate electrode creates a field that attracts electrons to the P-type silicon between the source and drain. That in turn changes that area to behave as if it were N-type silicon, creating a path for current to flow and turning the transistor “on.” Cutaway view of an NMOS transistor. A PMOS transistor works in a similar but opposite fashion. P-type silicon is used for the source and drain, with N-type silicon positioned between them. When a negative voltage is placed on the gate, the gate electrode creates a field that repels electrons from the N-type silicon between the source and drain. That in turn changes that area to behave as if it were P-type silicon, creating a path for current to flow and turning the transistor “on.” When both NMOS and PMOS field-effect transistors are combined in a complementary arrangement, power is used only when the transistors are switching, making dense, low-power circuit designs possible. Because of this, virtually all modern processors are designed using CMOS (Complementary Metal Oxide Semiconductor) technology.

Compared to a tube, a transistor is much more efficient as a switch and can be miniaturized to microscopic scale. Since the transistor was invented, engineers have strived to make it smaller and smaller. In 2003, NEC researchers unveiled a silicon transistor only 5 nanometers (billionths of a meter) in size. Other technology, such as Graphene and carbon nanotubes, are being explored to produce even smaller transistors, down to the molecular or even atomic scale. In 2008, British researchers unveiled a Graphene-based transistor only 1 atom thick and 10 atoms (1 nm) across, and in 2010, IBM researchers created Graphene transistors switching at a rate of 100 gigahertz, thus paving the way for future chips denser and faster than possible with silicon-based designs. Integrated Circuits: The Next Generation The third generation of modern computers is known for using integrated circuits instead of individual transistors. Jack Kilby at Texas Instruments and Robert Noyce at Fairchild are both credited with having invented the integrated circuit (IC) in 1958 and 1959.

An IC is a semiconductor circuit that contains more than one component on the same base (or substrate material), which are usually interconnected without wires. The first prototype IC constructed by Kilby at TI in 1958 contained only one transistor, several resistors, and a capacitor on a single slab of germanium, and it featured fine gold “flying wires” to interconnect them. However, because the flying wires had to be individually attached, this type of design was not practical to manufacture. By comparison, Noyce patented the “planar” IC design in 1959, where all the components are diffused in or etched on a silicon base, including a layer of aluminum metal interconnects. In 1960, Fairchild constructed the first planar IC, consisting of a flip-flop circuit with four transistors and five resistors on a circular die only about 20 mm2 in size. By comparison, the Intel Core i7 quad-core processor incorporates 731 million transistors (and numerous other components) on a single 263 mm2 die!

Create a new thread in the US Reviews comments forum about this subject Title The Transistor - History The Transistor in a Century of Electronics A Three Terminal Device Vacuum tubes were made containing several three terminal devices called triodes. The transistor is a three terminal, solid state electronic device. In a three terminal device we can control electric current or voltage between two of the terminals by applying an electric current or voltage to the third terminal. This three terminal character of the transistor is what allows us to make an amplifier for electrical signals, like the one in our radio. With the three-terminal transistor we can also make an electric switch, which can be controlled by another electrical switch. By cascading these switches (switches that control switches that control switches, etc.) we can build up very complicated logic circuits. These logic circuits can be built very compact on a silicon chip with 1,000,000 transistors per square centimeter. We can turn them on and off very rapidly by switching every 0.000000001 seconds. Such logic chips are at the heart of your personal computer and many other gadgets you use today. Light Bulbs and Vacuum Tubes The transistor was not the first three terminal device.

The vacuum tube triode preceded the transistor by nearly 50 years. Vacuum tubes played an important role in the emergence of home electronics and in the scientific discoveries and technical innovations which are the foundation for our modern electronic technology. Thomas Edison's light bulb was one of the first uses of vacuum tubes for electrical applications. Soon after the discovery of the light bulb, a third electrode was placed in the vacuum tube to investigate the effect that this electrode would have on "cathode rays," which were observed around the filament of the light bulb. Joseph John Thomson developed a vacuum tube to carefully investigate the nature of cathode rays, which resulted in his discovery, published in 1897. He showed that the cathode rays were really made up of particles, or "corpuscles" as Thomson called them, that were contained in all material. Thomson had discovered the electron, for which he received the Nobel Prize in Physics 1906 . Lee De Forest and The Radio Radio brought information rapidly to the masses and was the first widely used electronic device in the home. At the same time as physicists were trying to understand what cathode rays were, engineers were trying to apply them to make electronic devices.

In 1906, an American inventor and physicists, Lee De Forest, made the vacuum tube triode, or audion as he called it. The triode was a three terminal device that allowed him to make an amplifier for audio signals, making AM radio possible. Radio revolutionized the way in which information and entertainment reached the great majority of people. The vacuum tube triode also helped push the development of computers forward a great deal. Electronic tubes were used in several different computer designs in the late 1940's and early 1950's. But the limits of these tubes were soon reached. As the electric circuits became more complicated, one needed more and more triodes. Engineers packed several triodes into one vacuum tube (that is why the tube has so many legs) to make the tube circuits more efficient. Early Computers The largest computers based on vacuum tubes had racks and racks of tubes filling large rooms. The vacuum tubes tended to leak, and the metal that emitted electrons in the vacuum tubes burned out. The tubes also required so much power that big and complicated circuits were too large and took too much energy to run.

In the late 1940's, big computers were built with over 10,000 vacuum tubes and occupied over 93 square meters of space. The problems with vacuum tubes lead scientists and engineers to think of other ways to make three terminal devices. Instead of using electrons in vacuum, scientists began to consider how one might control electrons in solid materials, like metals and semiconductors. Already in the 1920's, scientists understood how to make a two terminal device by making a point contact between a sharp metal tip and a piece of semiconductor crystal. These point-contact diodes were used to rectify signals (change oscillating signals to steady signals), and make simple AM radio receivers (crystal radios). However, it took many years before the three terminal solid state device - the transistor - was discovered. The First Transistor The first point contact transistor made use of the semiconductor germanium. Paper clips and razor blades were used to make the device. In 1947, John Bardeen and Walter Brattain, working at Bell Telephone Laboratories, were trying to understand the nature of the electrons at the interface between a metal and a semiconductor.

They realized that by making two point contacts very close to one another, they could make a three terminal device - the first "point contact" transistor. They quickly made a few of these transistors and connected them with some other components to make an audio amplifier. This audio amplifier was shown to chief executives at Bell Telephone Company, who were very impressed that it didn't need time to "warm up" (like the heaters in vacuum tube circuits). They immediately realized the power of this new technology. This invention was the spark that ignited a huge research effort in solid state electronics. Bardeen and Brattain received the Nobel Prize in Physics, 1956, together with William Shockley, "for their researches on semiconductors and their discovery of the transistor effect." Shockley had developed a so-called junction transistor, which was built on thin slices of different types of semiconductor material pressed together. The junction transistor was easier to understand theoretically, and could be manufactured more reliably. Limits of Individual Transistors Individual electronic components were soldered on to printed circuit boards.

For many years, transistors were made as individual electronic components and were connected to other electronic components (resistors, capacitors, inductors, diodes, etc.) on boards to make an electronic circuit. They were much smaller than vacuum tubes and consumed much less power. Electronic circuits could be made more complex, with more transistors switching faster than tubes. However, it did not take long before the limits of this circuit construction technique were reached. Circuits based on individual transistors became too large and too difficult to assemble. There were simply too many electronic components to deal with. The transistor circuits were faster than vacuum tube circuits, and there were noticeable problems due to time delays for electric signals to propagate a long distance in these large circuits. To make the circuits even faster, one needed to pack the transistors closer and closer together. The Integrated Circuit Integrated circuits placed all components in one chip, drastically reducing the size of the circuit and its components. In 1958 and 1959, Jack Kilby at Texas Instruments and Robert Noyce at Fairchild Camera, came up with a solution to the problem of large numbers of components, and the integrated circuit was developed.

Instead of making transistors one-by-one, several transistors could be made at the same time, on the same piece of semiconductor. Not only transistors, but other electric components such as resistors, capacitors and diodes could be made by the same process with the same materials. For more than 30 years, since the 1960's, the number of transistors per unit area has been doubling every 1.5 years. This fantastic progression of circuit fabrication is known as Moore's law, after Gordon Moore, one of the early integrated circuit pioneers and founders of Intel Corporation. The Nobel Prize in Physics 2000 was awarded to Jack Kilby for the invention of the integrated circuit. From the dawn of the vacuum tube triode, to the discovery of the transistor and the development of the integrated circuit, the 20th century has certainly been the century of electronics.

By Professor David B Haviland First published 19 December 2002 An Outline of the History of the Transistor "The transistor was probably the most important invention of the 20th Century, and the story behind the invention is one of clashing egos and top secret research...." Quicktour Transistorized! This brief introduction outlines personalities and organizations involved in the history of the transistor. For a richer picture, please follow the links throughout this web site. Bell Laboratories, one of the world's largest industrial laboratories, was the research arm of the giant telephone company American Telephone and Telegraph ( AT&T ). In 1945, Bell Labs was beginning to look for a solution to a long-standing problem. 1907 - The Problem AT&T brought its former president, Theodore Vail, out of retirement to help it fight off competition erupting from the expiration of Alexander Graham Bell's telephone patents. Vail's solution: transcontinental telephone service. In 1906, the eccentric American inventor Lee De Forest developed a triode in a vacuum tube. It was a device that could amplify signals, including, it was hoped, signals on telephone lines as they were transferred across the country from one switch box to another.

AT&T bought De Forest's patent and vastly improved the tube. It allowed the signal to be amplified regularly along the line, meaning that a telephone conversation could go on across any distance as long as there were amplifiers along the way. But the vacuum tubes that made that amplification possible were extremely unreliable, used too much power and produced too much heat. In the 1930s, Bell Lab's director of research, Mervin Kelly, recognized that a better device was needed for the telephone business to continue to grow. He felt that the answer might lie in a strange class of materials called semiconductors . 1945 - The Solution After the end of World War II, Kelly put together a team of scientists to develop a solid-state semiconductor switch to replace the problematic vacuum tube . The team would use some of the advances in semiconductor research during the war that had made radar possible. A young, brilliant theoretician, Bill Shockley , was selected as the team leader. (See Shockley, Brattain and Bardeenthe team and the teammates ) Shockley drafted Bell Lab's Walter Brattain , an experimental physicist who could build or fix just about anything, and hired theoretical physicist John Bardeen from the University of Minnesota.

Shockley filled out his team with an eclectic mix of physicists, chemists and engineers. The group was diverse, yet close knit. Walter Brown, a physicist who joined the group in 1951, recalls hearing about exuberant parties and good lunches. Betty Sparks, Shockley's secretary, recalled the group's high spirits at her wedding to Morgan Sparks. They called their lab " Hell's Bells Laboratory ." In the spring of 1945, Shockley designed what he hoped would be the first semiconductor amplifier, relying on something called the " field effect ." His device was a small cylinder coated thinly with silicon, mounted close to a small, metal plate. It was, as University of Illinois Electrical Engineer Nick Holonyak said, a crazy idea. Indeed, the device didn't work, and Shockley assigned Bardeen and Brattain to find out why. According to author Joel Shurkin, the two largely worked unsupervised; Shockley spent most of his time working alone at home. Ensconced in Bell Labs' Murray Hill facilities, Bardeen and Brattain began a great partnership. Bardeen, the theoretician, suggested experiments and interpreted the results, while Brattain built and ran the experiments.

Technician Phil Foy recalls that as time went on with little success, tensions began to build within the lab group. In the fall of 1947, author Lillian Hoddeson says, Brattain decided to try dunking the entire apparatus into a tub of water. Surprisingly, it worked... a little bit. Brattain began to experiment with gold on germanium, eliminating the liquid layer on the theory that it was slowing down the device. It didn't work, but the team kept experimenting using that design as a starting point. Shortly before Christmas, Bardeen had an historic insight. Everyone thought they knew how electrons behaved in crystals, but Bardeen discovered they were wrong. The electrons formed a barrier on the surface. His breakthrough was what they needed. Without telling Shockley about the changes they were making to the investigation, Bardeen and Brattain worked on. On December 16, 1947, they built the point-contact transistor , made from strips of gold foil on a plastic triangle, pushed down into contact with a slab of germanium.

When Bardeen and Brattain called Shockley to tell him of the invention, Shockley was both pleased at the group's results and furious that he had not been directly involved. He decided that to preserve his standing, he would have to do Bardeen and Brattain one better. His device, the junction (sandwich) transistor , was developed in a burst of creativity and anger, mostly in a hotel room in Chicago. It took him a total of four weeks of working pen on paper, although it took another two years before he could actually build one. His device was more rugged and more practical than Bardeen and Brattain's point-contact transistor, and much easier to manufacture. It became the central artifact of the electronic age. Author Michael Riordan says Bardeen and Brattain got "pushed aside." That insult broke the team apart, turning a once cooperative environment into one that was highly competitive. The problems of whose names should be on the patent for the device, and who should be featured in publicity photographs, sent tensions higher still. Bell Labs decided to unveil the invention on June 30, 1948.

With the help of engineer John Pierce, who wrote science fiction in his spare time, Bell Labs settled on the name " transistor "-- combining the ideas of "trans-resistance" with the names of other devices like thermistors. The invention got little attention at the time, either in the popular press or in industry. But Shockley saw its potential. He left Bell Labs to found Shockley Semiconductor in Palo Alto, California. He hired superb engineers and physicists, but, according to physical chemist Harry Sello, Shockley's personality drove out eight of his best and brightest. Those " traitorous eight " founded a new company called Fairchild Semiconductor . Bob Noyce and Gordon Moore, two of the eight, went on to form Intel Corporation . They (and others at Texas Instruments ) co-invented the integrated circuit . Today, Intel produces billions of transistors daily on its integrated circuits, yet Bardeen, Brattain, and Shockley earned very little money from their research. Nonetheless, Shockley's company was the beginning of Silicon Valley. Bardeen left Bell Labs for the University of Illinois, where he won a second Nobel Prize. Brattain stayed on for several years, and then left to teach.

Shockley lost his company and taught at Stanford for a while, and then got involved in a notorious controversy over race, genetics and intelligence that destroyed his reputation. In the 1950s and 1960s, most U.S. companies chose to focus their attentions on the military market in producing transistor products. That left the door wide open for Japanese engineers like Masaru Ibuka and Akio Morita, who founded a new company named Sony Electronics that mass-produced tiny transistorized radios. Bell Labs' President Emeritus Ian Ross said that part of their success lay in developing the ability to quickly mass-produce transistors. The transistorized radio changed the world, opening up the information age. Information could quickly be scattered to the ends of the Earth, to the point that historian Charles Stewart heard about the assassination of Martin Luther King Jr. from Bedouin tribesmen in the Sahara shortly after it happened. The original three met several times after their breakup: once in Stockholm, Sweden, to receive the 1956 Nobel Prize for their contributions to physics, and once again at Bell Labs in 1972 to commemorate the 25th anniversary of their inventions.

They were celebrating something that they could not know when they first began working on the transistor -- that they were going to change the world. Back to top __________________ For additional reading, see Michael Riordan and Lillian Hoddeson's Crystal Fire: The Invention of the Transistor and Birth of the Information Age , New York, W. W. Norton (1998) Resources: Video news clip on this page produced by Bell Labs, copyright AT&T Bell Labs. Transistor Substitution Transistor Substitution De-soldering and replacing a transistor takes very little time. More time is spent figuring out which one to replace and, sometimes, what to replace it with! The original transistor might need special ordering, or be discontinued and unavailable. Knowing how to substitute an alternate transistor type can speed up your repair, or make an otherwise impossible repair possible. TRANSISTOR TYPES Transistors are given registered type numbers by the Joint Electron Device Engineering Council (JEDEC). The type number is often printed on the transistor, although product manufacturers sometimes have their own part numbers printed instead. In that case, the product's Service Manual or Parts List might provide you with the transistor type.

If you don't have that particular type, a transistor cross-reference guide can sometimes help you to find a compatible, generic part. The NTE Cross Reference is a good example - NTE branded replacements are widely available. But you might already have a suitable substitute in your parts drawers. We arrange our transistor stock, not by their type number, but by their specifications This makes it easy to go right to the drawers holding good candidates for substitution. You can find transistor specs in data sheets available online. Use your browser to search for the transistor's JEDEC type, or use a search engine like AllDataSheet.com . The first spec to consider is the Transistor Outline. This is the only spec you can actually see. BIPOLAR TRANSISTOR SPECS Bipolar (junction) transistors (BJTs) are the most common variety of transistor. The following electrical specs are important when choosing a substitute BJT. TRANSISTOR POLARITY A junction transistor is a sandwich of "N" and "P" type semiconductors. Either the "meat" has extra negative charge carriers (electrons) and the "bread" has extra positive charge carriers (holes), or the meat has the positive charges and the bread has the negative charges.

A substitute transistor must have the same polarity, either "PNP" or "NPN", as the original. If you install the wrong polarity, the sandwich won't work. MAXIMUM VOLTAGES If more than a maximum voltage rating is applied to a transistor, it can be permanently damaged. At the maximum voltage, also called a breakdown voltage (BV), electrons begin to avalanche in the transistor. During an avalanche, electrons in the P-N transition regions are accelerated to energies so high that they hit bound electrons with enough force to free them, creating additional charge carriers and greatly multiplying the transistor current. There are three breakdown voltages: VCB - the maximum voltage across the Collector-Base terminals VCE - the maximum voltage across the Collector-Emitter terminals VEB - the maximum voltage across the Emitter-Base terminals In each of these ratings, the 3rd terminal is assumed to be electrically open (unconnected). VCE, for example, may be written as VCEO, BVCEO, or most correctly as V(BR)CEO. The VEB rating isn't usually a factor in choosing a substitute transistor. VCB is always equal to or greater than VCE and you can use either of these maximum voltages to compare transistors.

Choose a substitute transistor with a breakdown voltage rating at least as high as the original. MAXIMUM CURRENT Maximum current is the maximum continuous collector current (IC) that a transistor can withstand without permanent damage. Small, TO-92 or TO-98 transistors, depending on their fabrication, can handle between about 100 and 1000mA. A TO-5 package might be rated as high as 5 amps; a TO-220, as high as 25A; and a TO-3, up to 500A. Be sure to choose a substitute transistor with a maximum current rating at least as high as the original. MAXIMUM POWER Maximum Power, called PD, is the overall power a transistor can dissipate, through heat, without burning up. Heat sinks and fans increase the ability of a transistor to dissipate heat. A TO-5 transistor with a PD of 3 watts might be able to dissipate 8-10 watts with a heat sink. Choose a substitute transistor with a maximum power rating at least as high as the original. CURRENT GAIN Current gain is only occasionally significant when choosing a substitute transistor. Actual circuit gain depends on other components.

But if the original transistor has a high gain, try to match it. Current gain falls off at higher frequencies, so a high-gain transistor can deliver a wider frequency response than a low-gain transistor. One measure of gain, called hFE, is often used for comparing transistors. The capital FE subscript refers to the Forward DC current transfer ratio in a common Emitter circuit. In other words, IC / IB. Data sheets often specify a minimum or typical value of hFE, or else a range of values that applies at a certain collector current (IC). "Darlington" transistors are made up of two transistors in series and have gains in the thousands, instead of the tens or hundreds. They also have double the input voltage drop since there are two semiconductor junctions in series. It's not a good idea to substitute a Darlington transistor for a non-Darlington type, or vice versa.

FIELD-EFFECT TRANSISTOR SPECS Field-effect transistors (FETs) come with the same transistor outlines as the BJTs but their electrical specs are somewhat different: Field-effect transistors come in three basic types: Type A - the Junction-gate FET (JFET) Type B - the Insulated-gate FET (IGFET) in Depletion mode Type C - the Insulated-gate FET (IGFET) in Enhancement mode IGFETs are usually called MOSFETs (Metal-Oxide Semiconductor FETs) because, originally, all IGFETs used metal gates coated with oxide insulators. Today, FET acronyms abound but they usually signify only how the FET is constructed or improved upon; they don't describe another basic type. Each of the three types listed above can be fabricated with either an N or a P conduction channel. So, altogether, there are six types of field-effect transistors. Be sure to substitute a FET of the same type as the original.

One of the following breakdown voltages is usually included in the specifications for a FET: BVGSS - the breakdown voltage between the Gate and the Source terminals when the drain is short-circuited to the source. (This rating is used primarily with JFETs.) BVDSS - the breakdown voltage between the Drain and the Source terminals when the gate is short-circuited to the source. (This rating is used primarily with Power MOSFETs.) A breakdown voltage can also be written as V(BR)GSS or simply VGSS. You can use either of the above voltages when comparing FETs. Just make sure to compare apples with apples. Pick a substitute FET with a rating at least as high as the original. CURRENT IDSS is the Drain to Source leakage current, often provided for small-signal FETs. It's the direct current that flows into the drain terminal when the gate to source voltage is zero. In a depletion type device, IDSS is an on-state current. In an enhancement type device, it's an off-state current. Minimum and maximum values are usually given. Select a substitute with the same general range of values.

ID(cont), the Continuous Drain Current, is usually provided for power MOSFETs. It's a maximum current rating so choose a substitute with a rating at least as high as the original. MAXIMUM POWER PD is the overall power the FET can dissipate through heat. This is the same spec used for bipolar transistors. Choose a substitute with a rating at least as high as the original. "ON" RESISTANCE rDS(on) is the DC resistance between the Drain and Source terminals when a specified gate to source voltage is applied to bias the FET to the on-state. For a depletion-type FET, the gate-source bias voltage might be 0 V (i.e., a gate to source short). rDS(on) could be important when replacing a power MOSFET. SUMMARY Finding a substitute replacement transistor isn't difficult if you know the specs of the original transistor and organize your stock by specs instead of type numbers. You'll find that fewer devices need to be stocked, and turnaround time can be reduced. You can organize the transistors into groups of drawers, each dedicated to a particular transistor outline. Subdivide the TO groups by electrical ratings like VCEO, IC, and PD, in whatever order you choose.

Then, when you need to pick a substitute for an original transistor, you can quickly home in on all your potential candidates.

Question: What did the transistor replace
Answer: Vacuum tube