[Repost] Popular Science of Components: Basic Knowledge of Transistors[Copy link]
What is a transistor? A transistor, also known as a bipolar junction transistor (BJT), is a current-driven semiconductor device that controls the flow of electric current, where a small current in the base lead controls a larger current between the collector and emitter. They can be used to amplify weak signals, as oscillators, or as switches. Transistors are usually made of silicon crystals, in the form of N and P type semiconductor layers sandwiched together. See Figure 1 below.
Figure 1: Figure 1a shows a 2N3904 TO-92 cutaway view showing the E - emitter, B - base, and C - collector leads connected to the silicon base. Figure 1b, from the May 1958 issue of Radio-Electronics magazine, shows the N and P type layers and their arrangement (germanium was used at the time). Transistors are enclosed and packaged in a plastic or metal cylindrical case with three leads (Figure 2). Figure 2: A comparison of popular package types and sizes. How do transistors work? We will use the NPN transistor as an example to explain how transistors work. A simple way to understand how this type of device works as a switch is to imagine water flowing through a pipe controlled by a valve. The water pressure represents the "voltage" and the water flowing through the pipe represents the "current" (Figure 3). The large pipe represents the collector/emitter junction, separated by a valve, which is represented by a gray oval in the figure. It acts like a movable flapper and is actuated by the water flowing through the small pipe representing the base. The valve maintains the water pressure from the collector to the emitter. When water flows through the smaller pipe (the base), it opens the valve between the collector/emitter junction and allows water to flow through the emitter to ground (the ground represents the return path for all the water or voltage/current). Figure 3: This diagram graphically illustrates how a transistor works. When water flows through the small pipe (base), it opens the valve between the collector/emitter junction, allowing water to flow through the emitter to ground. Choosing a Transistor for Your Application If you are simply looking to turn on a circuit or switch on a load, you should consider the following. Determine whether you want to bias or excite the transistor switch with a positive or negative current (i.e., NPN or PNP type, respectively). NPN transistors are driven (or turned on) by a positive current biased at the base to control the current from the collector to the emitter. PNP type transistors are driven by a negative current biased at the base to control the current from the emitter to the collector. (Note that the PNP polarity is opposite to the NPN.) See Figure 4 below for more details.
Figure 4: Schematic symbols for each transistor type.
Once the bias voltage is determined, the next variables needed are the voltage and current required for the load to operate. These variables will become the minimum voltage and current ratings for the transistor. Tables 1 and 2 below provide some common transistors and their key specifications, including their voltage and current limits. 361545 Transistors, NPN and PNP, Leaded and Surface Mount 361544 Transistors, NPN and PNP, Metal Canister Packages Example Transistor Circuits Figure 5 below shows an example circuit that turns on the collector-emitter junction by energizing the base, or biasing the transistor to turn it on by applying 5 volts to the base via a slide switch. This example will illuminate an LED used as a load. When biasing the base, resistors must be used properly to prevent overcurrent. I used leaded parts in a breadboard to test my example circuits. Most engineers using transistors in new product designs to be brought to market will use surface mount components (which are much smaller than the TO-92 package size). This link shows the various package sizes for the 3904 transistor. Since the 2N3904 is an NPN transistor, the base needs to be positively biased (proper voltage level and resistance) to turn on the collector-emitter junction for the proper current. It is also important to use a load resistor (R1) so that too much current does not flow through the LED and transistor. For more information on this transistor, see the 2N3904 datasheet.
Figure 5: Example of a 2N3904 circuit using an EG1218 slide switch to light an LED, including the C (collector), E (emitter), and B (base) pins (drawn using Scheme-it).
Figure 6: Example of a 2N3906 night-light circuit using a PDV-P5003 photocell to light an LED
A Brief History of the Invention of the Transistor Where did the transistor begin? Many people disagree on who actually invented the first working electrical prototype; however, it is undisputed that Alexander Graham Bell received the first patent on March 7, 1876, and subsequently founded the American Telephone and Telegraph Company (aka AT&T). Around 18941, Bell's patent expired. Although AT&T dominated the telephone market from the beginning until the early 20th century, AT&T was losing customers to other newly formed companies. As a result, AT&T realized the need to continue to control and expand the telephone market. In 1909, AT&T President Theodore Vail1 wanted to make telephone transmissions transcontinental (New York to California). But to do this, they needed high-quality amplifiers or repeaters to strengthen the signal over long distances. Back in 1906, Lee De Forest borrowed from John A.Forest built on the work of Thomas Edison, who had invented a vacuum tube device called the "oscillator tube" to detect radio waves, and improved on it, resulting in the triode, an inefficient 3-terminal vacuum tube that could be used as an amplifier. In 1912, Harold Arnold of the Western Electric Company (AT&T's manufacturer) invited Forest to demonstrate his invention. Although Forest's triode operated at low voltages, Arnold needed a triode that could operate at higher voltages in order to make a repeater that could effectively transmit voice over long distances. Arnold thought he could make a better triode, so he hired scientists to study how the device worked and how to improve it. In October 1913, he succeeded. Soon after, telephone lines began to be installed on a large scale. AT&T had been hiring top scientists for many years to conduct various research, and this investment made it realize that in-depth research would give them a competitive advantage, so in 1925, "Bell Telephone Laboratories" was established. Keeping telephone lines functioning required thousands of vacuum tubes and relays. However, vacuum tubes were power-hungry, bulky, and often burned out. Mervin Kelly, director of research at Bell Labs, was inspired by the development of crystal rectifiers (used to enable radar) during World War II and felt that the answer to creating a component to replace the expensive and unreliable vacuum tubes might lie in semiconductors (a solid-state device). Kelly approached William Shockley, one of the lab’s leading physicists, and presented his ideas for improving the components that would carry voice over the wires. Kelly said it would be wonderful if noisy mechanical relays and power-hungry vacuum tubes were replaced by solid-state electronics. The idea stuck with Shockley and became his main goal. Kelly charged Shockley with finding a way to make it happen. Although he was a brilliant theorist, he was not very good at turning his ideas into reality. Shockley tried several times to prove his theory that he could connect the two sides of a semiconductor by energizing a plate above it through field-effect electron transfer. He was unsuccessful. Frustrated, he sought help from two other Bell Labs physicists, John Bardeen (expert in semiconductor electronic theory) and Walter Brattain (expert in prototyping and using lab equipment). They then joined his team. Shockley agreed to let the two work on their own as a team. Over the years, they had tried many times to implement the theory of field effects, but all failed. They carefully checked their calculations and found that they should be possible in theory. Later, Bardeen and Brattain thought outside the box and experimented with thin sheets of silicon and germanium to try to make the field effect work. In the fall of 1947, Brattain encountered difficulties in condensing water on the surface of semiconductors. Instead of letting the water dry, he dripped water on top of the silicon and stimulated the plate above, finally observing the amplification effect. The water droplets helped to solve the surface barrier problem and help the electron flow, but the effect was too small to clearly amplify the sound signal to the level required for successful sound transmission. In December 1947 (known to the world as the Miracle Month), they figured out how to eliminate the field effect gap, remove the water, and make gold contacts to the semiconductor. They used germanium, which was easier to work with at the time, and used a thin, naturally occurring oxide film on the germanium to insulate it. However, many subsequent attempts failed. By mid-December, by accident, Walter Brattain accidentally washed away the oxygen coating, allowing the gold contacts to touch the germanium directly! Success!!! He observed good amplification and the transistor worked. Instead of the electrons being pulled to the semiconductor surface as Shockley's field effect theory had assumed, Brattain/Bardeen discovered that by using gold contacts to touch the semiconductor, they were injecting holes into the semiconductor, allowing current to flow. Around mid-December 1947, they began making a working prototype without Shockley's knowledge. Brattain found a triangular piece of plastic, wrapped the gold foil around the hypotenuse of the plastic, and made a razor-thin slit in the apex of the triangle. This was a very crude prototype design. They used paper clips to make springs to press a triangular plastic piece, with a lead at each end, onto a thin germanium semiconductor on top of a thin copper plate. The copper plate under the germanium could be used as a third lead if desired (Figure 7). The result was a prototype called a point-contact transistor. Brattain and Bardeen called Shockley to tell him the good news. My research suggests that Shockley had mixed feelings, happy that the experiment had worked but disappointed that he hadn’t created it himself. On December 23, 1947, a week after their breakthrough, they demonstrated it to Shockley’s superiors (the public announcement was on June 30, 1948). A photo was later taken to commemorate this historic moment (Figure 8). Shockley knew that the point-contact transistor was fragile and would not be easy to manufacture industrially, so he did everything he could (alone) to improve it. Shockley worked furiously to try to solve the problem his own way…by layering semiconductor materials together to make the transistor more compact, and he wrote down his thoughts as he went along. He did a lot more research, eventually completing the theory and patenting the junction transistor (filed on June 25, 1948). The functional NPN junction transistor was introduced on April 20, 1950 (with the help of Gordon Teal and Morgan Sparks). There is more detail about this history than you might think. On December 10, 1956, William Shockley, John Bardeen, and Walter Brattain shared the Nobel Prize for the invention of the transistor.
Figure 8: John Bardeen (left), William Shockley (center), Walter Brattain (right). (Reused with permission from Nokia Corporation) Reference Riordan, Michael and Lillian Hoddeson. 1997. Crystal Fire: The Invention of the Transistor and the Birth of the Information Age. New York, NY: WW Norton & Company, Inc. Ryder, RM 1958. “Ten years of Transistors,” Radio-Electronics Magazine, May, page 35. Houghton Mifflin Harcourt Publishing Company. 1991. “ALEX ANDER GRAHAM BELL". Retrieved Dec. 19, 2017. Riordan, Michael, Lillian Hoddeson, and Conyers Herring. 1999. "The Invention of the Transistor", Modern Physics, Vol 71, No. 2: Centenary.
A vacuum tube is actually an "incandescent lamp", so it is inevitable that it is large in size and has high power consumption. However, in terms of electrical performance, semiconductors seem to be more fragile than vacuum devices.
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