The Transistor
Bell Labs, the Point-Contact Device, and What Followed
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The audiobook explains the differences between various transistor types including BJTs and MOSFETs, discusses semiconductor materials like silicon, and examines packaging methods used in production. It also addresses naming conventions, licensing issues, and proprietary developments that shaped the industry's growth. Chapters cover both simplified operation principles and complex technical details needed for understanding modern integrated circuits.
This detailed exploration of transistor history and technology will interest electrical engineering students, professionals working in semiconductor design, and anyone curious about how the electronic devices we use daily came to be.
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The thermionic triode, invented in 1907, made amplified radio and long-distance telephony possible, but it was fragile and used a lot of power. In 1909, William Eccles discovered the crystal diode oscillator. Julius Edgar Lilienfeld filed a patent for a field-effect transistor in Canada in 1925, and then again in the United States in 1926 and 1928. He never published research or showed working prototypes, and because semiconductor technology wasn’t advanced enough, his ideas couldn’t be used in the 1920s or 1930s. In 1934, Oskar Heil patented a similar device in Europe.
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At Bell Labs in Murray Hill, New Jersey, John Bardeen and Walter Brattain observed during late November through December 23, 1947, that two gold point contacts on a germanium crystal produced a signal with greater output power than input. William Shockley, head of the Solid State Physics Group, recognized the importance and expanded knowledge of semiconductors over the following months. John R. Pierce coined the term "transistor" as a contraction of "transresistance." Shockley proposed filing a patent based on the field-effect concept, but lawyers at Bell Labs advised against it due to prior patents by Lilienfeld. What resulted from their efforts that year was the first point-contact transistor. In 1956, Shockley, Bardeen, and Brattain shared the Nobel Prize in Physics "for their researches on semiconductors and their discovery of the transistor effect." Initially attempting to build a field-effect transistor, Shockley's team encountered difficulties with surface states and materials, leading them instead to invent the bipolar point-contact and junction transistors.
In 1948, physicists Herbert Mataré and Heinrich Welker working at the Compagnie des Freins et Signaux Westinghouse in Paris independently invented the point-contact transistor. Mataré, who had worked on crystal detectors during World War II, began researching interference in 1947 and by June had achieved consistent results with germanium samples from Welker, similar to what Bardeen and Brattain had done earlier that December. Realizing Bell Labs had already invented the transistor, the company moved quickly to get its transistron into production for use in France’s telephone network, filing a patent application on August 13. Later that same year, William Shockley at Bell Labs applied for a patent for the first bipolar junction transistor, and on April 12, 1950, chemists Gordon Teal and Morgan Sparks successfully produced a working NPN germanium transistor. Bell announced this new sandwich transistor on July 4, 1951.
In 1953, Philco developed the first high-frequency transistor, a surface-barrier germanium device that could operate at up to 60 MHz. The process involved etching depressions into an n-type germanium base from both sides using jets of indium(III) sulfate until it was just a few ten-thousandths of an inch thick. Indium was then electroplated into these depressions to form the collector and emitter. That same year, AT&T began using transistors in their No. 4A Toll Crossbar Switching System, where they selected trunk circuits based on routing information encoded on translator cards. This system replaced an earlier model, the Western Electric No. 3A phototransistor, which read mechanical encoding from punched metal cards.
In 1953, Herbert Mataré’s company, INTERMETALL, showed the first prototype pocket transistor radio at a trade fair in Düsseldorf. The actual production model, the Regency TR-1, came out in October 1954, made by a partnership between Regency and Texas Instruments and built in Indianapolis. It was about pocket-sized, had four transistors and a germanium diode, and was designed by a Chicago firm. It came in six colors at first. The first all-transistor car radio appeared in 1955, developed by Chrysler and Philco and offered as an option for 1956 models, with showrooms getting them in October of that year.
In 1954, Morris Tanenbaum developed the first working silicon transistor at Bell Labs, on January 26. The same year, Texas Instruments announced the first production commercial silicon transistor, a breakthrough led by Gordon Teal, who had previously worked at Bell Labs and was known for his skill in growing high-purity crystals. That same year, Sony released the TR-63, the first mass-produced transistor radio, which helped drive widespread adoption of transistor radios. By the late 1950s, transistors had largely replaced vacuum tubes as the dominant electronic technology.
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The concept of the field-effect transistor began with physicist Julius Edgar Lilienfeld, who in 1926 filed a patent for a device resembling what would later be known as a MESFET. He returned to the idea two years later, in 1928, when he submitted a second patent for an insulated-gate field-effect transistor. The notion was revisited by engineer Oskar Heil during the 1930s and again by William Shockley in the 1940s.
In 1945, Heinrich Welker was granted a patent for the JFET, or junction field-effect transistor. Then in 1952, Shockley provided a theoretical explanation of how it worked. A practical version followed in 1953, created by George C. Dacey and Ian M. Ross.
In 1948, at Bell Labs, Bardeen and Brattain patented a device that would become the foundation for modern microchips—the insulated-gate FET, or IGFET, which used an inversion layer. That patent, along with the concept of the inversion layer itself, laid the groundwork for technologies we rely on today, like CMOS and DRAM.
In the beginning, the semiconductor industry centered on the junction transistor, a large and complicated device that couldn’t be made in bulk, which kept it limited to only a few specialized uses. Researchers began exploring field-effect transistors (FETs) as possible replacements, but they struggled to make them function correctly. The main problem was what researchers called the surface state barrier—a layer at the material’s surface that blocked electric fields from reaching the inside, preventing FETs from working as intended.
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In 1955, Carl Frosch and Lincoln Derick accidentally created a layer of silicon dioxide over a silicon wafer and noticed it had surface passivation effects. By 1957, using masking and predeposition techniques, they were able to produce silicon dioxide field-effect transistors—what would become the first planar transistors, with drain and source adjacent on the same surface. They demonstrated that silicon dioxide acted as an insulator and protective layer, preventing dopants from diffusing into the wafer. Later, in 1960, J. R. Ligenza and W. G. Spitzer examined the process of thermally grown oxides, built a high-quality Si/SiO2 stack, and published their findings.
In 1959, Mohamed Atalla and Dawon Kahng from Bell Labs came up with a new kind of transistor using silicon. The next year, they and their team showed it worked. E. E. LaBate and E. I. Povilonis built the device, while M. O. Thurston, L. A. D'Asaro, and J. R. Ligenza handled the processes that made it possible. H. K. Gummel and R. Lindner studied how it behaved. This MOSFET was smaller, used less power, and could fit more transistors into a chip than earlier versions. It made it possible to pack over 10,000 transistors onto one integrated circuit.
The idea of the inversion layer, first developed by Bardeen and Brattain in 1948, became the foundation for today’s CMOS technology. In 1963, Chih-Tang Sah and Frank Wanlass at Fairchild Semiconductor invented the CMOS process. Then in 1967, Dawon Kahng and Simon Sze reported the creation of a floating-gate MOSFET.
In 1967, researchers at Bell Labs including Robert Kerwin, Donald Klein, and John Sarace created the self-aligned gate, also known as the silicon-gate MOS transistor. Later, at Fairchild Semiconductor, Federico Faggin and Tom Klein used that design to build the first silicon-gate MOS integrated circuit.
In 1984, researchers Toshihiro Sekigawa and Yutaka Hayashi from the Electrotechnical Laboratory demonstrated a double-gate MOSFET. Then, in 1989, Digh Hisamoto and his team at Hitachi Central Research Laboratory developed the FinFET, a kind of 3D non-planar multi-gate MOSFET.
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Transistors stand as perhaps the most pivotal invention of the twentieth century, given their role as the core active parts in nearly all electronic devices today. The journey began at Bell Labs, where researchers sought to develop something better than the large, inefficient vacuum tubes that dominated early electronics. It was there that they created the point-contact device, a breakthrough that laid the groundwork for everything that followed. This small but powerful component would become the foundation of modern technology, enabling computers, smartphones, and countless other innovations we rely on daily. Without this invention, the electronic age as we know it simply wouldn’t exist. The point-contact transistor wasn't just an advance—it was the start of a new era in human communication and computation.
In 1947, the first transistor was invented at Bell Labs, a breakthrough later recognized with an IEEE Milestone in 2009. The same organization honored the junction transistor, developed the following year in 1948, with another milestone. Then in 1959, the MOSFET received the same recognition. Each of these inventions marked a major step forward in electronics and laid the groundwork for the modern age.
The MOSFET is the most widely used transistor today, found in everything from computers and smartphones to communication devices. Called by the US Patent and Trademark Office a "groundbreaking invention that transformed life and culture around the world," it has been central to modern digital electronics since the late 20th century. It enabled the digital age and is considered possibly the most important invention in electronics. Because it can be mass-produced using automated semiconductor fabrication from basic materials, its cost per unit is incredibly low. By 2018, more than 13 sextillion had been made—more than any other artificial object in history.
Today, billions of individual MOS transistors are made each year, but the vast majority end up inside integrated circuits, which are also called ICs, microchips, or simply chips. These chips combine transistors with other components like diodes and resistors to build complete electronic systems. A logic gate might use around 20 transistors, while advanced microprocessors, as of 2023, can contain as many as 92 billion transistors on a single die. Some chips even have double that number when using dual die, and the most advanced ones, by 2020, reached up to 2.6 trillion transistors. Transistors in these processors are organized into logic gates to carry out computations.
The transistor changed everything, making devices cheap, flexible, and reliable. It replaced old mechanical systems with new electronic ones, especially in appliances and machines. Today, it's simpler and less expensive to use a microcontroller and write code than build a mechanical system from scratch. This shift happened because of innovations at Bell Labs, where engineers worked on the point-contact device. The results were far-reaching, turning control systems into something much easier to design and produce. Transistorized circuits became the standard way to manage everything from home appliances to industrial machinery. What followed was a revolution in how we build and use technology.
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A transistor operates by using a small signal between certain terminals to control a much larger signal at others, a quality known as gain. It can boost a weak input—whether voltage or current—into a stronger output, functioning as an amplifier. At the same time, it acts like an electrically controlled switch, with its behavior governed by other components in the circuit. This device emerged from work done at Bell Labs, where researchers developed what came to be known as the point-contact device. The transistor’s ability to amplify and switch makes it a foundational element in modern electronics.
A bipolar junction transistor features three labeled terminals: base, collector, and emitter. By allowing a small current to flow between the base and emitter, it becomes possible to regulate or turn on a significantly larger flow of current from the collector to the emitter.
A field-effect transistor, or FET, is a device with three parts: gate, source, and drain. The voltage applied to the gate controls the flow of current between the source and the drain. This simple setup allows the FET to act as a switch or an amplifier in electronic circuits. It's one of the key components in modern electronics, forming the basis for many devices we use every day. The operation relies on the electric field created by the gate voltage to modulate the conductivity of the channel between source and drain. This principle makes the FET a fundamental building block in semiconductor technology.
In the diagram shown, you see a standard bipolar transistor operating within a circuit. The flow of electrical charge moves from the emitter to the collector, controlled by the base current. Since the connection between the base and emitter acts like a semiconductor diode, a voltage difference naturally forms between these two points. This voltage drop depends on the specific material used in the transistor and is known as VBE, or Base Emitter Voltage.
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Transistors function as electronic switches in digital circuits, capable of being either fully on or fully off. They are used in both high-power systems like switched-mode power supplies and low-power systems such as logic gates. When used this way, key performance factors include the amount of current they can handle, the voltage they can manage, and how fast they can switch, which is measured by rise and fall times.
When a transistor operates as a switch, it must closely mimic an ideal switch, acting like an open circuit when off and a short circuit when on, with a transition that happens instantly. Engineers design the system so that when the switch is off, only tiny leakage currents flow—too small to impact the rest of the circuit. In the on position, the transistor's resistance must be minimal, again not affecting how the rest of the circuit behaves. The change from off to on has to be swift enough that it doesn’t cause problems in the system’s operation.
In a grounded-emitter transistor circuit, like the light-switch example, raising the base voltage causes both emitter and collector currents to grow quickly. As this happens, the voltage between collector and emitter decreases due to lower resistance. If that voltage difference reaches zero—or close to it—the collector current is then limited only by the load resistance and the supply voltage. At this point, the transistor is said to be saturated, meaning current flows easily from collector to emitter. When that occurs, the switch is considered to be on.
When bipolar transistors are used as switches, they must be biased to operate in two regions: cut-off when off, and saturation when on. This requires a sufficient base drive current, which allows the transistor to control a much larger collector current. The ratio between these currents depends on the transistor type and the collector current itself. In a typical light-switch circuit, a resistor is selected to provide enough base current to keep the transistor saturated. The value of this base resistor is determined by the supply voltage, the voltage drop across the transistor’s collector-emitter junction, the desired collector current, and the transistor's amplification factor, beta.
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The common-emitter amplifier is a design where a small voltage change at the input, labeled Vin, controls a tiny base current in the transistor. Because of the transistor’s current amplification factor and how the circuit is built, these small fluctuations in input voltage lead to much larger variations in output voltage, Vout. This setup makes the transistor act as an effective signal booster, turning slight changes into meaningful swings in power. It's a key function that allows electronic devices to amplify weak signals into usable ones.
Transistor amplifiers can be built in several different ways, each offering a specific kind of gain—sometimes just current, sometimes just voltage, and sometimes both. These setups are called configurations, and they allow engineers to tailor the performance of a single transistor for different purposes. The flexibility in how transistors are connected makes them incredibly useful in electronic devices where signal strength needs to be increased. Whether it's boosting a weak electrical signal or controlling power output, these various arrangements provide the tools needed to shape the behavior of the transistor in exactly the way required. This adaptability is one of the key reasons why the transistor became so central to the development of modern electronics.
From mobile phones to televisions, countless devices rely on amplifiers to boost sound and process signals. The earliest audio amplifiers using discrete transistors delivered only a few hundred milliwatts of power. Over time, as transistor design improved and amplifier circuits grew more sophisticated, both the power output and audio quality rose steadily. These advances made possible the wide range of electronic products we use today, all built on the foundation laid by early work in semiconductor technology.
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A simple trick helps distinguish transistor types using the arrow in their symbol. For bipolar junction transistors, the arrow on an n–p–n design points away from the base, which you can remember as “Not Point iN.” In contrast, the arrow on a p–n–p version points toward the base, described as “Points iN Proudly.” But this rule doesn’t hold for MOSFET-based designs, where the arrow usually points in the opposite direction—specifically, the arrow for an n–p–n MOSFET points inside.
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The field-effect transistor, also known as a unipolar transistor, operates using either electrons in n-channel FETs or holes in p-channel FETs to carry electric current. This type of transistor has four distinct terminals: source, gate, drain, and body, which is also called the substrate. In most cases, the body terminal is internally connected to the source terminal within the device's packaging, and this setup will be assumed in the explanation that follows.
In a field-effect transistor, current flows from drain to source through a conducting channel, and that flow is controlled by applying voltage between the gate and source terminals. As the gate–source voltage (VGS) increases, the drain–source current (IDS) grows exponentially until it reaches a threshold, after which it rises at a roughly quadratic rate—represented as IDS proportional to (VGS − VT) squared, where VT is the threshold voltage. This behavior is seen in the space-charge-limited region above threshold. However, modern devices like those at the 65 nm technology node don’t show this quadratic response.
FETs come in two main types: junction FETs, or JFETs, and insulated gate FETs, which are more commonly called MOSFETs, short for metal–oxide–semiconductor FET. The MOSFET gets its name from its structure, with layers of metal, oxide, and semiconductor material. In contrast, the JFET’s gate connects directly to the channel through a p–n diode formed between the source and drain. This design makes the n-channel JFET similar in function to the vacuum tube triode, since both work in depletion-mode, have high input impedance, and control current using an input voltage.
Metal–semiconductor FETs, or MESFETs, are a type of JFET where the standard reverse biased p–n junction is swapped out for a metal–semiconductor junction. These devices, along with HEMTs—also known as high-electron-mobility transistors or HFETs, which use a two-dimensional electron gas with very high carrier mobility for charge transport, are especially suited for operating at very high frequencies, reaching several gigahertz.
Field-effect transistors, or FETs, come in two main types: depletion-mode and enhancement-mode, based on how they behave when there's no voltage applied between the gate and source. In enhancement mode, the channel is off until a voltage is applied to turn it on, while in depletion mode, the channel is already on at zero bias, and a voltage can reduce conduction. A more positive gate voltage increases current flow in n-channel devices and decreases it in p-channel ones. Most junction FETs are depletion-mode because making them enhancement-mode would cause the diode junctions to conduct unintentionally, whereas most insulated-gate FETs are enhancement-mode types.
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The metal–oxide–semiconductor field-effect transistor, or MOSFET, is a kind of field-effect transistor made by carefully oxidizing a semiconductor, usually silicon. It has an insulated gate that controls how well electricity flows through it, using voltage to change its conductivity. This makes it useful for amplifying or switching electronic signals. The MOSFET is the most common type of transistor in use today and forms the foundation of modern electronics. In fact, nearly all transistors—99.9% of them—are MOSFETs.
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Bipolar transistors get their name from the fact that they rely on both majority and minority carriers for conduction. The first kind of transistor to be produced on a large scale was the bipolar junction transistor. It’s made by combining two junction diodes, forming either an n–p–n structure—where a thin p-type semiconductor layer sits between two n-type layers—or a p–n–p structure, with n-type layers surrounding a p-type base. These arrangements create two p–n junctions: one between the base and emitter, and another between the base and collector. The area in between is called the base region. Simply wiring two diodes together without sharing a semiconductor layer won’t produce a transistor.
A bipolar junction transistor features three terminals—emitter, base, and collector—each connected to a distinct semiconductor layer. When functioning in its active mode, the device operates with the emitter–base junction forward-biased, allowing electrons and holes to recombine at that interface. Simultaneously, the base–collector junction is reverse-biased, pushing carriers away from it into the base region. Electrons injected into this narrow base diffuse toward the collector, where most are swept across due to the reverse bias. Only a small portion of the electrons recombine within the base, which contributes to the base current. The base is intentionally doped lightly compared to the emitter and collector, reducing recombination rates and allowing more carriers to reach the collector. By adjusting the number of electrons leaving the base, the collector current can be controlled. That current is approximately β times the base current, where β typically exceeds 100 in small-signal transistors but may be lower in high-power versions.
The bipolar junction transistor operates differently from the field-effect transistor, notably as a low-input-impedance device. When the base–emitter voltage (VBE) rises, both the base–emitter current and the collector–emitter current (ICE) grow exponentially. This behavior aligns with models known as the Shockley diode model and the Ebers–Moll model. Due to this exponential increase, the BJT achieves greater transconductance than the FET.
When light hits the base region of a bipolar transistor, photons are absorbed and create a photocurrent that functions as the base current. This causes the device to conduct electricity. The resulting collector current is roughly β times the magnitude of that photocurrent. Transistors built for this function feature a transparent window in their casing so light can reach the sensitive area. These special devices are known as phototransistors.
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The MOSFET is by far the most widely used transistor today, found in 99.9% of all transistors around the world. The bipolar junction transistor, or BJT, was the dominant type from the 1950s through the 1960s. Even after MOSFETs became common in the 1970s, BJTs were still preferred for analog uses like amplifiers because of their greater linearity. By the 1980s, newer MOSFET designs such as power MOSFETs, LDMOS, and RF CMOS began replacing BJTs in most power applications. In integrated circuits, MOSFETs quickly took over digital circuitry during the 1970s. Discrete MOSFETs are also used in analog circuits, voltage regulators, amplifiers, power transmitters, and motor drivers.
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Manufacturers sometimes use their own numbering systems, like CK722, but those don’t always reveal who made the device. Because components are often second-sourced, a prefix such as MPF in MPF102 might have originally stood for Motorola FET, yet now it’s not a dependable sign of origin. Some naming conventions incorporate elements from other systems: for example, a PN2222A may be a 2N2222A from Fairchild Semiconductor housed in a plastic case. However, a PN108 is actually a plastic version of a BC108, not a 2N108, and the PN100 doesn’t relate to other xx100 devices at all.
When manufacturers bought large quantities of the same type of part, they often had them labeled with internal designations instead of using standard industry numbers. For instance, Hewlett-Packard might order a part known as 1854,0053, which is actually a JEDEC 2N2218 transistor. That same device also carries a CV number: CV7763.
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At Bell Labs, where the transistor was born, there were many different ways to name parts, and when these names appeared on devices, they often got shortened or mixed up. This led to confusion. For example, two completely different transistors could end up with the same label—like J176. One was a low-power JFET called the J176, while another was a more powerful MOSFET labeled 2SJ176. These similarities in naming made it hard to tell them apart, especially when looking at the devices themselves.
When older through-hole transistors are replaced with surface-mount versions, each manufacturer assigns them different part numbers. That’s because every company has its own way of managing differences in pinout layouts and choices like having two matched devices in one package. So even if the original transistor—such as the 2N3904—was officially recognized by a standards group and widely known to engineers, the updated versions don’t stick to a single naming system.
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The first bipolar junction transistors were made from germanium. Today, silicon types are more common, but some advanced microwave and high-performance versions now use gallium arsenide or silicon–germanium. Single-element semiconductor materials like germanium and silicon are called elemental semiconductors.
The most common semiconductor materials used in transistors have rough parameters listed in a nearby table. Those numbers change depending on several conditions. Temperature plays a role, as does electric field strength. The level of impurities in the material affects it too, along with strain and other factors that can influence how these materials behave.
The junction forward voltage is the voltage needed across the emitter-base junction of a BJT to get a specific current flowing through the base. For a typical silicon device, that voltage is about 0.7 volts when the current is 1 milliampere. The current grows quickly as you increase the voltage, and lower voltages mean less power is used to run the transistor. This voltage goes down as temperature rises — specifically, for silicon, it drops by 2.1 millivolts for each degree Celsius. In some circuits, that change can cause problems, so engineers sometimes add special components called sensistors to keep things stable.
The electrical behavior of a MOSFET depends on the density of mobile carriers in its channel, which is influenced by the electric field that forms the channel and other factors like the impurity level within it. Some impurities, known as dopants, are intentionally added during the manufacturing process to control how the device functions. These dopants help determine the electrical properties of the MOSFET, making them essential for its operation.
When an electric field of one volt per meter is applied, electrons and holes move through semiconductor material at speeds reflected in the electron mobility and hole mobility values. Generally speaking, higher mobility means faster operation for transistors. According to the table, germanium outperforms silicon in this area. Still, germanium comes with four significant drawbacks when measured against both silicon and gallium arsenide.
Because electrons move more easily through semiconductor materials than holes do, n–p–n bipolar transistors operate faster than p–n–p ones. Among all semiconductors, gallium arsenide, or GaAs, offers the highest electron mobility, which is why it's used in high-frequency applications. A newer type of field-effect transistor, the high-electron-mobility transistor, or HEMT, uses a heterostructure made of aluminium gallium arsenide and gallium arsenide. This design achieves twice the electron mobility of a GaAs-metal junction. Due to their speed and low noise, HEMTs are found in satellite receivers working at around 12 gigahertz. Researchers are also developing HEMTs based on gallium nitride and aluminum gallium nitride, which offer even higher electron mobility for future uses.
When a transistor operates, it generates heat, and there’s a limit to how hot the junction can get before damage occurs. Maximum junction temperature values come from datasheets provided by various manufacturers. These temperatures are critical — if exceeded, the device may be permanently damaged. This is not just theoretical; it's a real concern for anyone working with transistors, especially in applications where heat buildup is possible. The data reflects real-world conditions and helps engineers design circuits that keep components within safe operating ranges. It’s a simple rule: don’t let the junction get too hot, or you risk destroying the transistor.
The Al–Si junction is a type of metal–semiconductor barrier diode, often called a Schottky diode because of its high-speed performance. It's included in certain tables due to its presence in some silicon power IGFETs. During the fabrication process, a parasitic reverse Schottky diode can form between the source and drain. While this diode may cause problems in circuit design, it’s sometimes intentionally used as part of the circuit’s function.
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Transistors are housed in different types of packages, with two main kinds being through-hole and surface-mount, or SMD for short. The ball grid array, or BGA, is a newer kind of surface-mount package. Instead of traditional leads, it uses solder balls on the bottom. These smaller designs offer better performance at high frequencies but handle less power than their through-hole counterparts.
Transistor packages come in different materials like glass, metal, ceramic, or plastic, and the type of package often affects how much power the transistor can handle and how well it works at certain frequencies. Power transistors usually have bigger packages that can be attached to heat sinks to keep them cool, and their collector or drain is often connected directly to the metal case. On the opposite end of the scale, some surface-mount microwave transistors are so tiny they could fit on the head of a pin.
Transistors come in different packages, and while those packages are mostly standardized, the way functions are assigned to the pins isn’t. That means that even within the same transistor type, the terminals might be used for different purposes depending on the specific model. For example, a part number like BC212L or BC212K shows how the function assignment can vary between similar transistors. So while the physical package may look the same, what’s inside and how it works can differ based on that terminal labeling.
Today, most transistors are made in small surface-mount packages, but back when things were different, through-hole transistors came in just a few standard forms. The list of those older-style packages is short, and here’s a look at some of the most common ones, presented in alphabetical order.
In the 1960s, IBM used a type of hybrid circuit module called the SLT, which included transistor and diode chips protected by glass. These chips, known as die, were not packaged but instead assembled into larger devices. Another way to handle these bare chips was through techniques like direct chip attach or chip-on-board, where the transistors were mounted directly onto boards without traditional casing. This approach allowed for more compact designs and helped move electronics forward during a time when miniaturization was becoming key.
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